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New research – temperature (July 26, 2016)

Posted by Ari Jokimäki on July 26, 2016

Some of the latest papers on temperature are shown below. First a few highlighted papers with abstracts and then a list of some other papers. If this subject interests you, be sure to check also the other papers – they are by no means less interesting than the highlighted ones.

Highlights

The rogue nature of hiatuses in a global warming climate (Sévellec, Sinha & Skliris, 2016)
http://onlinelibrary.wiley.com/doi/10.1002/2016GL068950/abstract

Abstract: The nature of rogue events is their unlikelihood and the recent unpredicted decade-long slowdown in surface warming, the so-called hiatus, may be such an event. However, given decadal variability in climate, global surface temperatures were never expected to increase monotonically with increasing radiative forcing. Here, surface air temperature from twenty climate models is analysed to estimate the historical and future likelihood of hiatuses and “surges” (faster than expected warming), showing that the global hiatus of the early 21st Century was extremely unlikely. A novel analysis of future climate scenarios suggests that hiatuses will almost vanish and surges will strongly intensify by 2100 under a “business as usual” scenario. For “CO2 stabilisation” scenarios, hiatus and surge characteristics revert to typical 1940s values. These results suggest to study the hiatus of the early 21st Century and future re-occurrences as rogue events, at the limit of the variability of current climate modelling capability.

Underestimated warming of northern Canada in the Berkeley Earth temperature product (Way, Oliva & Viau, 2016)
http://onlinelibrary.wiley.com/doi/10.1002/joc.4808/abstract

Abstract: The Berkeley Earth surface temperature (BERK) project provides gridded global temperature anomaly products using an automated geostatistical approach to adjust station data for systematic biases. Despite its widespread usage, the BERK data set has not been evaluated at the national-scale, especially in data-sparse high latitude environments. This study provides an evaluation of the BERK product across all of Canada using 333 climate stations made available from the homogenized Environment Canada station network (HTcan). Comparison between co-located monthly air temperature anomalies for the two data sets suggests small differences between the two products for mean surface (∼2 m) air temperature. However, the relatively minimal bias in mean temperature is a consequence of contrasting cold and warm biases in minimum and maximum air temperatures, respectively, that are larger but effectively even out when averaged together. The BERK product is shown to exhibit systematic underestimation of recent regional warming in northern Canada which when combined with an overestimation of warmth earlier in the record results in an observable reduction in warming rates for minimum and mean temperature anomalies since 1950. The temporal evolution and spatial pattern of the observed biases suggest that the BERK-automated adjustments to station data in northern Canada miss some inhomogeneities in the raw station data. These results highlight the need for enhanced data recovery and homogenization efforts in data-sparse high latitude regions and emphasize the importance of national-scale climate data sets for evaluating global gridded products. We also recommend caution when using the BERK minimum and maximum monthly air temperature products for long-term trend analyses.

Revisiting whether recent surface temperature trends agree with the CMIP5 ensemble (Lin & Huybers, 2016)
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0123.1

Abstract: A weaker trend in global mean temperature over the past 15 years relative to the preceding decades has been characterized as significantly lower than those contained within the CMIP5 ensemble. In this study, divergence between model simulations and observations is estimated using a fixed-intercept linear trend with a slope estimator that has one-third the noise variance compared to simple linear regression. Following the approach of Fyfe et al. (2013) where inter-model spread is used to assess the distribution of trends, but using the fixed-intercept trend metric, demonstrates that recently observed trends in global-mean temperature are consistent (p > 0.1) with the CMIP5 ensemble for all 15-year intervals of observation-model divergence since 1970. Significant clustering of global trends according to modeling center indicates that the spread in CMIP5 trends is better characterized using ensemble members drawn across models, as opposed to using ensemble members from a single model. Despite model-observation consistency at the global level, substantial regional discrepancies in surface temperature trends remain.

Climate change in the Kola Peninsula, Arctic Russia, during the last 50 years from meteorological observations (Marshall, Vignols & Rees, 2016)
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0179.1

Abstract: We provide a detailed climatology and evaluation of recent climate change in the Kola Peninsula, Arctic Russia, a region influenced by both the North Atlantic and Arctic Oceans. The analysis is based on 50 years of monthly surface air temperature (SAT), precipitation (PPN) and sea level pressure (SLP) data from ten meteorological stations for 1966-2015. Regional mean annual SAT is ~0°C: the moderating effect of the ocean is such that coastal (inland) stations have a positive (negative) value. Examined mean annual PPN totals rise from ~430 mm in the north-east of the region to ~600 mm in the west.

Annual SAT in the Kola Peninsula has increased by 2.3 ± 1.0 °C over the past 50 years. Seasonally, statistically significant warming has taken place in spring and fall, although the largest trend has occurred in winter. While there has been no significant change in annual PPN, spring has become significantly wetter and fall drier. The former is associated with the only significant seasonal SLP trend (decrease). A positive winter North Atlantic Oscillation (NAO) index is generally associated with a warmer and wetter Kola Peninsula while a positive Siberian High (SH) index has the opposite impact. The relationship between both the NAO and SH and SAT is broadly coherent across the region whereas their relationship with PPN varies markedly, although none of the relationships are temporally invariant. Reduced sea ice in the Barents and White Seas and associated circulation changes are likely to be the principal drivers behind the observed changes.

Water temperature increases in the river Rhine in response to climate change (Hardenbicker et al. 2016)
http://link.springer.com/article/10.1007%2Fs10113-016-1006-3

Abstract: The present study analyzes climate change effects on the water temperature of the Rhine, one of the largest rivers in Central Europe. Simulation calculations were performed based on a range of climate and river flow projections for the near (2021–2050) and for the far future (2071–2100) compared to a reference period (1961–1990). Changes in mean annual water temperature in the near future range between +0.6 and +1.4 °C and between +1.9 and +2.2 °C in the far future (average of nine stations). Monthly mean values of the far future change in a more differentiated way by +0.4 to +1.3 °C in spring and +2.7 to +3.4 °C in late summer. The length of periods of high water temperature, expressed as successive days with water temperatures over 27 °C, increases by a factor of four until 2100. These prolonged durations of periods with unusually high water temperatures may provoke changes in the food web and in the rates of biological processes in the Rhine.

Other papers

Assessing the uncertainty of CESM-LE in simulating the trends of mean and extreme temperature and precipitation over China (Li, Zhu & Dong, 2016)
http://onlinelibrary.wiley.com/doi/10.1002/joc.4837/abstract

Reconciling Observed and Modelled Temperature and Precipitation Trends over Europe by Adjusting for Circulation Variability (Saffioti et al. 2016)
http://onlinelibrary.wiley.com/doi/10.1002/2016GL069802/abstract

Geo-spatial analysis of temporal trends of temperature and its extremes over India using daily gridded (1°×1°) temperature data of 1969–2005 (Chakraborty et al. 2016)
http://rd.springer.com/article/10.1007%2Fs00704-016-1869-8

Sudden stratospheric warmings observed in the last decade by satellite measurements (Kishore et al. 2016)
http://www.sciencedirect.com/science/article/pii/S0034425716302656

Longitudinal Asymmetric Trends of Tropical Cold-point Tropopause Temperature and Their Link to Strengthened Walker Circulation (Hu et al. 2016)
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0851.1

An in situ-based analysis of the relationship between land surface ‘skin’ and screen-level air temperatures (Good, 2016)
http://onlinelibrary.wiley.com/doi/10.1002/2016JD025318/abstract

Spatiotemporal rainfall and temperature trends throughout the Brazilian Legal Amazon, 1973–2013 (Almeida et al. 2016)
http://onlinelibrary.wiley.com/doi/10.1002/joc.4831/abstract

Spatial variation of deterministic chaos in mean daily temperature and rainfall over Nigeria (Fuwape et al. 2016)
http://rd.springer.com/article/10.1007%2Fs00704-016-1867-x

Homogenisation of temperature and precipitation time series with ACMANT3: method description and efficiency tests (Domonkos & Coll, 2016)
http://onlinelibrary.wiley.com/doi/10.1002/joc.4822/abstract

The tropical Pacific as a key pacemaker of the variable rates of global warming (Kosaka & Xie, 2016)
http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2770.html

The changing shape of Northern Hemisphere summer temperature distributions (McKinnon et al. 2016)
http://onlinelibrary.wiley.com/doi/10.1002/2016JD025292/abstract

Future Decreases in Freezing Days Across North America (Rawlins et al. 2016)
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0802.1

Arctic warming, moisture increase and circulation changes observed in the Ny-Ålesund homogenized radiosonde record (Maturilli & Kayser, 2016)
http://rd.springer.com/article/10.1007%2Fs00704-016-1864-0

Trend analysis of air temperature time series in Greece and their relationship with circulation using surface and satellite data: recent trends and an update to 2013 (Feidas, 2016)
http://rd.springer.com/article/10.1007%2Fs00704-016-1854-2

Urban Heat Island traverses in the City of Adelaide, South Australia (Clay et al. 2016)
http://www.sciencedirect.com/science/article/pii/S2212095516300293

Recent Extreme Arctic Temperatures are due to a Split Polar Vortex (Overland & Wang, 2016)
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0320.1

Potential tropical Atlantic impacts on Pacific decadal climate trends (Chikamoto et al. 2016)
http://onlinelibrary.wiley.com/doi/10.1002/2016GL069544/abstract

Consistent land surface temperature data generation from irregularly spaced Landsat imagery (Fu & Weng, 2016)
http://www.sciencedirect.com/science/article/pii/S0034425716302565

Spatial and temporal variation in daily temperature indices in summer and winter seasons over India (1969–2012) (Kumar et al. 2016)
http://rd.springer.com/article/10.1007%2Fs00704-016-1844-4

Spatial patterns of recent Antarctic surface temperature trends and the importance of natural variability: lessons from multiple reconstructions and the CMIP5 models (Smith & Polvani, 2016)
http://rd.springer.com/article/10.1007%2Fs00382-016-3230-4

From Urban to National Heat Island: the effect of anthropogenic heat output on climate change in high population industrial countries (Murray & Heggie, 2016)
http://onlinelibrary.wiley.com/doi/10.1002/2016EF000352/abstract

From accelerated warming to warming hiatus in China (Xie, Huang & Liu, 2016)
http://onlinelibrary.wiley.com/doi/10.1002/joc.4809/abstract

Weekly cycles in peak time temperatures and urban heat island intensity (Earl, Simmonds & Tapper, 2016)
http://iopscience.iop.org/article/10.1088/1748-9326/11/7/074003/meta

Radiative and Dynamical Influences on Polar Stratospheric Temperature Trends (Ivy, Solomon & Rieder, 2016)
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0503.1

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Papers on ice sheet collapse

Posted by Ari Jokimäki on February 3, 2016

This is a list of papers on ice sheet collapse. The list is not complete, and will most likely be updated in future in order to make it more thorough and more representative.

Potential sea-level rise from Antarctic ice-sheet instability constrained by observations – Ritz et al. (2015)
Abstract: Large parts of the Antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine-ice-sheet instability (MISI), a self-sustaining retreat of the grounding line triggered by oceanic or atmospheric changes. There is growing evidence that MISI may be underway throughout the Amundsen Sea embayment (ASE), which contains ice equivalent to more than a metre of global sea-level rise. If triggered in other regions, the centennial to millennial contribution could be several metres. Physically plausible projections are challenging: numerical models with sufficient spatial resolution to simulate grounding-line processes have been too computationally expensive to generate large ensembles for uncertainty assessment, and lower-resolution model projections rely on parameterizations that are only loosely constrained by present day changes. Here we project that the Antarctic ice sheet will contribute up to 30 cm sea-level equivalent by 2100 and 72 cm by 2200 (95% quantiles) where the ASE dominates. Our process-based, statistical approach gives skewed and complex probability distributions (single mode, 10 cm, at 2100; two modes, 49 cm and 6 cm, at 2200). The dependence of sliding on basal friction is a key unknown: nonlinear relationships favour higher contributions. Results are conditional on assessments of MISI risk on the basis of projected triggers under the climate scenario A1B (ref. 9), although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss. We find that contributions are restricted by a combination of these constraints, calibration with success in simulating observed ASE losses, and low assessed risk in some basins. Our assessment suggests that upper-bound estimates from low-resolution models and physical arguments (up to a metre by 2100 and around one and a half by 2200) are implausible under current understanding of physical mechanisms and potential triggers.
Citation: Catherine Ritz, Tamsin L. Edwards, Gaël Durand, Antony J. Payne, Vincent Peyaud, Richard C. A. Hindmarsh, Nature 528, 115–118 (03 December 2015) doi:10.1038/nature16147.

Marine Ice Sheet Collapse Potentially Under Way for the Thwaites Glacier Basin, West Antarctica – Joughin et al. (2014)
Abstract: Resting atop a deep marine basin, the West Antarctic Ice Sheet has long been considered prone to instability. Using a numerical model, we investigated the sensitivity of Thwaites Glacier to ocean melt and whether its unstable retreat is already under way. Our model reproduces observed losses when forced with ocean melt comparable to estimates. Simulated losses are moderate (1 mm per year of sea-level rise) collapse in the different simulations within the range of 200 to 900 years.
Citation: Ian Joughin, Benjamin E. Smith, Brooke Medley, Science 16 May 2014: Vol. 344, Issue 6185, pp. 735-738, DOI: 10.1126/science.1249055.

Where might we find evidence of a Last Interglacial West Antarctic Ice Sheet collapse in Antarctic ice core records? – Bradley et al. (2012) [FULL TEXT]
Abstract: Abundant indirect evidence suggests that the West Antarctic Ice Sheet (WAIS) reduced in size during the Last Interglacial (LIG) compared to the Holocene. This study explores this possibility by comparing, for the first time, ice core stable isotope records for the LIG with output from a glacio-isostatic adjustment (GIA) model. The results show that ice core records from East Antarctica are remarkably insensitive to vertical movement of the solid land motion driven by a simulated hypothetical collapse of the WAIS. However, new and so far unexplored sites are identified which are sensitive to the isostatic signal associated with WAIS collapse and so ice core proxy data from these sites would be effective in testing this hypothesis further.
Citation: S.L. Bradley, M. Siddall, G.A. Milne, V. Masson-Delmotte, E. Wolff, Global and Planetary Change, Volumes 88–89, May 2012, Pages 64–75, doi: 10.1016/j.gloplacha.2012.03.004.

Stability of the West Antarctic ice sheet in a warming world – Joughin & Alley (2011) [FULL TEXT]
Abstract: Ice sheets are expected to shrink in size as the world warms, which in turn will raise sea level. The West Antarctic ice sheet is of particular concern, because it was probably much smaller at times during the past million years when temperatures were comparable to levels that might be reached or exceeded within the next few centuries. Much of the grounded ice in West Antarctica lies on a bed that deepens inland and extends well below sea level. Oceanic and atmospheric warming threaten to reduce or eliminate the floating ice shelves that buttress the ice sheet at present. Loss of the ice shelves would accelerate the flow of non-floating ice near the coast. Because of the slope of the sea bed, the consequent thinning could ultimately float much of the ice sheet’s interior. In this scenario, global sea level would rise by more than three metres, at an unknown rate. Simplified analyses suggest that much of the ice sheet will survive beyond this century. We do not know how likely or inevitable eventual collapse of the West Antarctic ice sheet is at this stage, but the possibility cannot be discarded. For confident projections of the fate of the ice sheet and the rate of any collapse, further work including the development of well-validated physical models will be required.
Citation: Ian Joughin, Richard B. Alley, Nature Geoscience 4, 506–513 (2011) doi:10.1038/ngeo1194.

