AGW Observer

Observations of anthropogenic global warming

New research from last week 16/2011

Posted by Ari Jokimäki on April 25, 2011

Here is the new research published last week. I’m not including everything that was published but just some papers that got my attention. Those who follow my Facebook page (and/or Twitter) have already seen most of these, as I post these there as soon as they are published. Here, I’ll just put them out in one batch. Sometimes I might also point out to some other news as well, but the new research will be the focus here. Here’s the archive for the news of previous weeks. By the way, if this sort of thing interests you, be sure to check out A Few Things Illconsidered, they have a weekly posting containing lots of links to new research and other climate related news. Planet 3.0 also reports new research.

Published last week:

Global river flow has decreased

The impact of climate, CO2, nitrogen deposition and land use change on simulated contemporary global river flow – Shi et al. (2011) “We investigated how climate, rising atmospheric CO2 concentration, increasing anthropogenic nitrogen deposition and land use change influenced continental river flow over the period 1948–2004 using the Community Land Model version 4 (CLM4) with coupled river transfer model (RTM), a global river routing scheme. The model results indicate that the global mean river flow shows significant decreasing trend and climate forcing likely functions as the dominant controller of the downward trend during the study period. Nitrogen deposition and land use change account for about 5% and 2.5% of the decrease in simulated global scale river flow, respectively, while atmospheric CO2 accounts for an upward trend. However, the relative role of each driving factor is heterogeneous across regions in our simulations. The trend in river flow for the Amazon River basin is primarily explained by CO2, while land use change accounts for 27.4% of the downward trend in river flow for the Yangtze rive basin. Our simulations suggest that to better understand the trends of river flow, it is not only necessary to take into account the climate, but also to consider atmospheric composition, carbon-nitrogen interaction and land use change, particularly for regional scales.” Shi, X., J. Mao, P. E. Thornton, F. M. Hoffman, and W. M. Post (2011), Geophys. Res. Lett., 38, L08704, doi:10.1029/2011GL046773.

Changes in lake area in Alaska

Mechanisms influencing changes in lake area in Alaskan boreal forest – Roach et al. (2011) “During the past ∼50 years, the number and area of lakes have declined in several regions in boreal forests. However, there has been substantial finer-scale heterogeneity; some lakes decreased in area, some showed no trend, and others increased. The objective of this study was to identify the primary mechanisms underlying heterogeneous trends in closed-basin lake area. Eight lake characteristics (δ18O, electrical conductivity, surface:volume index, bank slope, floating mat width, peat depth, thaw depth at shoreline, and thaw depth at the forest boundary) were compared for 15 lake pairs in Alaskan boreal forest where one lake had decreased in area since ∼1950, and the other had not. Mean differences in characteristics between paired lakes were used to identify the most likely of nine mechanistic scenarios that combined three potential mechanisms for decreasing lake area (talik drainage, surface water evaporation, and terrestrialization) with three potential mechanisms for non-decreasing lake area (sub-permafrost groundwater recharge through an open talik, stable permafrost, and thermokarst). A priori expectations of the direction of mean differences between decreasing and non-decreasing paired lakes were generated for each scenario. Decreasing lakes had significantly greater electrical conductivity, greater surface:volume indices, shallower bank slopes, wider floating mats, greater peat depths, and shallower thaw depths at the forest boundary. These results indicated that the most likely scenario was terrestrialization as the mechanism for lake area reduction combined with thermokarst as the mechanism for non-decreasing lake area. Terrestrialization and thermokarst may have been enhanced by recent warming which has both accelerated permafrost thawing and lengthened the growing season, thereby increasing plant growth, floating mat encroachment, transpiration rates, and the accumulation of organic matter in lake basins. The transition to peatlands associated with terrestrialization may provide a transient increase in carbon storage enhancing the role of northern ecosystems as major stores of global carbon.” Jennifer Roach, Brad Griffith, Dave Verbyla, Jeremy Jones, Global Change Biology, DOI: 10.1111/j.1365-2486.2011.02446.x.