A new projection of sea level change in response to collapse of marine sectors of the Antarctic Ice Sheet – Gomez et al. (2010) [FULL TEXT]
Abstract: We present gravitationally self-consistent predictions of sea level change that would follow the disappearance of either the West Antarctic Ice Sheet (WAIS) or marine sectors of the East Antarctic Ice Sheet (EAIS). Our predictions are based on a state-of-the-art pseudo-spectral sea level algorithm that incorporates deformational, gravitational and rotational effects on sea level, as well as the migration of shorelines due to both local sea-level variations and changes in the extent of marine-based ice cover. If we define the effective eustatic value (EEV) as the geographically uniform rise in sea level once all marine-based sectors have been filled with water, then we find that some locations can experience a sea level rise that is ∼40 per cent higher than the EEV. This enhancement is due to the migration of water away from the zone of melting in response to the loss of gravitational attraction towards the ice sheet (load self-attraction), the expulsion of water from marine areas as these regions rebound due to the unloading, and the feedback into sea level of a contemporaneous perturbation in Earth rotation. In the WAIS case, this peak enhancement is twice the value predicted in a previous projection that did not include expulsion of water from exposed marine-sectors of the West Antarctic or rotational feedback. The peak enhancements occur over the coasts of the United States and in the Indian Ocean in the WAIS melt scenario, and over the south Atlantic and northwest Pacific in the EAIS scenario. We conclude that accurate projections of the sea level hazard associated with ongoing global warming should be based on a theory that includes the complete suite of physical processes described above.
Citation: Natalya Gomez, Jerry X. Mitrovica, Mark E. Tamisiea, Peter U. Clark, Geophys. J. Int. (2010) 180(2):623-634. doi: 10.1111/j.1365-246X.2009.04419.x.

Record of a Mid-Pleistocene depositional anomaly in West Antarctic continental margin sediments: an indicator for ice-sheet collapse? – Hillenbrand & Frederichs (2009)
Abstract: Modern global warming is likely to cause future melting of Earth’s polar ice sheets that may result in dramatic sea-level rise. A possible collapse of the West Antarctic Ice Sheet (WAIS) alone, which is considered highly vulnerable as it is mainly based below sea level, may raise global sea level by up to 5–6 m. Despite the importance of the WAIS for changes in global sea level, its response to the glacial–interglacial cycles of the Quaternary is poorly constrained. Moreover, the geological evidence for the disintegration of the WAIS at some time within the last ca. 750 kyr, possibly during Marine Isotope Stage (MIS) 11 (424–374 ka), is ambiguous. Here we present physical properties, palaeomagnetic, geochemical and clay mineralogical data from a glaciomarine sedimentary sequence that was recovered from the West Antarctic continental margin in the Amundsen Sea and spans more than the last 1 Myr. Within the sedimentary sequence, proxies for biological productivity (such as biogenic opal and the barium/aluminum ratio) and the supply of lithogenic detritus from the West Antarctic hinterland (such as ice-rafted debris and clay minerals) exhibit cyclic fluctuations in accordance with the glacial–interglacial cycles of the Quaternary. A prominent depositional anomaly spans MIS 15–MIS 13 (621–478 ka). The proxies for biological productivity and lithogenic sediment supply indicate that this interval has the characteristics of a single, prolonged interglacial period. Even though no proxy suggests environmental conditions much different from today, we conclude that, if the WAIS collapsed during the last 800 kyr, then MIS 15–MIS 13 was the most likely time period. Apparently, the duration rather than the strength of interglacial conditions was the crucial factor for the WAIS drawdown. A comparison with various marine and terrestrial climate archives from around the world corroborates that unusual environmental conditions prevailed throughout MIS 15–MIS 13. Some of these anomalies are observed in the pelagic Southern Ocean and the South Atlantic and might originate in major ice-sheet drawdown in Antarctica, but further research is required to test this hypothesis.
Citation: C.-D. Hillenbrand, G. Kuhn, T. Frederichs, Quaternary Science Reviews, Volume 28, Issues 13–14, June 2009, Pages 1147–1159, doi: 10.1016/j.quascirev.2008.12.010.

Reassessment of the Potential Sea-Level Rise from a Collapse of the West Antarctic Ice Sheet – Bamber et al. (2009) [FULL TEXT]
Abstract: Theory has suggested that the West Antarctic Ice Sheet may be inherently unstable. Recent observations lend weight to this hypothesis. We reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution. We obtain a value for the global, eustatic sea-level rise contribution of about 3.3 meters, with important regional variations. The maximum increase is concentrated along the Pacific and Atlantic seaboard of the United States, where the value is about 25% greater than the global mean, even for the case of a partial collapse.
Citation: Jonathan L. Bamber, Riccardo E. M. Riva, Bert L. A. Vermeersen, Anne M. LeBrocq, Science 15 May 2009: Vol. 324, Issue 5929, pp. 901-903, DOI: 10.1126/science.1169335.

The Sea-Level Fingerprint of West Antarctic Collapse – Mitrovica et al. (2009) [FULL TEXT]
Abstract: Recent projections of sea-level rise after a future collapse of theWest Antarctic Ice Sheet (for example, the Fourth Intergovernmental Panel on Climate Change Assessment Report) assume that meltwater will spread uniformly (that is, eustatically) across the oceans once marine-based sectors of the West Antarctic are filled. A largely neglected 1977 study predicted that peak values would be 20% higher than the eustatic in the North Pacific and 5 to 10% higher along the U.S. coastline. We show, with use of a state-of-the-art theory, that the sea-level rise in excess of the eustatic value will be two to three times higher than previously predicted for U.S. coastal sites.
Citation: Jerry X. Mitrovica, Natalya Gomez, Peter U. Clark, Science 06 Feb 2009: Vol. 323, Issue 5915, pp. 753, DOI: 10.1126/science.1166510.

Modelling West Antarctic ice sheet growth and collapse through the past five million years – Pollard & DeConto (2009) [FULL TEXT]
Abstract: The West Antarctic ice sheet (WAIS), with ice volume equivalent to ~5 m of sea level, has long been considered capable of past and future catastrophic collapse. Today, the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams. Grounding lines are several hundred metres below sea level and the bed deepens upstream, raising the prospect of runaway retreat. Projections of future WAIS behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms. Its variation since the Last Glacial Maximum is best known, with grounding lines advancing to the continental-shelf edges around ~15 kyr ago before retreating to near-modern locations by ~3 kyr ago. Prior collapses during the warmth of the early Pliocene epoch9 and some Pleistocene interglacials have been suggested indirectly from records of sea level and deep-sea-core isotopes, and by the discovery of open-ocean diatoms in subglacial sediments. Until now, however, little direct evidence of such behaviour has been available. Here we use a combined ice sheet/ice shelf model capable of high-resolution nesting with a new treatment of grounding-line dynamics and ice-shelf buttressing to simulate Antarctic ice sheet variations over the past five million years. Modelled WAIS variations range from full glacial extents with grounding lines near the continental shelf break, intermediate states similar to modern, and brief but dramatic retreats, leaving only small, isolated ice caps on West Antarctic islands. Transitions between glacial, intermediate and collapsed states are relatively rapid, taking one to several thousand years. Our simulation is in good agreement with a new sediment record (ANDRILL AND-1B) recovered from the western Ross Sea, indicating a long-term trend from more frequently collapsed to more glaciated states, dominant 40-kyr cyclicity in the Pliocene, and major retreats at marine isotope stage 31 (approx1.07 Myr ago) and other super-interglacials.
Citation: David Pollard, Robert M. DeConto, Nature 458, 329-332 (19 March 2009) | doi:10.1038/nature07809.

West Antarctic Ice Sheet collapse – the fall and rise of a paradigm – Vaughan (2008) [FULL TEXT]
Abstract: It is now almost 30 years since John Mercer (1978) first presented the idea that climate change could eventually cause a rapid deglaciation, or “collapse,” of a large part of the West Antarctic ice sheet (WAIS), raising world sea levels by 5 m and causing untold economic and social impacts. This idea, apparently simple and scientifically plausible, created a vision of the future, sufficiently alarming that it became a paradigm for a generation of researchers and provided an icon for the green movement. Through the 1990s, however, a lack of observational evidence for ongoing retreat in WAIS and improved understanding of the complex dynamics of ice streams meant that estimates of likelihood of collapse seemed to be diminishing. In the last few years, however, satellite studies over the relatively inaccessible Amundsen Sea sector of West Antarctica have shown clear evidence of ice sheet retreat showing all the features that might have been predicted for emergent collapse. These studies are re-invigorating the paradigm, albeit in a modified form, and debate about the future stability of WAIS. Since much of WAIS appears to be unchanging, it may, no longer be reasonable to suggest there is an imminent threat of a 5-m rise in sea level resulting from complete collapse of the West Antarctic ice sheet, but there is strong evidence that the Amundsen Sea embayment is changing rapidly. This area alone, contains the potential to raise sea level by around ~1.5 m, but more importantly it seems likely that it could, alter rapidly enough, to make a significant addition to the rate of sea-level rise over coming two centuries. Furthermore, a plausible connection between contemporary climate change and the fate of the ice sheet appears to be developing. The return of the paradigm presents a dilemma for policy-makers, and establishes a renewed set of priorities for the glaciological community. In particular, we must establish whether the hypothesized instability in WAIS is real, or simply an oversimplification resulting from inadequate understanding of the feedbacks that allow ice sheets to achieve equilibrium: and whether there is any likelihood that contemporary climate change could initiate collapse.
Citation: David G. Vaughan, Climatic Change, November 2008, Volume 91, Issue 1, pp 65-79, DOI: 10.1007/s10584-008-9448-3.

Glacier Surge After Ice Shelf Collapse – De Angelis & Skvarca (2003) [FULL TEXT]
Abstract: The possibility that the West Antarctic Ice Sheet will collapse as a consequence of ice shelf disintegration has been debated for many years. This matter is of concern because such an event would imply a sudden increase in sea level. Evidence is presented here showing drastic dynamic perturbations on former tributary glaciers that fed sections of the Larsen Ice Shelf on the Antarctic Peninsula before its collapse in 1995. Satellite images and airborne surveys allowed unambiguous identification of active surging phases of Boydell, Sjögren, Edgeworth, Bombardier, and Drygalski glaciers. This discovery calls for a reconsideration of former hypotheses about the stabilizing role of ice shelves.
Citation: Hernán De Angelis, Pedro Skvarca, Science 07 Mar 2003: Vol. 299, Issue 5612, pp. 1560-1562, DOI: 10.1126/science.1077987.

Risk Estimation of Collapse of the West Antarctic Ice Sheet – Vaughan & Spouge (2002) [FULL TEXT]
Abstract: Complete collapse of the West Antarctic Ice Sheet (WAIS) would raise global sea level by around 5 m, but whether collapse is likely, or even possible, has been `glaciology’s grand unsolved problem’ for more than two decades. Collapse of WAIS may result from readjustments continuing since the last glacial maximum, or more recent climate change, but it is also possible that collapse will result from internal flow instabilities, or not occur at all in the present inter-glacial. Such complexity led the Intergovernmental Panel on Climate Change to conclude in its Second Assessment Report that `estimating the likelihood of a collapse during the next century is not yet possible’. However, a refusal by scientists to estimate the risk leaves policy-makers with no sound scientific basis on which to respond to legitimate public concerns. Here we present a discussion of the likelihood of WAIS-collapse, drawing input from an interdisciplinary panel of experts. The results help to summarise the state of scientific knowledge and uncertainty. While the overall opinion of the panel was that WAIS most likely will not collapse in the next few centuries, their uncertainty retains a 5% probability of WAIS causing sea level rise at least 10 mm/year within 200 years. Since this uncertainty reflects both the unpredictability of the physical system and the scientific uncertainty, it will undoubtedly change as a better understanding is established.
Citation: David G. Vaughan, John R. Spouge, Climatic Change, January 2002, Volume 52, Issue 1, pp 65-91, DOI: 10.1023/A:1013038920600.

No evidence for a Pleistocene collapse of the West Antarctic Ice Sheet from continental margin sediments recovered in the Amundsen Sea – Hillenbrand et al. (2002)
Abstract: Records of glaciomarine deposition recovered from the West Antarctic continental margin in the Amundsen Sea allow the reconstruction of the behaviour of the West Antarctic Ice Sheet (WAIS) in response to the natural climatic changes of the last 1.8 million years. Contents of gravel-sized and lithogenic components represent the input and redeposition of glaciogenic debris, whereas variations in the proportions of the calcareous sediment fraction reflect palaeoproductivity changes. All proxies, which are regarded as sensitive to a WAIS collapse, changed markedly during the global climatic cycles, but do not confirm a complete disintegration of the WAIS during the Pleistocene.
Citation: Claus-Dieter Hillenbrand, Dieter K. Fütterer, Hannes Grobe, Thomas Frederichs, Geo-Marine Letters, July 2002, Volume 22, Issue 2, pp 51-59, DOI: 10.1007/s00367-002-0097-7.

Pleistocene Collapse of the West Antarctic Ice Sheet – Scherer et al. (1998)
Abstract: Some glacial sediment samples recovered from beneath the West Antarctic ice sheet at ice stream B contain Quaternary diatoms and up to 108 atoms of beryllium-10 per gram. Other samples contain no Quaternary diatoms and only background levels of beryllium-10 (less than 106 atoms per gram). The occurrence of young diatoms and high concentrations of beryllium-10 beneath grounded ice indicates that the Ross Embayment was an open marine environment after a late Pleistocene collapse of the marine ice sheet.
Citation: Reed P. Scherer, Ala Aldahan, Slawek Tulaczyk, Göran Possnert, Hermann Engelhardt, Barclay Kamb, Science 03 Jul 1998: Vol. 281, Issue 5373, pp. 82-85, DOI: 10.1126/science.281.5373.82.

Active volcanism beneath the West Antarctic ice sheet and implications for ice-sheet stability – Blankenship et al. (1993)
Abstract: IT is widely understood that the collapse of the West Antarctic ice sheet (WAIS) would cause a global sea level rise of 6 m, yet there continues to be considerable debate about the detailed response of this ice sheet to climate changel–3. Because its bed is grounded well below sea level, the stability of the WAIS may depend on geologically controlled conditions at the base which are independent of climate. In particular, heat supplied to the base of the ice sheet could increase basal melting and thereby trigger ice streaming, by providing the water for a lubricating basal layer of till on which ice streams are thought to slide4,5. Ice streams act to protect the reservoir of slowly moving inland ice from exposure to oceanic degradation, thus enhancing ice-sheet stability. Here we present aerogeophysical evidence for active volcanism and associated elevated heat flow beneath the WAIS near the critical region where ice streaming begins. If this heat flow is indeed controlling ice-stream formation, then penetration of ocean waters inland of the thin hot crust of the active portion of the West Antarctic rift system could lead to the disappearance of ice streams, and possibly trigger a collapse of the inland ice reservoir.
Citation: Donald D. Blankenship, Robin E. Bell, Steven M. Hodge, John M. Brozena, John C. Behrendt, Carol A. Finn, Nature 361, 526 – 529 (11 February 1993); doi:10.1038/361526a0.

Irregular oscillations of the West Antarctic ice sheet – Macayeal (1992) [FULL TEXT]
Abstract: Model simulations of the West Antarctic ice sheet suggest that sporadic, perhaps chaotic, collapse (complete mobilization) of the ice sheet occurred throughout the past one million years. The irregular behaviour is due to the slow equilibration time of the distribution of basal till, which lubricates ice-sheet motion. This nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history, and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change.
Citation: Douglas R. MacAyeal, Nature 359, 29 – 32 (03 September 1992); doi:10.1038/359029a0.

Posted in Climate claims, Climate science, Global warming effects | Leave a Comment »

Papers on early 20th century warming

Posted by Ari Jokimäki on August 29, 2013

This is a list of papers on early 20th century warming. List contains both observational and theoretical studies. The list is not complete, and will most likely be updated in future in order to make it more thorough and more representative.

UPDATE (April 15, 2015): Thompson et al. (2015) added.
UPDATE (April 8, 2014): Suo et al. (2013), Kelly et al. (1980), Petterssen (1949), Ahlmann (1948) added.

Early twentieth-century warming linked to tropical Pacific wind strength – Thompson et al. (2015)
“Of the rise in global atmospheric temperature over the past century, nearly 30% occurred between 1910 and 1940 when anthropogenic forcings were relatively weak. This early warming has been attributed to internal factors, such as natural climate variability in the Atlantic region, and external factors, such as solar variability and greenhouse gas emissions. However, the warming is too large to be explained by external factors alone and it precedes Atlantic warming by over a decade. For the late twentieth century, observations and climate model simulations suggest that Pacific trade winds can modulate global temperatures, but instrumental data are scarce in the early twentieth century. Here we present a westerly wind reconstruction (1894–1982) from seasonally resolved measurements of Mn/Ca ratios in a western Pacific coral that tracks interannual to multidecadal Pacific climate variability. We then reconstruct central Pacific temperatures using Sr/Ca ratios in a coral from Jarvis Island, and find that weak trade winds and warm temperatures coincide with rapid global warming from 1910 to 1940. In contrast, winds are stronger and temperatures cooler between 1940 and 1970, when global temperature rise slowed down. We suggest that variations in Pacific wind strength at decadal timescales significantly influence the rate of surface air temperature change.”
Diane M. Thompson, Julia E. Cole, Glen T. Shen, Alexander W. Tudhope & Gerald A. Meehl, Nature Geoscience 8, 117–121 (2015) doi:10.1038/ngeo2321.

External forcing of the early 20th century Arctic warming – Suo et al. (2013) “The observed Arctic warming during the early 20th century was comparable to present-day warming in terms of magnitude. The causes and mechanisms for the early 20th century Arctic warming are less clear and need to be better understood when considering projections of future climate change in the Arctic. The simulations using the Bergen Climate Model (BCM) can reproduce the surface air temperature (SAT) fluctuations in the Arctic during the 20th century reasonably well. The results presented here, based on the model simulations and observations, indicate that intensified solar radiation and a lull in volcanic activity during the 1920s–1950s can explain much of the early 20th century Arctic warming. The anthropogenic forcing could play a role in getting the timing of the peak warming correct. According to the model the local solar irradiation changes play a crucial role in driving the Arctic early 20th century warming. The SAT co-varied closely with local solar irradiation changes when natural external forcings are included in the model either alone or in combination with anthropogenic external forcings. The increased Barents Sea warm inflow and the anomalous atmosphere circulation patterns in the northern Europe and north Atlantic can also contribute to the warming. In summary, the early 20th century warming was largely externally forced.” Lingling Suo, Odd Helge Otterå, Mats Bentsen, Yongqi Gao, Ola M. Johannessen, Tellus A 2013, 65, 20578, http://dx.doi.org/10.3402/tellusa.v65i0.20578. [Full text]

Early 20th century warming in the Arctic: A review – Yamanouchi (2011) “From the 1920s to the 1940s, the Artic experienced significant warming that is comparable to the recent 30-year warming. The former warming was concentrated mostly in high latitudes, in contrast to the recent 30-year warming, which has occurred in all latitudes. Several explanations have been proposed; however, one of these proposed explanations, single external forcing, which could once explain the global average, failed to explain the early 20th century scenario. A second possible explanation was internal atmospheric variability with low frequency. Another candidate for the explanation was still forcing by black carbon deposited on snow and ice surfaces. The answer is most likely to be a combination of intrinsic internal natural climate variability and positive feedbacks that amplified the radiative and atmospheric forcing. We must continue our study by discovering historical data, analyzing ice cores, reanalyzing the Arctic system together with long-term reanalysis dating back to the 1880s, and also determine the contributions of each factor.” Takashi Yamanouchi, Polar Science, Volume 5, Issue 1, April 2011, Pages 53–71, http://dx.doi.org/10.1016/j.polar.2010.10.002.

Early 20th century Arctic warming in retrospect – Wood & Overland (2010) “The major early 20th century climatic fluctuation (∼1920–1940) has been the subject of scientific enquiry from the time it was detected in the 1920s. The papers of scientists who studied the event first-hand have faded into obscurity but their insights are relevant today. We review this event through a rediscovery of early research and new assessments of the instrumental record. Much of the inter-annual to decadal scale variability in surface air temperature (SAT) anomaly patterns and related ecosystem effects in the Arctic and elsewhere can be attributed to the superposition of leading modes of variability in the atmospheric circulation. Meridional circulation patterns were an important factor in the high latitudes of the North Atlantic during the early climatic fluctuation. Sea surface temperature (SST) anomalies that appeared during this period were congruent with low-frequency variability in the climate system but were themselves most likely the result of anomalous forcing by the atmosphere. The high-resolution data necessary to verify this hypothesis are lacking, but the consistency of multiple lines of evidence provides strong support. Our findings indicate that early climatic fluctuation is best interpreted as a large but random climate excursion imposed on top of the steadily rising global mean temperature associated with anthropogenic forcing.” Kevin R. Wood, James E. Overland, International Journal of Climatology, Volume 30, Issue 9, pages 1269–1279, July 2010, DOI: 10.1002/joc.1973.

Influence of volcanic activity and changes in solar irradiance on surface air temperatures in the early twentieth century – Shiogama et al. (2006) “Causes of the global surface air temperature warming in the early half of the 20th century are examined using a climate model and an optimal detection/attribution methodology. While the anthropogenic response seems to be underestimated in our model, our previous study detected the influence due to natural external forcing, including the combined effects of solar irradiance changes and the recovery from large volcanic activity. We further partition the responses between these two natural external factors, detecting both the solar and the volcanic signal in the observed early warming. A diagnosis of the sensitivity to solar forcing and a volcanic super-eruption simulation suggest that our model possesses larger climate sensitivities to solar forcing and longer relaxation times to volcanic forcing than HadCM3, enabling us to detect both the solar and volcanic forcing responses.” Hideo Shiogama, Tatsuya Nagashima, Tokuta Yokohata, Simon A. Crooks, Toru Nozawa, Geophysical Research Letters, Volume 33, Issue 9, May 2006, DOI: 10.1029/2005GL025622.

Detecting natural influence on surface air temperature change in the early twentieth century – Nozawa et al. (2005) “We analyze surface air temperature datasets simulated by a coupled climate model forced with different external forcings, to diagnose the relative importance of these forcings to the observed warming in the early 20th century. The geographical distribution of linear temperature trends in the simulations forced only by natural contributions (volcanic eruptions and solar variability) shows better agreement with observed trends than that does the simulations forced only by well-mixed greenhouse gases. Using an optimal fingerprinting technique we robustly detect a significant natural contribution to the early 20th century warming. In addition, the amplitude of our simulated natural signal is consistent with the observations. Over the same period, however, we could not detect a greenhouse gas signal in the observed surface temperature in the presence of the external natural forcings. Hence our analysis suggests that external natural factors caused more warming in the early 20th century than anthropogenic factors.” Toru Nozawa, Tatsuya Nagashima, Hideo Shiogama, Simon A. Crooks, Geophysical Research Letters, Volume 32, Issue 20, October 2005, DOI: 10.1029/2005GL023540. [Full text]

The Early Twentieth-Century Warming in the Arctic—A Possible Mechanism – Bengtsson et al. (2004) “The huge warming of the Arctic that started in the early 1920s and lasted for almost two decades is one of the most spectacular climate events of the twentieth century. During the peak period 1930–40, the annually averaged temperature anomaly for the area 60°–90°N amounted to some 1.7°C. Whether this event is an example of an internal climate mode or is externally forced, such as by enhanced solar effects, is presently under debate. This study suggests that natural variability is a likely cause, with reduced sea ice cover being crucial for the warming. A robust sea ice–air temperature relationship was demonstrated by a set of four simulations with the atmospheric ECHAM model forced with observed SST and sea ice concentrations. An analysis of the spatial characteristics of the observed early twentieth-century surface air temperature anomaly revealed that it was associated with similar sea ice variations. Further investigation of the variability of Arctic surface temperature and sea ice cover was performed by analyzing data from a coupled ocean–atmosphere model. By analyzing climate anomalies in the model that are similar to those that occurred in the early twentieth century, it was found that the simulated temperature increase in the Arctic was related to enhanced wind-driven oceanic inflow into the Barents Sea with an associated sea ice retreat. The magnitude of the inflow is linked to the strength of westerlies into the Barents Sea. This study proposes a mechanism sustaining the enhanced westerly winds by a cyclonic atmospheric circulation in the Barents Sea region created by a strong surface heat flux over the ice-free areas. Observational data suggest a similar series of events during the early twentieth-century Arctic warming, including increasing westerly winds between Spitsbergen and Norway, reduced sea ice, and enhanced cyclonic circulation over the Barents Sea. At the same time, the North Atlantic Oscillation was weakening.” Bengtsson, Lennart, Vladimir A. Semenov, Ola M. Johannessen, 2004: The Early Twentieth-Century Warming in the Arctic—A Possible Mechanism. J. Climate, 17, 4045–4057. doi: http://dx.doi.org/10.1175/1520-0442(2004)0172.0.CO;2. [Full text]

Solar and Greenhouse Gas Forcing and Climate Response in the Twentieth Century – Meehl et al. (2003) “Ensemble experiments with a global coupled climate model are performed for the twentieth century with time-evolving solar, greenhouse gas, sulfate aerosol (direct effect), and ozone (tropospheric and stratospheric) forcing. Observed global warming in the twentieth century occurred in two periods, one in the early twentieth century from about the early 1900s to the 1940s, and one later in the century from, roughly, the late 1960s to the end of the century. The model’s response requires the combination of solar and anthropogenic forcing to approximate the early twentieth-century warming, while the radiative forcing from increasing greenhouse gases is dominant for the response in the late twentieth century, confirming previous studies. Of particular interest here is the model’s amplification of solar forcing when this acts in combination with anthropogenic forcing. This difference is traced to the fact that solar forcing is more spatially heterogeneous (i.e., acting most strongly in areas where sunlight reaches the surface) while greenhouse gas forcing is more spatially uniform. Consequently, solar forcing is subject to coupled regional feedbacks involving the combination of temperature gradients, circulation regimes, and clouds. The magnitude of these feedbacks depends on the climate’s base state. Over relatively cloud-free oceanic regions in the subtropics, the enhanced solar forcing produces greater evaporation. More moisture then converges into the precipitation convergence zones, intensifying the regional monsoon and Hadley and Walker circulations, causing cloud reductions over the subtropical ocean regions, and, hence, more solar input. An additional response to solar forcing in northern summer is an enhancement of the meridional temperature gradients due to greater solar forcing over land regions that contribute to stronger West African and South Asian monsoons. Since the greenhouse gases are more spatially uniform, such regional circulation feedbacks are not as strong. These regional responses are most evident when the solar forcing occurs in concert with increased greenhouse gas forcing. The net effect of enhanced solar forcing in the early twentieth century is to produce larger solar-induced increases of tropical precipitation when calculated as a residual than for early century solar-only forcing, even though the size of the imposed solar forcing is the same. As a consequence, overall precipitation increases in the early twentieth century in the Asian monsoon regions are greater than late century increases, qualitatively consistent with observed trends in all-India rainfall. Similar effects occur in West Africa, the tropical Pacific, and the Southern Ocean tropical convergence zones.” Meehl, Gerald A., Warren M. Washington, T. M. L. Wigley, Julie M. Arblaster, Aiguo Dai, 2003: Solar and Greenhouse Gas Forcing and Climate Response in the Twentieth Century. J. Climate, 16, 426–444. doi: http://dx.doi.org/10.1175/1520-0442(2003)0162.0.CO;2. [Full text]

Estimation of natural and anthropogenic contributions to twentieth century temperature change – Tett et al. (2002) “Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well-mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well-mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well-mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an “optimal detection” methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well-mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near-surface temperature from well-mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near-surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.” Simon F. B. Tett, Gareth S. Jones, Peter A. Stott, David C. Hill, John F. B. Mitchell, Myles R. Allen, William J. Ingram, Tim C. Johns, Colin E. Johnson, Andy Jones, David L. Roberts, David M. H. Sexton, Margaret J. Woodage, Journal of Geophysical Research: Atmospheres (1984–2012), Volume 107, Issue D16, pages ACL 10-1–ACL 10-24, 27 August 2002, DOI: 10.1029/2000JD000028. [Full text]

Simulation of Early 20th Century Global Warming – Delworth & Knutson (2000) “The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model’s climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.” Thomas L. Delworth, Thomas R. Knutson, Science 24 March 2000: Vol. 287 no. 5461 pp. 2246-2250, DOI: 10.1126/science.287.5461.2246.