Hirantian glaciation in Iran Zagros Mountains

Stratigraphic evidence for the Hirnantian (latest Ordovician) glaciation in the Zagros Mountains, Iran – Ghavidel-syooki et al. (2011) “High-latitude Hirnantian diamictites (Dargaz Formation) and lower–Silurian kerogenous black shales (Sarchahan Formation) are locally exposed in the Zagros Mountains. The glaciogenic Dargaz deposits consist of three progradational/retrogradational cycles, each potentially controlled by the regional advance and retreat of the Hirnantian ice sheet. Glacial incisions of sandstone packages change laterally from simple planar to high-relief (< 40 m deep) scalloped truncating surfaces that join laterally forming complex polyphase unconformities that scour into the underlying Seyahou Formation. The glaciated source area was to the present-day west, in the region of the Arabian Shield, where numerous tunnel valleys have been reported. Based on a study of palynomorphs and graptolites, the glaciomarine Dargaz diamictites are dated as Hirnantian, whereas the youngest Sarchahan black shales are diachronous throughout the Zagros, ranging from the Hirnantian persculptus to the earliest Aeronian (Llandovery) triangulatus zones. The diachronism is related to onlapping geometries capping an inherited glaciogenic palaeorelief that preserved different depth incisions and source areas. Our data suggest the presence of Hirnantian satellite ice caps adjacent the Zagros margin of Arabia and allow us to fill a gap in the present knowledge of the peripheral extension of the Late Ordovician ice sheet." Mohammad Ghavidel-syooki, J. Javier Álvaro, Leonid Popov, Mansoureh Ghobadi Pour, Mohammad H. Ehsani and Anna Suyarkova, Palaeogeography, Palaeoclimatology, Palaeoecology, doi:10.1016/j.palaeo.2011.04.011.

Climate variability during last 800kyr

Interglacial and glacial variability from the last 800 ka in marine, ice and terrestrial archives – Lang & Wolff (2011) “We have compiled 37 ice, marine and terrestrial palaeoclimate records covering the last 800 000 years in order to assess the pattern of glacial and interglacial strength, and termination amplitude. Records were selected based on their length, completeness and resolution, and their age models were updated, where required, by alignment to the LR04 benthic δ18O stack. The resulting compilation allows comparison of individual glacial to interglacial transitions with confidence, but the level of synchronisation is inadequate for discussion of temporal phasing. The comparison of interglacials and glacials concentrates on the peaks immediately before and after terminations; particularly strong and weak glacials and interglacials have been identified. This confirms that strong interglacials are confined to the last 450 ka, and that this is a globally robust pattern; however weak interglacials (i.e. marine isotope stage 7) can still occur in this later period. Strong glacial periods are also concentrated in the recent half of the records, although marine isotope stage 16 is strong in many δ18O records. Strong interglacials, particularly in the marine isotopic records, tend to follow strong glacials, suggesting that we should not expect interglacial strength to be strongly influenced by the instantaneous astronomical forcing. Many interglacials have a complex structure, with multiple peaks and troughs whose origin needs to be understood. However this compilation emphasises the under-representation of terrestrial environments and highlights the need for long palaeoclimate records from these areas. The main result of this work is the compiled datasets and maps of interglacial strength which provide a target for modelling studies and for conceptual understanding.” Lang, N. and Wolff, E. W., Clim. Past, 7, 361-380, doi:10.5194/cp-7-361-2011, 2011. [full text]