Changes in atmospheric circulation over northern hemisphere oceans associated with the rapid warming of the 1920s – Fu et al. (1999)
Global mean surface temperature has increased since the late 19th century. The warming occurred largely during two periods: 1920–1940, and since the mid-1970s. Although most recent studies have focused on the latter period, it is of interest to analyse the earlier period and compare its major features to the recent warming episode. The warming during 1920–1940 occurred most rapidly during the 1920s. It was strongest at high northern latitudes in winter, a pattern now believed to be characteristic of ‘greenhouse warming’. This warming of the Arctic was much discussed during the 1930s and 1940s, but the data available at that time were mostly derived from land areas. In this paper, we use the COADS marine data set and recent compilations of land surface temperature data sets to examine the behaviour of the surface fields over the ocean during this event. Considering the thermal and atmospheric fields at the surface, the strongest signal occurs in the North Atlantic Ocean during winter, being distinct but more gradual in the other oceans and seasons. The Northern Hemisphere continental record shows that both middle and high latitudes experienced rapid warming in the early 20th century warming interval (the 1920s and 1930s, hereafter referred to as ETCW). Temperature data for northern tropics, while displaying similar general characteristics, exhibit some differences with regard to timing and rates of change. There is a suggestion of weakening of the westerlies and the trade wind system in the 1930s, following an intensification of the westerlies across the North Atlantic during the previous two decades. This weakening may be related to a lessening of atmospheric baroclinicity in association with the fact that the amplitude of warming at high latitudes was much greater than that in low latitudes, reducing the mean meridional thermal gradient, and therefore the geostrophic pressure gradient. There is some indication that the North Atlantic and North Pacific high-pressure systems shifted northward. Coincident with this northward shift of the subtropical highs, typhoons in the Northwest Pacific and hurricanes in the North Atlantic became more numerous in this period of rising temperature, which we suggest is linked to a northward shift of the respective near-equatorial convergence zones. Concomitant to the weakening of the westerlies and trade wind systems, the Asian monsoon troughs deepened substantially, a situation generally favourable to the development of active monsoons. It is thought that the combination of these two features—enhanced continental monsoons and implied lowered vertical wind shear over the oceans—would tend to enhance the release of latent heat in the tropics, representing strengthened Hadley and Walker circulations, which may have been at least partly responsible for greater aridity in subtropical land areas of both hemispheres during this period. The latter is also consistent with an expansion and/or strengthening of the subtropical high-pressure belt into the continents.” Congbin Fu, Henry F. Diaz, Dongfeng Dong, Joseph O. Fletcher, International Journal of Climatology, Volume 19, Issue 6, pages 581–606, May 1999, DOI: 10.1002/(SICI)1097-0088(199905)19:63.0.CO;2-P.

Solar Forcing of Global Climate Change Since The Mid-17th Century – Reid et al. (1997) “Spacecraft measurements of the sun’s total irradiance since 1980 have revealed a long-term variation that is roughly in phase with the 11-year solar cycle. Its origin is uncertain, but may be related to the overall level of solar magnetic activity as well as to the concurrent activity on the visible disk. A low-pass Gaussian filtered time series of the annual sunspot number has been developed as a suitable proxy for solar magnetic activity that contains a long-term component related to the average level of activity as well as a short-term component related to the current phase of the 11-year cycle. This time series is also assumed to be a proxy for solar total irradiance, and the irradiance is reconstructed for the period since 1617 based on the estimate from climatic evidence that global temperatures during the Maunder Minimum of solar activity, which coincided with one of the coldest periods of the Little Ice Age, were about 1 °C colder than modern temperatures. This irradiance variation is used as the variable radiative forcing function in a one-dimensional ocean–climate model, leading to a reconstruction of global temperatures over the same period, and to a suggestion that solar forcing and anthropogenic greenhouse-gas forcing made roughly equal contributions to the rise in global temperature that took place between 1900 and 1955. The importance of solar variability as a factor in climate change over the last few decades may have been underestimated in recent studies.” George C. Reid, Climatic Change, October 1997, Volume 37, Issue 2, pp 391-405, DOI: 10.1023/A:1005307009726.

Changes in Global Surface Temperature From 1880 to 1977 Derived From Historical Records of Sea Surface Temperature – Paltridge & Woodruff (1981) “A preliminary analysis based primarily on historical records of sea surface temperature (SST) gives estimates of the change since 1880 of global, hemispheric and zonal average surface temperatures. The global change with time is roughly similar in shape and magnitude to that derived by Mitchell from land station data alone, but lags the Mitchell curve by 10-20 years. That is, the present data show a minimumof temperature somewhere between 1900 and 1925 and a maximum somewhere between 1945 and 1970. Comparing the means of these 25-year periods, the rise from minimum to maximum was (roughly) 0.6 K for the Northern Hemisphere and 0.9 K for the Southern Hemisphere. Comparing the means of the 50 years before 1930 and the 48 years from 1930 to 1977, the rise was 0.3 K for the Northern Hemisphere and 0.6 K for the Southern Hemisphere. The figures do not take into account the polar regions which, on linear extrapolation from lower latitudes, may have risen in temperature by twice the hemispheric averages. The temperature of the tropical zone (l0°N-10°S) has not changed over the years, so that the meridionaltemperature gradient has decreased in both hemispheres. The detail of the various conclusions may be revised later in the light of further analysis of the errors associated with the SST data sets. This furtheranalysis is underway at the Environmental Research Laboratories of NOAA.” Paltridge, G., S. Woodruff, 1981: Changes in Global Surface Temperature From 1880 to 1977 Derived From Historical Records of Sea Surface Temperature. Mon. Wea. Rev., 109, 2427–2434. doi: http://dx.doi.org/10.1175/1520-0493(1981)1092.0.CO;2. [Full text]

Variations in Surface Air Temperatures: Part 2. Arctic Regions, 1881–1980 – Kelly et al. (1980) “We describe annual and seasonal changes in air temperatures over high latitudes of the Northern Hemisphere during the period 1881–1980. Trends (that is, fluctuations on time scales greater than 20 years) in the average temperature of the Arctic are compared with those of the Northern Hemisphere. Seasonal and regional departures from the long-term trends in the average temperature of the Arctic are identified. Spatial patterns of variation in the Arctic temperature field are determined by principal component analysis and the major characteristics of the time series of the dominant patterns are summarized. Trends in Arctic temperatures have been broadly similar to those for the Northern Hemisphere during the study period. The Arctic variations were, however, greater in magnitude and more rapid. The spatial pattern of change associated with the trend in Arctic temperatures is clearly identified by principal component analysis. It shows that the trends have, in general, been Arctic-wide, but that certain regions are particularly sensitive to long-term variations, most notably northwest Greenland and around the Kara Sea. There is some evidence that the pattern of Arctic cooling that occurred after 1940 was more complex than the warming that affected the whole Arctic during the 1920’s and 1930’s. Warming of the Arctic has occurred during the 1970’s, but is not yet of sufficient duration to be considered long term, except, perhaps, in spring. The average temperature of the Arctic during the 1970’s was equal to that of the 1960’s, indicating a cessation of the long-term cooling trend but not, as yet, a shift to long-term warming. Short-term variations in temperature appear to be most pronounced close to major regions of sea-ice production and decay.” Kelly, P. M., P. D. Jones, C. B. Sear, B. S. G. Cherry, R. K. Tavakol, 1982: Variations in Surface Air Temperatures: Part 2. Arctic Regions, 1881–1980. Mon. Wea. Rev., 110, 71–83. doi: http://dx.doi.org/10.1175/1520-0493(1982)1102.0.CO;2. [Full text]

Temperature fluctuations and trends over the earth – Callendar (1961) “The annual temperature deviations at over 400 meteorological stations are combined on a regional basis to give the integrated fluctuations over large areas and zones. These are shown in graphical form, and it is concluded that a solar or atmospheric dust hypothesis is necessary to explain the world-wide fluctuations of a few years duration. An important change in the relationships of the zonal fluctuations has occurred since 1920. The overall temperature trends found from the data are considered in relation to the homogeneity of recording, and also to the evidence of glacial recession in different zones. It is concluded that the rising trend, shown by the instruments during recent decades, is significant from the Arctic to about 45°S lat., but quite small in most regions below 35°N. and not yet apparent in some. It is thought that the regional and zonal distribution of recent climatic trends is incompatible with the hypothesis of increased solar heating as the cause. On the other hand, the major features of this distribution are not incompatible with the hypothesis of increased carbon dioxide radiation, if the rate of atmospheric mixing between the hemispheres is a matter of decades rather than years.” G. S. Callendar, Quarterly Journal of the Royal Meteorological Society, Volume 87, Issue 371, pages 1–12, January 1961, DOI: 10.1002/qj.49708737102.

Changes in the General Circulation Associated with the Recent Climatic Variation – Petterssen (1949) No abstract. S. Petterssen, Geografiska Annaler, Vol. 31, Glaciers and Climate: Geophysical and Geomorphological Essays (1949), pp. 212-221.

The Present Climatic Fluctuation – Ahlmann (1948) No abstract. Hans W:Son Ahlmann, The Geographical Journal, Vol. 112, No. 4/6 (Oct. – Dec., 1948), pp. 165-193.

The artificial production of carbon dioxide and its influence on temperature – Callendar (1938) “By fuel combustion man has added about 150,000 million tons of carbon dioxide to the air during the past half century. The author estimates from the best available data that approximately three quarters of this has remained in the atmosphere. The radiation absorption coefficients of carbon dioxide and water vapour are used to show the effect of carbon dioxide on “sky radiation.” From this the increase in mean temperature, due to the artificial production of carbon dioxide, is estimated to be at the rate of 0.003°C. per year at the present time. The temperature observations at 200 meteorological stations are used to show that world temperatures have actually increased at an average rate of 0.005°C. per year during the past half century.” G. S. Callendar, Quarterly Journal of the Royal Meteorological Society, Volume 64, Issue 275, pages 223–240, April 1938, DOI: 10.1002/qj.49706427503. [Full text]

Posted in AGW evidence, Climate claims, Climate science | 3 Comments »

Global warming has not stopped

Posted by Ari Jokimäki on August 19, 2013

The apparent lack of warming in Earth’s surface temperature measurements since 1998 is not yet significant from climatic perspective. Surface temperature also seems to be changing according to IPCC projections. Climate model simulations show similar warming breaks, and have done so even before current break started, even if they include the effect of carbon dioxide. Models also can re-create the current break and the cause for the break seems to be known: warming has gone to the oceans instead of warming the surface. The ocean warming has been observed. Also the continuing warming effect of greenhouse gases has been observed. Global warming as a whole seems to continue despite the apparent break in surface measurements.

Several decades of long-term warming is evident in recent surface temperature measurements. However, since 1998 surface temperature records don’t show clear warming. This is not very clear because the time period is not very long and the possible trends might not be statistically significant. Skeptical Science trend calculator shows warming trends since 1998 but they are not statistically significant. Santer and others (2011) estimated that it takes 17 years of satellite measurements, before effect of mankind to lower atmosphere can be detected. In some cases, 15 years has been mentioned for the limit of statistical significance, so the situation seems to be quite borderline. For example, from 1983 and 1998, which was time of rapid warming, SkS trend calculator still shows a trend that is not statistically significant. From 1982 to 1998 trend is significant. (Even if statistically enough time would pass without warming, it still wouldn’t mean that increases in greenhouse gases wouldn’t have a warming effect. This we will see below in more detail.)

Climate is usually considered as average weather over longer period of time. Standard length for the climatic time period is 30 years. Let us see what this means for the surface temperature. Following figure shows Earth’s surface temperature as a running 30 year mean (this means that the value of each point in the graph is the average of the surrounding 30 years of temperature values, for example, the running 30 year mean value for year 1990 is the average of temperatures of 30 years between 1975 and 2004):

30yrTemp

As we can see from the graph, there are no signs of global warming stopping or even slowing down in this kind of inspection. From climatic perspective global warming still continues. Those with sharp eyes notice that the time shown in X axis stops in the graph before 2000. This is because in 30 year running mean the year 1998 is currently last one that can be shown. However, the temperature evolution after 1998 is included and is affecting the graph starting from 1983. The fact that graph stops at 1998 means that we have to wait 15 years before we know how climate has evolved since 1998.

Although global surface temperature evolution since 1998 is not yet climatically important, there has been quite a lot of research on the issue. One question appearing in public has been that does the temperature follow earlier projections. Rahmstorf and others (2012) have analyzed how IPCC projections match the surface temperature measurements. Here is a graph of their results:

1998ipcc
The evolution of Earth’s surface temperature and IPCC projections. Surface temperature without corrections is shown in pink (one year running mean averaged from all surface temperature analyses) and corrected surface temperature is shown in red (corrections are explained in text). Blue area and blue lines are IPCC third assessment report projections. Green area and green lines are IPCC fourth assessment report (AR4) projections.

As we can see from the graph, surface temperature changes shown in pink sometimes go outside the projections. This is because some factors are not included in IPCC projections. Such factors are solar activity changes and eruptions of volcanoes. Additionally, the variation of El Niño/La Niña is random and therefore it doesn’t change in simulations at the same time it does in real life. IPCC projections are combined results from many simulations, so the El Niño/La Niña variations of different simulations tend to cancel out when simulation results are combined. This means that the projections don’t actually include El Niño/La Niña variation either. It should be noted that even if surface temperature shown in pink doesn’t stay within limits of projections, it does stay within limits of all individual simulations (not shown in the Figure above).

The effects of the Sun, volcanoes, and El Niño/La Niña variation have been corrected for in the temperature evolution shown in red. This graph stays quite well within limits of projections especially during last few years. It sometimes goes outside the projections in the beginning of the time period. There still might be some factors which would need to be corrected for. On the average it does seem to follow the projections even in the beginning of the time period (at least it doesn’t deviate permanently to one direction).

Already happened temperature evolution can be recreated with models also so that internal variability of climate system and changes in solar activity and in volcanoes are included (afterwards we have knowledge for example when a volcanic eruption has happened). Lean & Rind (2009) have done this and the following Figure shows their results:

1998lean
a) The observed surface temperature (black) and simulation of surface temperature from a simple model (orange). b) The factors affecting surface temperature. Graphs are from Lean & Rind (2009).

The result of the simple model shown in the graph is so close to observed surface temperature evolution that we have good reason to suspect this study might be able to answer us why surface temperature has not apparently increased since 1998. Lower part of the Figure shows the factors affecting surface temperature, and from there we can see that ENSO (that is, the variation of El Niño and La Niña) seems to have varied in quite a similar manner than surface temperature in the period in question. Also the cause of the longer-term temperature rise seems to be clear: the effect of mankind is the only one of the factors, which shows long-term increase.

Also the future projections of climate models show periods where surface temperature doesn’t increase, even if models have the effect of greenhouse gases included. Here are some examples of such simulations:

1998malli
Projections of climate models show periods of slowed/paused warming similar to that in the observations since 1998. On the left: simulations with three different emission scenarios from IPCC AR4. Upper right: simulation example from Easterling & Wehner (2009). Lower right: simulation example from Meehl and others (2011).

All shown simulation examples show periods where long-term warming trend is paused even for decades, and warming continues after that. The Figure also shows simulation examples from model runs for IPCC AR4 projections. These show similar pauses. AR4 discusses the expected temperature evolution rather carelessly. From the texts of the report one might get an impression that surface temperature should rise certain amount in each decade. They of course mean that on average certain rise in temperature is expected per decade, even if it doesn’t occur during each decade. Some people have used the carelessly worded texts in IPCC AR4 to distort the issue, even when the same report shows the simulations presented above, where the truth in the matter can be seen.

Simulation examples shown above are all quite recent. Model simulations have shown similar features also earlier. Here we see an example from IPCC second assessment report (SAR), which was published in 1995, before current warming pause apparently started:

1998ipcc2
Model simulations from IPCC second assessment report. Year 0 means year 1990.

One interesting detail in the SAR model simulations is that one of them shows strong spike around 1995. This is comparable to the 1998 peak in observed temperatures. Even if the post-1995 evolution is difficult to see in the graph, we can be sure that the simulation in question shows quite long pause in the warming after 1995. In principle, we could say that the simulation in question predicted the warming pause, albeit being off by few years. It’s not genuine prediction of course, but just a coincidence. Nevertheless, it’s a quite curious detail.

So, the simulations of climate models show clearly that while the increase of greenhouse gases in the atmosphere increases Earth’s surface temperature in the long run, other factors cause pauses to the warming every now and then. Similarly, those other factors also speed up the warming in other times. This has been explicitly stated in Easterling & Wehner (2009): “We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming”.

At the moment it seems that the factor causing the pause has been ENSO, which in practice means that the warming effect of greenhouse gases has gone deeper to the oceans instead of warming the surface. This has been the subject of some recent studies.

Already shown above were the results of Lean & Rind (2009) and Meehl and others (2011). Similar results have also been reported by Kaufmann and others (2011), Hunt (2011), Guemas and others (2013), and Watanabe and others (2013). According to all these studies, the primary cause for the apparent pause in the surface warming is that the warming has gone to the oceans. Solar activity has also been said to have played some role on the issue. Also, Solomon and others (2010) have suggested that changes in water vapor content in the atmosphere might have speeded up the warming during the 1990’s and slowed the warming during 2000’s.

The warming going into oceans has also been observed. Following Figure shows the ocean heat content in the top layer (0-700 m) of the oceans (from Lyman and others, 2010):

1998meri

The graph shows that after 1998 the heat content in the oceans has increased substantially.

But when can we expect surface warming to continue? Surface warming continues when the sum of all factors affecting Earth’s surface temperature has a warming effect. It can take decades and decades, as long as there are other factors that have large enough cooling effect to mask the warming effect from greenhouse gases. However, our current knowledge suggests that there are no such factors that could have large enough cooling effect in order to make this pause much longer.

There are reasons to think that warming might continue soon. Earth’s surface temperature has been very high recently, close to record temperatures, while solar activity has been very low and La Niña has been the prevailing state of ENSO. Without the effect of greenhouse gases these factors would have cooled Earth’s surface substantially. We haven’t seen such cooling. When La Niña changes to El Niño, it is expected that warming will continue.

So it seems that the warming effect of greenhouse gases seems to be still there. Fortunately, we don’t need to guess this, as we also have observations of the effect of greenhouse gases, as we see next.

A group of researchers have studied spectral measurements of outgoing long-wave radiation taken from satellites (Chapman and others, 2013). They found out that the warming effect of carbon dioxide has continued to increase during the 2000’s. They calculated from the spectral measurements that between 2002 and 2012 the amount of outgoing long-wave radiation decreased in the characteristic absorption frequencies of greenhouse gases. This was the case at least for carbon dioxide, ozone, and methane. Largest warming effect was from carbon dioxide. Observed decreases in the outgoing long-wave radiation matched the expectation from the increased greenhouse gas concentrations during the study period. Here are their results in a graph:

1998olr

It should be noted that the study of Chapman and others was presented in a conference in April 2013, and apparently official research paper has not been published yet. Information presented here is from the conference paper.

References:

IPCC second assessment report (over 50 MB PDF file, the graph shown here is on the PDF page 314, page 300 of the report).

IPCC AR4 simulations: Figure 10.5 with caption.

D. Chapman, P. Nguyen, M. Halem, A decade of measured greenhouse forcings from AIRS, Proc. SPIE 8743, Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery XIX, 874313 (May 18, 2013); doi:10.1117/12.2017019. [abstract]

John A. Church, Neil J. White, Leonard F. Konikow, Catia M. Domingues, J. Graham Cogley, Eric Rignot, Jonathan M. Gregory, Michiel R. van den Broeke, Andrew J. Monaghan, Isabella Velicogna, Revisiting the Earth’s sea-level and energy budgets from 1961 to 2008, Geophysical Research Letters, Volume 38, Issue 18, 28 September 2011, DOI: 10.1029/2011GL048794. [abstract, full text]

David R. Easterling, Michael F. Wehner, 2009, Is the climate warming or cooling? Geophysical Research Letters, Volume 36, Issue 8, April 2009, DOI: 10.1029/2009GL037810. [abstract, full text]

Virginie Guemas, Francisco J. Doblas-Reyes, Isabel Andreu-Burillo & Muhammad Asif, Retrospective prediction of the global warming slowdown in the past decade, Nature Climate Change, 3, 649–653 (2013) doi:10.1038/nclimate1863. [abstract]

B. G. Hunt, The role of natural climatic variation in perturbing the observed global mean temperature trend, Climate Dynamics, February 2011, Volume 36, Issue 3-4, pp 509-521, DOI: 10.1007/s00382-010-0799-x. [abstract]

Robert K. Kaufmann, Heikki Kauppi, Michael L. Mann, and James H. Stock, Reconciling anthropogenic climate change with observed temperature 1998–2008, PNAS July 19, 2011 vol. 108 no. 29 11790-11793, doi: 10.1073/pnas.1102467108. [abstract, full text]

John M. Lyman, Simon A. Good, Viktor V. Gouretski, Masayoshi Ishii, Gregory C. Johnson, Matthew D. Palmer, Doug M. Smith, & Josh K. Willis, Robust warming of the global upper ocean, Nature 465, 334–337 (20 May 2010) doi:10.1038/nature09043. [abstract, full text]

Gerald A. Meehl, Julie M. Arblaster, John T. Fasullo, Aixue Hu & Kevin E. Trenberth, 2011, Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods, Nature Climate Change, 1, 360–364 (2011) doi:10.1038/nclimate1229. [abstract, full text]

Stefan Rahmstorf et al 2012, Comparing climate projections to observations up to 2011, Environ. Res. Lett. 7 044035 doi:10.1088/1748-9326/7/4/044035. [abstract, full text]

B. D. Santer, C. Mears, C. Doutriaux, P. Caldwell, P. J. Gleckler, T. M. L. Wigley, S. Solomon, N. P. Gillett, D. Ivanova, T. R. Karl, J. R. Lanzante, G. A. Meehl, P. A. Stott, K. E. Taylor, P. W. Thorne, M. F. Wehner, F. J. Wentz, 2011, Separating signal and noise in atmospheric temperature changes: The importance of timescale, Journal of Geophysical Research: Atmospheres (1984–2012), Volume 116, Issue D22, November 2011, DOI: 10.1029/2011JD016263. [abstract, full text]

Susan Solomon, Karen H. Rosenlof, Robert W. Portmann, John S. Daniel, Sean M. Davis, Todd J. Sanford, Gian-Kasper Plattner, Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming, Science 5 March 2010: Vol. 327 no. 5970 pp. 1219-1223, DOI: 10.1126/science.1182488. [abstract, full text]

Masahiro Watanabe, Youichi Kamae, Masakazu Yoshimori, Akira Oka, Makiko Sato, Masayoshi Ishii, Takashi Mochizuki, Masahide Kimoto, Strengthening of ocean heat uptake efficiency associated with the recent climate hiatus, Geophysical Research Letters, Volume 40, Issue 12, pages 3175–3179, 28 June 2013, DOI: 10.1002/grl.50541. [abstract]

Posted in Climate claims, Climate science | 7 Comments »

Papers on global surface temperature since 1998

Posted by Ari Jokimäki on August 2, 2013

This is a list of papers on global surface temperature since 1998. The list is not complete, and will most likely be updated in future in order to make it more thorough and more representative.

UPDATE (February 22, 2014): England et al. (2014) added.
UPDATE (November 14, 2013): Otto et al. (2013) and Cowtan & Way (2013) added.
UPDATE (October 3, 2013): Kosaka & Xie (2013) and Fyfe et al. (2013) added.

See also the list on global surface temperature for additional relevant papers.

Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus – England et al. (2014) “Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing. Here we show that a pronounced strengthening in Pacific trade winds over the past two decades—unprecedented in observations/reanalysis data and not captured by climate models—is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline. At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions. The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.” Matthew H. England, Shayne McGregor, Paul Spence, Gerald A. Meehl, Axel Timmermann, Wenju Cai, Alex Sen Gupta, Michael J. McPhaden, Ariaan Purich & Agus Santoso, Nature Climate Change (2014), doi:10.1038/nclimate2106. [Full text]

Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends – Cowtan & Way (2013) “Incomplete global coverage is a potential source of bias in global temperature reconstructions if the unsampled regions are not uniformly distributed over the planet’s surface. The widely used HadCRUT4 dataset covers on average about 84% of the globe over recent decades, with the unsampled regions being concentrated at the poles and over Africa. Three existing reconstructions with near-global coverage are examined, each suggesting that HadCRUT4 is subject to bias due to its treatment of unobserved regions. Two alternative approaches for reconstructing global temperatures are explored, one based on an optimal interpolation algorithm and the other a hybrid method incorporating additional information from the satellite temperature record. The methods are validated on the basis of their skill at reconstructing omitted sets of observations. Both methods provide superior results than excluding the unsampled regions, with the hybrid method showing particular skill around the regions where no observations are available. Temperature trends are compared for the hybrid global temperature reconstruction and the raw HadCRUT4 data. The widely quoted trend since 1997 in the hybrid global reconstruction is two and a half times greater than the corresponding trend in the coverage-biased HadCRUT4 data. Coverage bias causes a cool bias in recent temperatures relative to the late 1990s which increases from around 1998 to the present. Trends starting in 1997 or 1998 are particularly biased with respect to the global trend. The issue is exacerbated by the strong El Niño event of 1997-1998, which also tends to suppress trends starting during those years.” Kevin Cowtan, Robert G. Way, Quarterly Journal of the Royal Meteorological Society, DOI: 10.1002/qj.2297.

Energy budget constraints on climate response – Otto et al. (2013) “The rate of global mean warming has been lower over the past decade than previously. It has been argued that this observation might require a downwards revision of estimates of equilibrium climate sensitivity, that is, the long-term (equilibrium) temperature response to a doubling of…” Alexander Otto, Friederike E. L. Otto, Olivier Boucher, John Church, Gabi Hegerl, Piers M. Forster, Nathan P. Gillett, Jonathan Gregory, Gregory C. Johnson, Reto Knutti, Nicholas Lewis, Ulrike Lohmann, Jochem Marotzke, Gunnar Myhre, Drew Shindell, Bjorn Stevens & Myles R. Allen, Nature Geoscience 6, 415–416 (2013), doi:10.1038/ngeo1836.

Recent global-warming hiatus tied to equatorial Pacific surface cooling – Kosaka & Xie (2013) “Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century, challenging the prevailing view that anthropogenic forcing causes climate warming. Various mechanisms have been proposed for this hiatus in global warming, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase.” Yu Kosaka, Shang-Ping Xie, Nature 501,403–407 (19 September 2013) doi:10.1038/nature12534.

Overestimated global warming over the past 20 years – Fyfe et al. (2013) “Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability.” John C. Fyfe, Nathan P. Gillett, Francis W. Zwiers, Nature Climate Change 3,767–769(2013)doi:10.1038/nclimate1972. [Full text]

Strengthening of ocean heat uptake efficiency associated with the recent climate hiatus – Watanabe et al. (2013) “The rate of increase of global-mean surface air temperature (SATg) has apparently slowed during the last decade. We investigated the extent to which state-of-the-art general circulation models (GCMs) can capture this hiatus period by using multimodel ensembles of historical climate simulations. While the SATg linear trend for the last decade is not captured by their ensemble means regardless of differences in model generation and external forcing, it is barely represented by an 11-member ensemble of a GCM, suggesting an internal origin of the hiatus associated with active heat uptake by the oceans. Besides, we found opposite changes in ocean heat uptake efficiency (κ), weakening in models and strengthening in nature, which explain why the models tend to overestimate the SATg trend. The weakening of κ commonly found in GCMs seems to be an inevitable response of the climate system to global warming, suggesting the recovery from hiatus in coming decades.” Masahiro Watanabe, Youichi Kamae, Masakazu Yoshimori, Akira Oka, Makiko Sato, Masayoshi Ishii, Takashi Mochizuki, Masahide Kimoto, Geophysical Research Letters, Volume 40, Issue 12, pages 3175–3179, 28 June 2013, DOI: 10.1002/grl.50541.

Retrospective prediction of the global warming slowdown in the past decade – Guemas et al. (2013) “Despite a sustained production of anthropogenic greenhouse gases, the Earth’s mean near-surface temperature paused its rise during the 2000–2010 period. To explain such a pause, an increase in ocean heat uptake below the superficial ocean layer has been proposed to overcompensate for the Earth’s heat storage. Contributions have also been suggested from the deep prolonged solar minimum, the stratospheric water vapour, the stratospheric and tropospheric aerosols. However, a robust attribution of this warming slowdown has not been achievable up to now. Here we show successful retrospective predictions of this warming slowdown up to 5 years ahead, the analysis of which allows us to attribute the onset of this slowdown to an increase in ocean heat uptake. Sensitivity experiments accounting only for the external radiative forcings do not reproduce the slowdown. The top-of-atmosphere net energy input remained in the [0.5–1] W m−2 interval during the past decade, which is successfully captured by our predictions. Most of this excess energy was absorbed in the top 700 m of the ocean at the onset of the warming pause, 65% of it in the tropical Pacific and Atlantic oceans. Our results hence point at the key role of the ocean heat uptake in the recent warming slowdown. The ability to predict retrospectively this slowdown not only strengthens our confidence in the robustness of our climate models, but also enhances the socio-economic relevance of operational decadal climate predictions.” Virginie Guemas, Francisco J. Doblas-Reyes, Isabel Andreu-Burillo & Muhammad Asif, Nature Climate Change, 3, 649–653 (2013) doi:10.1038/nclimate1863.