Positive feedback from atmospheric methane chemistry

Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions – Isaksen et al. (2011) “The magnitude and feedbacks of future methane release from the Arctic region are unknown. Despite limited documentation of potential future releases associated with thawing permafrost and degassing methane hydrates, the large potential for future methane releases calls for improved understanding of the interaction of a changing climate with processes in the Arctic and chemical feedbacks in the atmosphere. Here we apply a “state of the art” atmospheric chemistry transport model to show that large emissions of CH4 would likely have an unexpectedly large impact on the chemical composition of the atmosphere and on radiative forcing (RF). The indirect contribution to RF of additional methane emission is particularly important. It is shown that if global methane emissions were to increase by factors of 2.5 and 5.2 above current emissions, the indirect contributions to RF would be about 250% and 400%, respectively, of the RF that can be attributed to directly emitted methane alone. Assuming several hypothetical scenarios of CH4 release associated with permafrost thaw, shallow marine hydrate degassing, and submarine landslides, we find a strong positive feedback on RF through atmospheric chemistry. In particular, the impact of CH4 is enhanced through increase of its lifetime, and of atmospheric abundances of ozone, stratospheric water vapor, and CO2 as a result of atmospheric chemical processes. Despite uncertainties in emission scenarios, our results provide a better understanding of the feedbacks in the atmospheric chemistry that would amplify climate warming.” Isaksen, I. S. A., M. Gauss, G. Myhre, K. M. Walter Anthony, and C. Ruppel (2011), Global Biogeochem. Cycles, 25, GB2002, doi:10.1029/2010GB003845.

Black carbon layers in atmosphere

Free tropospheric black carbon aerosol measurements using high altitude balloon: Do BC layers build “their own homes” up in the atmosphere? – Babu et al. (2011) “First ever in-situ measurements of black carbon (BC) aerosols in the troposphere (up to 9 km) made over central India and the resulting atmospheric impact as revealed by the environment lapse rate are presented. The altitude distribution of BC showed multiple peaks; two surprisingly large peaks, one at ∼4.5 km, and another above 8 km. Associated with these, rapid decrease in the environmental lapse rate and a sharp increase in the atmosphere stability were observed, probably caused by the atmospheric warming by the BC layers. This important observation calls for extensive high altitude profiling of BC to quantify the resultant warming, increase in stability and consequent increase in BC lifetime.” Babu, S. S., K. K. Moorthy, R. K. Manchanda, P. R. Sinha, S. K. Satheesh, D. P. Vajja, S. Srinivasan, and V. H. A. Kumar (2011), Geophys. Res. Lett., 38, L08803, doi:10.1029/2011GL046654.

Himalayan glaciers – melting

Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) derived from stereo imagery – Bolch et al. (2011) “Mass loss of Himalayan glaciers has wide-ranging consequences such as changing runoff distribution, sea level rise and an increasing risk of glacial lake outburst floods (GLOFs). The assessment of the regional and global impact of glacier changes in the Himalaya is, however, hampered by a lack of mass balance data for most of the range. Multi-temporal digital terrain models (DTMs) allow glacier mass balance to be calculated. Here, we present a time series of mass changes for ten glaciers covering an area of about 50 km2 south and west of Mt. Everest, Nepal, using stereo Corona spy imagery (years 1962 and 1970), aerial images and recent high resolution satellite data (Cartosat-1). This is the longest time series of mass changes in the Himalaya. We reveal that the glaciers have been significantly losing mass since at least 1970, despite thick debris cover. The specific mass loss for 1970–2007 is 0.32 ± 0.08 m w.e. a−1, however, not higher than the global average. Comparisons of the recent DTMs with earlier time periods indicate an accelerated mass loss. This is, however, hardly statistically significant due to high uncertainty, especially of the lower resolution ASTER DTM. The characteristics of surface lowering can be explained by spatial variations of glacier velocity, the thickness of the debris-cover, and ice melt due to exposed ice cliffs and ponds.” Bolch, T., Pieczonka, T., and Benn, D. I., The Cryosphere, 5, 349-358, doi:10.5194/tc-5-349-2011, 2011. [full text]