Separating signal and noise in atmospheric temperature changes: The importance of timescale – Santer et al. (2011) “We compare global-scale changes in satellite estimates of the temperature of the lower troposphere (TLT) with model simulations of forced and unforced TLT changes. While previous work has focused on a single period of record, we select analysis timescales ranging from 10 to 32 years, and then compare all possible observed TLT trends on each timescale with corresponding multi-model distributions of forced and unforced trends. We use observed estimates of the signal component of TLT changes and model estimates of climate noise to calculate timescale-dependent signal-to-noise ratios (S/N). These ratios are small (less than 1) on the 10-year timescale, increasing to more than 3.9 for 32-year trends. This large change in S/N is primarily due to a decrease in the amplitude of internally generated variability with increasing trend length. Because of the pronounced effect of interannual noise on decadal trends, a multi-model ensemble of anthropogenically-forced simulations displays many 10-year periods with little warming. A single decade of observational TLT data is therefore inadequate for identifying a slowly evolving anthropogenic warming signal. Our results show that temperature records of at least 17 years in length are required for identifying human effects on global-mean tropospheric temperature.” B. D. Santer, C. Mears, C. Doutriaux, P. Caldwell, P. J. Gleckler, T. M. L. Wigley, S. Solomon, N. P. Gillett, D. Ivanova, T. R. Karl, J. R. Lanzante, G. A. Meehl, P. A. Stott, K. E. Taylor, P. W. Thorne, M. F. Wehner, F. J. Wentz, Journal of Geophysical Research: Atmospheres (1984–2012), Volume 116, Issue D22, November 2011, DOI: 10.1029/2011JD016263. [Full text]

Reconciling anthropogenic climate change with observed temperature 1998–2008 – Kaufmann et al. (2011) “Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations. As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.” Robert K. Kaufmann, Heikki Kauppi, Michael L. Mann, and James H. Stock, PNAS July 19, 2011 vol. 108 no. 29 11790-11793, doi: 10.1073/pnas.1102467108. [Full text]

The role of natural climatic variation in perturbing the observed global mean temperature trend – Hunt (2011) “Controversy continues to prevail concerning the reality of anthropogenically-induced climatic warming. One of the principal issues is the cause of the hiatus in the current global warming trend. There appears to be a widely held view that climatic change warming should exhibit an inexorable upwards trend, a view that implies there is no longer any input by climatic variability in the existing climatic system. The relative roles of climatic change and climatic variability are examined here using the same coupled global climatic model. For the former, the model is run using a specified CO2 growth scenario, while the latter consisted of a multi-millennial simulation where any climatic variability was attributable solely to internal processes within the climatic system. It is shown that internal climatic variability can produce global mean surface temperature anomalies of ±0.25 K and sustained positive and negative anomalies sufficient to account for the anomalous warming of the 1940s as well as the present hiatus in the observed global warming. The characteristics of the internally-induced negative temperature anomalies are such that if this internal natural variability is the cause of the observed hiatus, then a resumption of the observed global warming trend is to be expected within the next few years.” B. G. Hunt, Climate Dynamics, February 2011, Volume 36, Issue 3-4, pp 509-521, DOI: 10.1007/s00382-010-0799-x.

Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods – Meehl et al. (2011) “There have been decades, such as 2000–2009, when the observed globally averaged surface-temperature time series shows little increase or even a slightly negative trend (a hiatus period). However, the observed energy imbalance at the top-of-atmosphere for this recent decade indicates that a net energy flux into the climate system of about 1 W m−2 (refs 2, 3) should be producing warming somewhere in the system. Here we analyse twenty-first-century climate-model simulations that maintain a consistent radiative imbalance at the top-of-atmosphere of about 1 W m−2 as observed for the past decade. Eight decades with a slightly negative global mean surface-temperature trend show that the ocean above 300 m takes up significantly less heat whereas the ocean below 300 m takes up significantly more, compared with non-hiatus decades. The model provides a plausible depiction of processes in the climate system causing the hiatus periods, and indicates that a hiatus period is a relatively common climate phenomenon and may be linked to La Niña-like conditions.” Gerald A. Meehl, Julie M. Arblaster, John T. Fasullo, Aixue Hu & Kevin E. Trenberth, Nature Climate Change, 1, 360–364 (2011) doi:10.1038/nclimate1229. [Full text]

Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming – Solomon et al. (2010) “Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.” Susan Solomon, Karen H. Rosenlof, Robert W. Portmann, John S. Daniel, Sean M. Davis, Todd J. Sanford, Gian-Kasper Plattner, Science 5 March 2010: Vol. 327 no. 5970 pp. 1219-1223, DOI: 10.1126/science.1182488. [Full text]

An imperative for climate change planning: tracking Earth’s global energy – Trenberth (2009) “Planned adaptation to climate change requires information about what is happening and why. While a long-term trend is for global warming, short-term periods of cooling can occur and have physical causes associated with natural variability. However, such natural variability means that energy is rearranged or changed within the climate system, and should be traceable. An assessment is given of our ability to track changes in reservoirs and flows of energy within the climate system. Arguments are given that developing the ability to do this is important, as it affects interpretations of global and especially regional climate change, and prospects for the future.” Kevin E Trenberth, Current Opinion in Environmental Sustainability, Volume 1, Issue 1, October 2009, Pages 19–27, http://dx.doi.org/10.1016/j.cosust.2009.06.001. [Full text]

How will Earth’s surface temperature change in future decades? – Lean & Rind (2009) “Reliable forecasts of climate change in the immediate future are difficult, especially on regional scales, where natural climate variations may amplify or mitigate anthropogenic warming in ways that numerical models capture poorly. By decomposing recent observed surface temperatures into components associated with ENSO, volcanic and solar activity, and anthropogenic influences, we anticipate global and regional changes in the next two decades. From 2009 to 2014, projected rises in anthropogenic influences and solar irradiance will increase global surface temperature 0.15 ± 0.03°C, at a rate 50% greater than predicted by IPCC. But as a result of declining solar activity in the subsequent five years, average temperature in 2019 is only 0.03 ± 0.01°C warmer than in 2014. This lack of overall warming is analogous to the period from 2002 to 2008 when decreasing solar irradiance also countered much of the anthropogenic warming. We further illustrate how a major volcanic eruption and a super ENSO would modify our global and regional temperature projections.” Judith L. Lean, David H. Rind, Geophysical Research Letters, Volume 36, Issue 15, 16 August 2009, DOI: 10.1029/2009GL038932. [Full text]

Is the climate warming or cooling? – Easterling & Wehner (2009) “Numerous websites, blogs and articles in the media have claimed that the climate is no longer warming, and is now cooling. Here we show that periods of no trend or even cooling of the globally averaged surface air temperature are found in the last 34 years of the observed record, and in climate model simulations of the 20th and 21st century forced with increasing greenhouse gases. We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming.” David R. Easterling, Michael F. Wehner, Geophysical Research Letters, Volume 36, Issue 8, April 2009, DOI: 10.1029/2009GL037810. [Full text]

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Has global warming stopped?

Posted by Ari Jokimäki on January 28, 2013

For a long time there has been claims about global warming having stopped, and that there hasn’t been warming in N years or since year X. Current fashion is to claim that there hasn’t been any warming in 16 years. All of this is of course irrelevant to the anthropogenic global warming, which according to Easterling & Wehner (2009) “can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer‐term warming”.

Standard time-period of significance to climate is 30 years. Below global surface temperature anomalies are presented as running 30-year mean.

30yrTemp

Do you see any signs of global warming stopping?

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Climate skeptic claims prebunked by Keeling

Posted by Ari Jokimäki on August 23, 2012

If you have followed discussions about global warming, you probably have seen claims that because atmospheric carbon dioxide concentration is measured on the top of a volcano (actually on the side – the measurement station is at the elevation of 3400m while the top of the mountain is at the elevation of over 4100m), and that volcanos emit carbon dioxide, the carbon dioxide measurements cannot be trusted.  Thus the argument concludes that we don’t know if atmospheric carbon dioxide concentration is rising or not. Here we take a look at what Charles David Keeling (1928 – 2005) wrote long ago.

Charles David Keeling in the lab

As many of you probably know, Keeling was the one who arranged atmospheric carbon dioxide measurements starting in the 1950s, that resulted in the famous Keeling Curve, which describes the evolution of atmospheric carbon dioxide as measured in Hawaii, on top of a volcano called Mauna Loa. The measurement site was selected, because Hawaii is located in the middle of an ocean and far away from disturbing carbon dioxide sources. Also, the elevation of the location of the station ensures that measured air masses are representative of large areas. However, as the Keeling Curve is famous and is drawn from the measurements that originate at a site that has volcanic vents nearby, it is quite natural that there are claims made that the presence of the volcano somehow invalidates all our knowledge of atmospheric carbon dioxide. Many readers of course know that it isn’t so, but here we look at what Keeling wrote about the issue in 1960. It turns out he had already debunked the claims, even before those claims were made.

In 1960, Keeling published the paper The Concentration and Isotopic Abundances of Carbon Dioxide in the Atmosphere (full text freely available in the linked abstract page). In this paper he discussed results from carbon dioxide measurements during a few years of observtions. Early in the paper he says:

“Three gas analysers, as described by SMITH (1953), equipped with strip chart recorders, have been employed to measure the concentration of carbon dioxide continuously at stations in Antarctica, Hawaii, and California.”

So already from the beginning of Keeling’s measurements there were other measurement stations besides Mauna Loa. End of story? Well, pretty much so for those claims, but anyway, let’s take a look at what other things Keeling has to say on the subject.

Despite the reality, what if Mauna Loa was the only atmospheric carbon dioxide measuring station as those claims suggest? Would carbon dioxide measurements be suspect because of that? Keeling discusses this too. Keeling notes that there was local contamination in all of the measurement stations. In the Antarctica station there was some combustion of fuel near the station:

“It could be readily spotted from the significant fluctuations in the otherwise steady trace of the recorder pen and was eliminated from consideration in the initial reading of the charts.”

In Mauna Loa, there also was some extra variability in the measurement record:

“This is attributed to release of carbon dioxide by nearby volcanic vents; combustion on the island associated with agricultural, industrial, and domestic activities; and lower concentration of carbon dioxide in the air transported to the station by upslope winds.”

It seems that Keeling was well aware that “Mauna Loa is a volcano”. He also identified some other sources of contamination. Not only was Keeling aware of the situation, he also knew how to correct it:

“The values reported here are averages of data for periods of downslope winds or strong lateral winds when the concentration remained nearly constant for several hours or more.”

At a California measurement station there was an interesting situation. The measured carbon dioxide concentration was found to be highly variable:

“Highest concentrations occur during light winds from the north, from the direction of Los Angeles; lowest concentrations when the wind is from the west or southwest and of moderate force or greater.”

Highest concentrations when winds were blowing from a large city? Why is that? Could it be that there is carbon dioxide coming from human actions?

Keeling then proceeds to discuss seasonal variation which he finds in the Northern Hemisphere but not in the Southern Hemisphere. Based on carbon-13 measurements, he concludes that the seasonal variation found in the Northern Hemisphere is from the activity of land plants. He also notes that timing of maximum and minimum concentrations match the timing of the land vegetation activity in the Northern Hemisphere. Keeling explains the absence of seasonal variability in the Southern Hemisphere by the smaller area of vegetation in the Southern Hemisphere.

Getting back to the claims of CO2 record contamination, Keeling then discusses the interannual trends in carbon dioxide concentration:

“Where data extend beyond one year, averages for the second year are higher than for the first year. At the South Pole, where the longest record exists, the concentration has increased at the rate of about 1.3 p.p.m. per year.”

Note that Keeling reported the first clear increase in carbon dioxide concentration from the South Pole, not from Mauna Loa.

So, already back in 1960, Keeling knew that Mauna Loa was a volcano. He also knew how to correct the problems that the volcano caused for carbon dioxide measurements. He also knew that the volcanic problems wouldn’t matter anyway because there were other measurement stations that were nowhere near any volcanos, and those stations show the same thing as Mauna Loa station – atmospheric carbon dioxide concentration is rising.

Posted in AGW evidence, Climate claims, Climate science | 3 Comments »

Short and long term water vapor feedback

Posted by Ari Jokimäki on May 17, 2011

There’s a lot of discussion on water vapor feedback in different media. Relating to this discussion, I’d like to note an interesting paper which seems to have gone largely unnoticed (also in scientific literature). The paper in question is “An Elicitation of the Dynamic Nature of Water Vapor Feedback in Climate Change Using a 1D Model” by Hallegatte et al. (2006).

Hallegatte et al. are studying the different response times of feedback processes and how they affect the overall water vapor feedback. They say:

Another difficulty in the interpretation of feedback processes arises from the speed of the different responses. Some processes participating in the feedback mechanism may be fast, others may be very slow. This problem is usually avoided by analyzing only the new equilibrium reached by the system after a perturbation has been applied. This current practice might, however, ignore important dynamical components of the response, especially when the forcing is varying.

They study this question with a simple 1D model for atmospheric water vapor. The simplicity of the model of course introduces some uncertainties (they can’t model the dynamic changes for example in Hadley circulation and convection is accounted for only indirectly).

The result of their analysis is shown in the following graph, which shows the evolution of water vapor feedback after a step change in the forcing (doubling of carbon dioxide concentration):

Feedback factor having value below 1 represents negative water vapor feedback. As we can see from the graph, the end result is strongly positive water vapor feedback after 10 years. However, especially noteworthy feature is the apparently negative short-term feedback. It seems that water vapor feedback turns positive only after 2-3 years.

Hallegatte et al. explain this fast negative feedback like this:

The fast pole corresponds to the lowering of latent heat flux due to rainfall decrease, which comes from the rising temperature (corresponding to a decrease in RH). This mechanism constitutes one path of the WV feedback: any transient trajectory showing an increase in atmospheric absolute humidity requires an imbalance between precipitation and evaporation, and hence necessitates an increase in atmospheric latent energy content compared with the equilibrium state. In consequence, the WV feedback process should involve a rapid atmospheric cooling, as formalized in our model, with a time response of about a few weeks.

They also study how the water vapor feedback works for more realistic change in the forcing instead of step change. In this case, the fast negative feedback is hardly detectable, but it only reduces the initial warming a bit.

Their conclusion:

In our model, the WV feedback is found to have a positive static gain of 36%, and a characteristic time longer than 4 yr, making the WV feedback fully active only in response to perturbations that last at least 10 yr.

It seems that based on this study, studying water vapor feedback in the context of anthropogenic global warming should be done in decadal time scales.

Reference: Hallegatte, Stéphane, Alain Lahellec, Jean-Yves Grandpeix, 2006: An Elicitation of the Dynamic Nature of Water Vapor Feedback in Climate Change Using a 1D Model. J. Atmos. Sci., 63, 1878–1894. [abstract, full text]

See also: On quick feedback determinations

Posted in Climate claims, Climate science | 4 Comments »

Cosmic ray contribution to global warming negligible

Posted by Ari Jokimäki on April 26, 2011

There has been claims that cosmic rays could have contributed significantly to global warming. According to a new study that is not the case. Instead, during the last 50 years, cosmic rays seemed to have caused warming of about 0.002°C – a negligible amount compared to observed warming.

Cosmic rays have been claimed to be a significant source for the formation of cloud condensation nuclei and through that they have been claimed to affect Earth’s climate significantly. There has been many studies debunking these claims and currently it seems that the possible effect of cosmic rays on the climate is small. It is however likely that there are some mechanisms by which cosmic rays do affect cloud formation at least a little. For example, recently a correlation was found in Europe between diurnal temperature range (difference between daily maximum and minimum temperatures) and strong cosmic ray flux changes (Forbush decreases and ground level enhancements).

A new study by Erlykin et al. evaluates the effects of cosmic rays to cloud cover. The paper discusses previous cosmic ray research, and performs some new analysis. They use cloud cover measurements of International Satellite Cloud Climatology Project (ISCCP). There has been criticisms of ISCCP data, but the researchers believe that ISCCP problems do not affect their results.

Evidence of cosmic ray effects

Solar activity changes during the sunspot cycle and that affects Earth’s temperature. According to Erlykin et al. the change in temperature then affects the height of low clouds. The change of height makes some of the low clouds migrate to medium cloud region. This would then cause a decrease in low cloud amount. This would also be a candidate for the origin of the correlation between cosmic ray flux and low cloud cover (because solar activity changes are known to affect also cosmic ray flux). When analysing cloud cover changes it would make sense to look at the changes in both the low and medium clouds. When both low and medium clouds are included to the correlation analysis, the correlation with cosmic ray flux is poor.

Forbush decreases (few percent decrease in cosmic ray flux that lasts few days) have been claimed to cause cloud cover changes. However, the observed correlations between cloud cover changes and cosmic ray flux during a few Forbush decreases could have been coincidental. Additionally, delay between the changes in cosmic ray flux and in cloud cover seems to have been too long for a physical cause. Some of the strongest Forbush decreases have also been associated with strong changes in solar activity, which also places more clouds over the cosmic ray hypothesis.

Nevertheless there are some evidence relating to Forbush decreases that suggests some kind of effect in cloud cover. The strongest effect seems to lie in the stratosphere, but there is also some evidence for an effect in troposphere. It is unclear if these effects are due to cosmic rays or solar activity.

For cosmic ray flux ground level enhancements there also is some evidence of cloud effects. The strongest effect seems to occur in the poles which might suggest genuine cosmic ray involvement. Erlykin et al. estimate a global effect of about 1 % to cloud cover during ground level enhancements.

In a recent analysis it was found out that in mid-latitudes rapid changes in cloud cover are associated with changes in cosmic ray flux and in surface temperature. Strong reaction here only occurs during the rapid cloud cover changes which are rare events, so the total effect of cosmic rays still seems to be small. Here too it is unclear if the observed effects are due to cosmic rays or simultaneous changes in solar activity.

In the case of these rapid changes, the rate of change in clouds and in cosmic rays have been different which speaks against the cosmic ray origin. On the other hand, linking these cloud changes to solar activity is also problematic, because most of the UV radiation stops in the stratosphere and therefore would not be able to cause changes to most of the cloud cover. It is possible that the explanation for cloud changes is found in surface temperature changes caused by solar activity changes.

There is lot of evidence from polar regions on stratospheric cosmic ray effects, which usually are related to solar flares. It seems that cosmic rays have an effect on stratospheric aerosols, ozone, wind, temperature, pressure and ionisation. There are also some evidence of possible effect on stratospheric clouds during Forbush decreases and other cosmic ray flux changes, but the size of the possible effect is unclear.

Correlation between cosmic ray flux and cloud cover

Analysis of cloud cover correlation between cosmic rays and UV radiation reveals that correlation is significant only in these cases: low cloud cover with cosmic rays (positive correlation) and with UV radiation (negative), medium cloud cover with UV radiation (positive), and high cloud cover with cosmic rays (negative). For the case of UV radiation, the different sign for the correlation with low and medium cloud cover might be explainable by part of low cloud cover changing to medium cloud cover when surface temperature changes. For cosmic rays, the negative correlation with high cloud cover is not what would be expected from ionization mechanism.

The correlation of cosmic rays with low cloud cover is the correlation that has been claimed to cause the global warming. However, the spatial distribution of the correlation does not fit to the expectations from cosmic ray origin. Correlation is strong in mid-latitudes, but weak in the poles and in the equator. Mid-latitude correlation is about 8 times higher than the correlation in equator and in poles. This distribution of correlation results a possible effect to global cloud cover which is smaller than 1%.

According to Erlykin et al. the cosmic ray flux has decreased about 0.6% in last 50 years. Assuming 1% effect of the cosmic rays to cloud cover, this would cause a change of 0.002°C in global surface temperature. This is negligible compared to the observed global warming in last 50 years. In conclusion, while cosmic rays do seem to have some minor effects to different things in the atmosphere, they do not seem to have anything to do with global warming.

Source: A.D. Erlykin, B.A. Laken and A.W. Wolfendale, Cosmic ray effects on cloud cover and their relevance to climate change, Journal of Atmospheric and Solar-Terrestrial Physics, doi:10.1016/j.jastp.2011.03.001. [abstract]

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Papers on sea level in small island countries

Posted by Ari Jokimäki on December 29, 2010

This is a list of papers on sea level changes in small island countries such as Maldives. Emphasis is on the sea level observations, not its consequences. There are plenty of papers dealing with vulnerability of the island countries but so far only couple of such papers are included here. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (November 17, 2011): Merrifield (2011) and Becker et al. (2012) added.

Sea level variations at tropical Pacific islands since 1950 – Becker et al. (2012) “The western tropical Pacific is usually considered as one of the most vulnerable regions of the world under present-day and future global warming. It is often reported that some islands of the region already suffer significant sea level rise. To clarify the latter concern, in the present study we estimate sea level rise and variability since 1950 in the western tropical Pacific region (20°S–15°N; 120°E–135°W). We estimate the total rate of sea level change at selected individual islands, as a result of climate variability and change, plus vertical ground motion where available. For that purpose, we reconstruct a global sea level field from 1950 to 2009, combining long (over 1950–2009) good quality tide gauge records with 50-year-long (1958–2007) gridded sea surface heights from the Ocean General Circulation Model DRAKKAR. The results confirm that El Niño-Southern Oscillation (ENSO) events have a strong modulating effect on the interannual sea level variability of the western tropical Pacific, with lower/higher-than-average sea level during El Niño/La Niña events, of the order of ± 20–30 cm. Besides this sub-decadal ENSO signature, sea level of the studied region also shows low-frequency (multi decadal) variability which superimposes to, thus in some areas amplifies current global mean sea level rise due to ocean warming and land ice loss. We use GPS precise positioning records whenever possible to estimate the vertical ground motion component that is locally superimposed to the climate-related sea level components. Superposition of global mean sea level rise, low-frequency regional variability and vertical ground motion shows that some islands of the region suffered significant ‘total’ sea level rise (i.e., that felt by the population) during the past 60 years. This is especially the case for the Funafuti Island (Tuvalu) where the “total” rate of rise is found to be about 3 times larger than the global mean sea level rise over 1950–2009.” M. Becker, B. Meyssignac, C. Letetrel, W. Llovel, A. Cazenave, T. Delcroix, Global and Planetary Change, Volumes 80-81, January 2012, Pages 85-98, doi:10.1016/j.gloplacha.2011.09.004.

A Shift in Western Tropical Pacific Sea Level Trends during the 1990s – Merrifield (2011) “Pacific Ocean sea surface height trends from satellite altimeter observations for 1993–2009 are examined in the context of longer tide gauge records and wind stress patterns. The dominant regional trends are high rates in the western tropical Pacific and minimal to negative rates in the eastern Pacific, particularly off North America. Interannual sea level variations associated with El Niño–Southern Oscillation events do not account for these trends. In the western tropical Pacific, tide gauge records indicate that the recent high rates represent a significant trend increase in the early 1990s relative to the preceding 40 years. This sea level trend shift in the western Pacific corresponds to an intensification of the easterly trade winds across the tropical Pacific. The wind change appears to be distinct from climate variations centered in the North Pacific, such as the Pacific decadal oscillation. In the eastern Pacific, tide gauge records exhibit higher-amplitude decadal fluctuations than in the western tropical Pacific, and the recent negative sea level trends are indistinguishable from these fluctuations. The shifts in trade wind strength and western Pacific sea level rate resemble changes in dominant global modes of outgoing longwave radiation and sea surface temperature. It is speculated that the western Pacific sea level response indicates a general strengthening of the atmospheric circulation over the tropical Pacific since the early 1990s that has developed in concert with recent warming trends.” Merrifield, Mark A., 2011: A Shift in Western Tropical Pacific Sea Level Trends during the 1990s. J. Climate, 24, 4126–4138, doi: 10.1175/2011JCLI3932.1. [Full text]

Patterns of Indian Ocean sea-level change in a warming climate – Han et al. (2010) “Global sea level has risen during the past decades as a result of thermal expansion of the warming ocean and freshwater addition from melting continental ice. However, sea-level rise is not globally uniform. Regional sea levels can be affected by changes in atmospheric or oceanic circulation. As long-term observational records are scarce, regional changes in sea level in the Indian Ocean are poorly constrained. Yet estimates of future sea-level changes are essential for effective risk assessment. Here we combine in situ and satellite observations of Indian Ocean sea level with climate-model simulations, to identify a distinct spatial pattern of sea-level rise since the 1960s. We find that sea level has decreased substantially in the south tropical Indian Ocean whereas it has increased elsewhere. This pattern is driven by changing surface winds associated with a combined invigoration of the Indian Ocean Hadley and Walker cells, patterns of atmospheric overturning circulation in the north–south and east–west direction, respectively, which is partly attributable to rising levels of atmospheric greenhouse gases. We conclude that—if ongoing anthropogenic warming dominates natural variability—the pattern we detected is likely to persist and to increase the environmental stress on some coasts and islands in the Indian Ocean.” Weiqing Han, Gerald A. Meehl, Balaji Rajagopalan, John T. Fasullo, Aixue Hu, Jialin Lin, William G. Large, Jih-wang Wang, Xiao-Wei Quan, Laurie L. Trenary, Alan Wallcraft, Toshiaki Shinoda & Stephen Yeager, Nature Geoscience 3, 546 – 550 (2010), doi:10.1038/ngeo901. [Full text]

Wind Effects on Past and Future Regional Sea Level Trends in the Southern Indo-Pacific – Timmermann et al. (2010) “Global sea level rise due to the thermal expansion of the warming oceans and freshwater input from melting glaciers and ice sheets is threatening to inundate low-lying islands and coastlines worldwide. At present the global mean sea level rises at 3.1 ± 0.7 mm yr−1 with an accelerating tendency. However, the magnitude of recent decadal sea level trends varies greatly spatially, attaining values of up to 10 mm yr−1 in some areas of the western tropical Pacific. Identifying the causes of recent regional sea level trends and understanding the patterns of future projected sea level change is of crucial importance. Using a wind-forced simplified dynamical ocean model, the study shows that the regional features of recent decadal and multidecadal sea level trends in the tropical Indo-Pacific can be attributed to changes in the prevailing wind regimes. Furthermore, it is demonstrated that within an ensemble of 10 state-of-the-art coupled general circulation models, forced by increasing atmospheric CO2 concentrations over the next century, wind-induced redistributions of upper-ocean water play a key role in establishing the spatial characteristics of projected regional sea level rise. Wind-related changes in near-surface mass and heat convergence near the Solomon Islands, Tuvalu, Kiribati, the Cook Islands, and French Polynesia oppose—but cannot cancel—the regional signal of global mean sea level rise.” Timmermann, Axel, Shayne McGregor, Fei-Fei Jin, 2010, J. Climate, 23, 4429–4437, doi: 10.1175/2010JCLI3519.1.

The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis of island change in the Central Pacific – Webb & Kench (2010) “Low-lying atoll islands are widely perceived to erode in response to measured and future sea-level rise. Using historical aerial photography and satellite images this study presents the first quantitative analysis of physical changes in 27 atoll islands in the central Pacific over a 19 to 61 yr period. This period of analysis corresponds with instrumental records that show a rate of sea-level rise of 2.0 mm yr−1 in the Pacific. Results show that 86% of islands remained stable (43%) or increased in area (43%) over the timeframe of analysis. Largest decadal rates of increase in island area range between 0.1 to 5.6 ha. Only 14% of study islands exhibited a net reduction in island area. Despite small net changes in area, islands exhibited larger gross changes. This was expressed as changes in the planform configuration and position of islands on reef platforms. Modes of island change included: ocean shoreline displacement toward the lagoon; lagoon shoreline progradation; and, extension of the ends of elongate islands. Collectively these adjustments represent net lagoonward migration of islands in 65% of cases. Results contradict existing paradigms of island response and have significant implications for the consideration of island stability under ongoing sea-level rise in the central Pacific. First, islands are geomorphologically persistent features on atoll reef platforms and can increase in island area despite sea-level change. Second, islands are dynamic landforms that undergo a range of physical adjustments in responses to changing boundary conditions, of which sea level is just one factor. Third, erosion of island shorelines must be reconsidered in the context of physical adjustments of the entire island shoreline as erosion may be balanced by progradation on other sectors of shorelines. Results indicate that the style and magnitude of geomorphic change will vary between islands. Therefore, island nations must place a high priority on resolving the precise styles and rates of change that will occur over the next century and reconsider the implications for adaption.” Arthur P. Webb and Paul S. Kench, Global and Planetary Change, Volume 72, Issue 3, June 2010, Pages 234-246, doi:10.1016/j.gloplacha.2010.05.003. [Full text]

Submerged reef terraces of the Maldives (Indian Ocean) – Fürstenau et al. (2009) “There is limited knowledge about the record of sea-level rise from the last glacial maximum (LGM) until the onset of Holocene reef growth in the Maldives archipelago. Multibeam data show that atoll slopes between 130 and 55 mbsl (meters below sea level) are characterized by a step-like morphology. In parts, these terraces show a raised rim and a crenate geometry. Atoll margin features can be interpreted as successive reef-growth and -drowning stages, which are attributable to changes in the rate of sea-level rise. These changes can tentatively be correlated to known records of global sea-level change since the LGM. In addition to terraces between 97 and 55 mbsl, which can be associated with the initiation of meltwater pulses MWP-1A and -1B, several reef-drowning stages between 94 and 68 mbsl are proposed. As the Maldives can be considered a tectonically stable, far-field site, the submerged reef terraces inferred from the first multibeam dataset for this region likely represent a valuable archive for global deglacial sea-level history in the Indian Ocean.” Jörn Fürstenau, Sebastian Lindhorst, Christian Betzler and Christian Hübscher, Geo-Marine Letters, Volume 30, Number 5, 511-515, DOI: 10.1007/s00367-009-0174-2.

Late Quaternary reef growth and sea level in the Maldives (Indian Ocean) – Gischler et al. (2008) “Based on rotary drilling and radiometric and U-series dating, we present the first comprehensive data on Holocene reef anatomy and sea-level rise as well as nature and age of underlying Pleistocene limestone in the Maldives. Holocene reefs in Rasdhoo Atoll, central Maldives, are composed of four facies including (1) robust-branching coral facies, (2) coralline algal facies, (3) domal coral facies, and (4) detrital sand and rubble facies. Branching coral and coralline algal facies predominate the marginal reefs and domal corals and detrital facies preferentially occur in a lagoon reef. In addition, microbialite crusts are found in lower core sections of marginal reefs. Microbialites formed during the early Holocene in reef cavities. Holocene reef thickness ranges from 14.5 m to > 22 m. Reef growth started as early as 8.5 kyr BP. Marginal reefs accreted in the keep-up mode with rates of > 15 m/kyr. Rate of sea-level rise significantly slowed down from 7–6 kyr BP and subsequently gradually rose with rates < 1 m/kyr. The lagoon reef accreted in the catch-up mode with rates of around 4 m/kyr. Even though no indications of a higher than present sea level were found during this study, it is not entirely clear from the data whether the sea gradually rose to or exceeded present level in the late Holocene. Submarine cementation in Holocene reefs studied is rather weak, presumably as a consequence of high accretion-rates, i.e., short time available for consolidation. Pleistocene coral grainstone was encountered in one core at 14.5 m below present level and three U-series dates indicate deposition during marine isotope stage 5e ca. 135 kyr BP.” Eberhard Gischler, J. Harold Hudson and Andrzej Pisera, Marine Geology, Volume 250, Issues 1-2, 21 April 2008, Pages 104-113, doi:10.1016/j.margeo.2008.01.004.

Reef-island topography and the vulnerability of atolls to sea-level rise – Woodroffe (2008) “Low-lying reef islands on the rim of atolls are perceived as particularly vulnerable to the impacts of sea-level rise. Three effects are inferred: erosion of the shoreline, inundation of low-lying areas, and saline intrusion into the freshwater lens. Regional reconstruction of sea-level trends, supplementing the short observational instrumental record, indicates that monthly mean sea level is rising in the eastern Indian and western Pacific Oceans. This paper reviews the morphology and substrate characteristics of reef islands on Indo-Pacific atolls, and summarises their topography. On most atolls across this region, there is an oceanward ridge built by waves to a height of around 3 m above MSL; in a few cases these are topped by wind-blown dunes. The prominence of these ridges, together with radiocarbon dating and multi-temporal studies of shoreline position, indicate net accretion rather than long-term erosion on most of these oceanward shores. Less prominent lagoonward ridges occur, but their morphology and continuity are atoll-specific, being a function of the processes operating in each lagoon. Low-lying central areas are a feature of many islands, often locally excavated for production of taro. These lower-lying areas are already subject to inundation, which seems certain to increase as the sea rises. Tropical storms play an important role in the geomorphology of reef islands in those regions where they are experienced. Topographical differences, as well as features such as emergence of the reef flat and the stability of the substrate, mean that islands differ in terms of their susceptibility to sea-level rise. Further assessment of variations in shoreline vulnerability based on topography and substrate could form the basis for enhancing the natural resilience of these islands.” Colin D. Woodroffe, Global and Planetary Change, Volume 62, Issues 1-2, May 2008, Pages 77-96, doi:10.1016/j.gloplacha.2007.11.001. [Full text]

Sea-level rise at tropical Pacific and Indian Ocean islands – Church et al. (2006) “Historical and projected sea-levels for islands in the tropical Pacific and Indian oceans are a subject of considerable interest and some controversy. The large variability (e.g. El Niño) signals and the shortness of many of the individual tide-gauge records contribute to uncertainty of historical rates of sea-level rise. Here, we determine rates of sea-level rise from tide gauges in the region. We also examine sea-level data from the TOPEX/Poseidon satellite altimeter and from a reconstruction of sea level in order to put the sparse (in space and time) tide-gauge data into context. For 1993 to 2001, all the data show large rates of sea-level rise over the western Pacific and eastern Indian Ocean (approaching 30 mm yr−1) and sea-level falls in the eastern Pacific and western Indian Ocean (approaching − 10 mm yr−1). Over the region 40°S to 40°N, 30°E to 120°W, the average rise is about 4 mm yr−1. For 1950 to 2001, the average sea-level rise (relative to land) from the six longest tide-gauge records is 1.4 mm yr−1. After correcting for glacial isostatic adjustment and atmospheric pressure effects, this rate is 2.0 mm yr−1, close to estimates of the global average and regional average rate of rise. The long tide-gauge records in the equatorial Pacific indicate that the variance of monthly averaged sea-level after 1970 is about twice that before 1970. We find no evidence for the fall in sea level at the Maldives as postulated by Mörner et al. (2004). Our best estimate of relative sea-level rise at Funafuti, Tuvalu is 2 ± 1 mm yr−1 over the period 1950 to 2001. The analysis clearly indicates that sea-level in this region is rising. We expect that the continued and increasing rate of sea-level rise and any resulting increase in the frequency or intensity of extreme sea-level events will cause serious problems for the inhabitants of some of these islands during the 21st century.” John A. Church, Neil J. White, and John R. Hunter, Global and Planetary Change, Volume 53, Issue 3, September 2006, Pages 155-168, doi:10.1016/j.gloplacha.2006.04.001. [Full text]

Have there been large recent sea level changes in the Maldive Islands? – Woodworth (2005) “The Maldive Islands are often used as case studies within research into the impacts of potential future sea level change. Therefore, if such studies are to be realistic, it is important that the past and future variations of sea level in the islands are understood as well as possible. That objective led a fieldwork team to the Maldives, and resulted in a conclusion that sea level in the islands fell by approximately 30 cm during the past few decades. In the present paper, the suggestion of such a fall has been examined from meteorological and oceanographic perspectives and found to be implausible. A number of met-ocean data sets and regional climate indices have been examined, at least one of which would have been expected to reflect a large sea level fall, without any supporting evidence being found. In particular, a suggestion that an increase in evaporation could have caused the fall has been demonstrated to be incorrect. Without any real evidence for a hitherto-unrecognised process which could lead to a sea level change as significant as that proposed by the fieldwork team, one concludes that a rise in sea level of approximately half a metre during the 21st century, as suggested by the Intergovernmental Panel on Climate Change Third Assessment Report, remains the most reliable scenario to employ in future studies of the islands.” Philip L. Woodworth, Global and Planetary Change, Volume 49, Issues 1-2, November 2005, Pages 1-18, doi:10.1016/j.gloplacha.2005.04.001.

Late Quaternary sea-level highstands in the central and eastern Indian Ocean: A review – Woodroffe (2005) “The relative sea-level history of several atolls in the central and eastern Indian Ocean, including the Cocos (Keeling) Islands, Chagos Archipelago, and the Maldives–Laccadive Archipelagoes, has been debated for over a century but takes on a particular significance in the face of anticipated climate change. For each of these central and eastern Indian Ocean atolls Pleistocene limestone is encountered at depths of 6–20 m below sea level. On the Cocos (Keeling) Islands this has been dated to Last Interglacial age. Conglomerate platform underlies the reef islands on Cocos within which a sequence of fossil microatolls of massive and branching Porites records a gradual fall of sea level relative to the atoll. In the Maldives, the significance of outcrops of ‘reef rock’ has been vigorously debated without resolving sea-level history. Although in situ Heliopora occurs on the reef flat of Addu Atoll, dated at around 2700 radiocarbon yrs BP, other evidence for higher sea level remains poorly constrained. Conglomerates of a similar age have been described from the Chagos Archipelago, but it has not been unequivocally demonstrated that they formed under conditions of relatively higher sea level. In contrast to reefs further west in the Indian Ocean, each of these atolls has living microatolls of massive Porites that have been constrained in their upward growth by sea level. Interpretation of the upper surface of two such specimens from the Cocos (Keeling) Islands indicates broad fluctuations in the sea surface over the past century; similar microatolls are described from the Maldives implying little change in sea level over recent years. Regardless of minor past fluctuations, most reef islands in the Maldives are particularly low-lying and appear vulnerable to inundation, and extracting a more detailed sea-level history remains an important challenge.” Colin D. Woodroffe, Global and Planetary Change, Volume 49, Issues 1-2, November 2005, Pages 121-138, doi:10.1016/j.gloplacha.2005.06.002.

Holocene sea-level changes in the Indo-Pacific – Woodroffe & Horton (2005) “Holocene sea-level reconstructions exist from many locations in the Indo-Pacific region. Despite being a large geographical region, the nature of Holocene sea-level change is broadly similar in all locations. Differences do exist, however, in the timing and magnitude of the Mid-Holocene High Stand (MHHS) and the nature of late Holocene sea level fall across the region. When the Indo-Pacific is subdivided into smaller regions, these discrepancies do not disappear, and in some cases the discrepancies are large within a single coastline. It is clear from this analysis that the fundamental criteria to produce accurate local relative sea-level curves are hardly ever met. There are serious problems associated with the correct interpretation of sea-level indicators and their relationship to mean sea level, and with the quality of age determinations. A consistent methodology throughout the Indo-Pacific for the analysis of sea level data is lacking. Future sea-level analysis from far field locations must involve the application of a consistent methodology in order to allow meaningful comparison between studies. This should help to resolve the ongoing debate about the magnitude and timing of the Mid-Holocene High Stand, and the nature of late Holocene sea-level fall across the region.” S. A. Woodroffe and B. P. Horton, Journal of Asian Earth Sciences, Volume 25, Issue 1, April 2005, Pages 29-43, doi:10.1016/j.jseaes.2004.01.009.

New perspectives for the future of the Maldives – Mörner et al. (2004) “Novel prospects for the Maldives do not include a condemnation to future flooding. The people of the Maldives have, in the past, survived a higher sea level of about 50–60 cm. The present trend lack signs of a sea level rise. On the contrary, there is firm morphological evidence of a significant sea level fall in the last 30 years. This sea level fall is likely to be the effect of increased evaporation and an intensification of the NE-monsoon over the central Indian Ocean.” Nils-Axel Mörner, Michael Tooley, and Göran Possnert, Global and Planetary Change, Volume 40, Issues 1-2, January 2004, Pages 177-182, doi:10.1016/S0921-8181(03)00108-5. [Full text]

Coral microatolls and 20th century sea level in the eastern Indian Ocean – Smithers & Woodroffe (2001) “Coral microatolls are discoid intertidal corals that are limited in their upward growth by subaerial exposure during low tides. Microatoll upper surface morphology preserves a filtered record of changes in the height of living coral (HLC), the upper limit to which corals can grow, and by proxy a historical record of former constraining water levels. Chronologies for these variations in HLC were established in this study using annual skeletal density bands revealed when skeletal slices were X-radiographed, supplemented by annual fluorescent bands visible when samples were illuminated with ultra-violet light. The upper surface morphologies of two large microatolls from separate reef-flat sites on the Cocos (Keeling) Islands are well correlated and indicate that the upper limit to coral growth has fluctuated by more than 5 cm since the early 1900s. The upper surfaces of these microatolls also indicate that there has been little net rise in sea level in the eastern Indian Ocean during the 20th century. Microatoll surface morphology suggests that average rates of sea-level rise in the eastern Indian Ocean over this period were less than 0.35 mm yr−1, a rate considerably lower than the rate of average global sea-level change determined from aggregated tide-gauge data. The broad surface undulations do not appear to correlate directly with either El Niño-Southern Oscillation events or occurrence of the Indian Ocean dipole mode of ocean-atmosphere circulation. Microatolls provide a simple and effective method for extrapolating broad variations in sea level beyond the tide-gauge record in remote mid-oceanic settings.” Scott G. Smithers and Colin D. Woodroffe, Earth and Planetary Science Letters, Volume 191, Issues 1-2, 30 August 2001, Pages 173-184, doi:10.1016/S0012-821X(01)00417-4.

Illuminating Sea-Level Fall around AD 1220–1510 (730-440 cal yr BP) in the Pacific Islands: Implications for Environmental Change and Cultural Transformation – Nunn (2000) “This paper focuses on the climatic transition between the Little Climatic Optimum (approximately AD 750–1300 or 1200-650 cal yr BP) and the Little Ice Age (approximately AD 1300–1800 or 650-150 cal yr BP) in the Pacific Islands. This transition was marked by rapid temperature and sea-level fall, and perhaps by sharply-increased precipitation associated with an increase in El Nino frequency. Examples from throughout the Pacific Islands demonstrate the possible and/or likely effects of sea-level fall at this time on inland horticulture through water-table fall; on coral reefs and lagoons through the emergence of reef surfaces and the consequent reduction of nearshore water circulation; on the emergence of reef islets and the conversion of tidal inlets to brackish lakes. The effects of such changes on human lifestyles are explored.” Patrick D. Nunn, New Zealand Geographer, Volume 56, Issue 1, pages 46–54, April 2000, DOI: 10.1111/j.1745-7939.2000.tb00559.x.

Holocene Sea-Level Record on Funafuti and Potential Impact of Global Warming on Central Pacific Atolls – Dickinson (1999) “Geomorphic features inherited from the mid-Holocene glacio-hydro-isostatic sea-level highstand that affected the central Pacific region influence the susceptibility of atoll islets to potentially enhanced wave erosion associated with rise in sea level from global warming. Shoreline morphology on multiple islets of Funafuti atoll in central Tuvalu reflects a relative mid-Holocene sea-level highstand 2.2–2.4 m above modern sea level. Typical islets are composed of unconsolidated post-mid-Holocene sediment resting disconformably on cemented coral rubble formed beneath now-emergent mid-Holocene reef flats. Exposed remnants of the lithified islet foundations serve as resistant buttresses protecting the flanks of atoll islets from wave attack. Islets lacking cemented mid-Holocene deposits as part of their internal structure are migratory sand cays with unstable shorelines. Any future sea-level rise ≥0.75 m, bringing high tide above the elevation of mid-Holocene low tide, might trigger enhanced wave erosion of stable atoll islets by overtopping the indurated mid-Holocene reef platforms. As analogous threshold relations are inferred for other central Pacific atolls, the risk of future inundation of island nations cannot be evaluated solely in terms of expected sea-level rise with respect to gross islet elevations.” William R. Dickinson, Quaternary Research, Volume 51, Issue 2, March 1999, Pages 124-132, doi:10.1006/qres.1998.2029.

Submarine topography of Maldivian atolls suggests a sea level of 130 metres below present at the last glacial maximum – Anderson (1998) “This present study reports the results of an echo sounding survey around four Maldivian atolls which suggests that local sea level was reduced to about 130 m below current levels during the last glacial maximum. At that time present-day atolls would have been exposed as large, steeply clifted islands.” R. C. Anderson, Coral Reefs, Volume 17, Number 4, 339-341, DOI: 10.1007/s003380050135.

Morphology and evolution of reef islands in the Maldives – Woodroffe (1992). From Mörner et al. (2004): “More recently, Woodroffe (1992) presented the first sea level curve for the Maldives. He claimed that the islands were predominantly formed by catch-up coral reef growth.” Woodroffe, C.D., 1992, Proc. 7th Int. Coral Reef Symp., Gaum 2, 1217– 1226.

Microatolls and recent sea level change on coral atolls – Woodroffe & McLean (1990) “MICROATOLLS are colonies of corals, commonly Porties, which are dead on top but living around their perimeter, and are found in intertidal environments on coral atolls. They can grow to several metres in diameter. Their upward growth is constrained by sea level through prolonged exposure at the lowest spring tides, and their dead upper surfaces have been limited by past sea levels. They act as natural recorders of sea level, which is of particular significance for coral atolls thought to be susceptible to inundation and erosion if sea level rises in response to global warming. X-radiographs of vertical slices through microatolls from the Maldives and Cocos (Keeling) Islands (Indian Ocean) and Kiribati (Pacific Ocean) record sea-level fluctuations over the past few decades. There is a high degree of reproducibility between adjacent corals, although on Cocos we noted geographical variation in the pattern of change around the atoll. The majority of microatolls sampled on these atolls record a slight fall in sea level over the past ten years.” Colin Woodroffe & Roger McLean, Nature 344, 531 – 534 (05 April 1990); doi:10.1038/344531a0.

Sea level rise: Some implications for Tuvalu – Lewis (1989) “Much current evidence suggests that global temperatures are slowly increasing. It is believed that this increase will be associated with a sea level rise. Tuvalu, approximately 1000 km north of Fiji in the South Pacific Ocean, is one of six countries, all of them island states, that could face total destruction when sea levels rise. If sea level rises occur anywhere near the extreme projections that have been made, we can write these nations off the map. (Pernetta, 1988). This paper examines possible implications to the people of Tuvalu.” James Lewis, The Environmentalist, Volume 9, Number 4, 269-275, DOI: 10.1007/BF02241827.

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