Methane sources and sinks 2006-2008

Source attribution of the changes in atmospheric methane for 2006–2008 – Bousquet et al. (2011) “The recent increase of atmospheric methane is investigated by using two atmospheric inversions to quantify the distribution of sources and sinks for the 2006–2008 period, and a process-based model of methane emissions by natural wetland ecosystems. Methane emissions derived from the two inversions are consistent at a global scale: emissions are decreased in 2006 (−7 Tg) and increased in 2007 (+21 Tg) and 2008 (+18 Tg), as compared to the 1999–2006 period. The agreement on the latitudinal partition of the flux anomalies for the two inversions is fair in 2006, good in 2007, and not good in 2008. In 2007, a positive anomaly of tropical emissions is found to be the main contributor to the global emission anomalies (~60–80%) for both inversions, with a dominant share attributed to natural wetlands (~2/3), and a significant contribution from high latitudes (~25%). The wetland ecosystem model produces smaller and more balanced positive emission anomalies between the tropics and the high latitudes for 2006, 2007 and 2008, mainly due to precipitation changes during these years. At a global scale, the agreement between the ecosystem model and the inversions is good in 2008 but not satisfying in 2006 and 2007. Tropical South America and Boreal Eurasia appear to be major contributors to variations in methane emissions consistently in the inversions and the ecosystem model. Finally, changes in OH radicals during 2006–2008 are found to be less than 1% in inversions, with only a small impact on the inferred methane emissions.” Bousquet, P., Ringeval, B., Pison, I., Dlugokencky, E. J., Brunke, E.-G., Carouge, C., Chevallier, F., Fortems-Cheiney, A., Frankenberg, C., Hauglustaine, D. A., Krummel, P. B., Langenfelds, R. L., Ramonet, M., Schmidt, M., Steele, L. P., Szopa, S., Yver, C., Viovy, N., and Ciais, P., Atmos. Chem. Phys., 11, 3689-3700, doi:10.5194/acp-11-3689-2011, 2011. [full text]

Greenland melted rapidly also between 1920-1960

A reconstruction of annual Greenland ice melt extent, 1784–2009 – Frauenfeld et al. (2011) “The total extent of ice melt on the Greenland ice sheet has been increasing during the last three decades. The melt extent observed in 2007 in particular was the greatest on record according to several satellite-derived records of total Greenland melt extent. Total annual observed melt extent across the Greenland ice sheet has been shown to be strongly related to summer temperature measurements from stations located along Greenland’s coast, as well as to variations in atmospheric circulation across the North Atlantic. We make use of these relationships along with historical temperature and circulation observations to develop a near-continuous 226 year reconstructed history of annual Greenland melt extent dating from 2009 back into the late eighteenth century. We find that the recent period of high-melt extent is similar in magnitude but, thus far, shorter in duration, than a period of high melt lasting from the early 1920s through the early 1960s. The greatest melt extent over the last 2 1/4 centuries occurred in 2007; however, this value is not statistically significantly different from the reconstructed melt extent during 20 other melt seasons, primarily during 1923–1961.” Frauenfeld, O. W., P. C. Knappenberger, and P. J. Michaels (2011), A reconstruction of annual Greenland ice melt extent, 1784–2009, J. Geophys. Res., 116, D08104, doi:10.1029/2010JD014918.

See also Jason Box’s comments on this paper.

Small climatic effect of methane release from hydrates in next 100 years

Rising Arctic Ocean temperatures cause gas hydrate destabilization and ocean acidification – Biastoch et al. (2011) “Vast amounts of methane hydrates are potentially stored in sediments along the continental margins, owing their stability to low temperature – high pressure conditions. Global warming could destabilize these hydrates and cause a release of methane (CH4) into the water column and possibly the atmosphere. Since the Arctic has and will be warmed considerably, Arctic bottom water temperatures and their future evolution projected by a climate model were analyzed. The resulting warming is spatially inhomogeneous, with the strongest impact on shallow regions affected by Atlantic inflow. Within the next 100 years, the warming affects 25% of shallow and mid-depth regions containing methane hydrates. Release of methane from melting hydrates in these areas could enhance ocean acidification and oxygen depletion in the water column. The impact of methane release on global warming, however, would not be significant within the considered time span.” Biastoch, A., et al. (2011), Geophys. Res. Lett., 38, L08602, doi:10.1029/2011GL047222..

Permafrost melting threatens Siberian ecosystems

Dynamics of the larch taiga–permafrost coupled system in Siberia under climate change – Zhang et al. (2011) “Larch taiga, also known as Siberian boreal forest, plays an important role in global and regional water–energy–carbon (WEC) cycles and in the climate system. Recent in situ observations have suggested that larch-dominated taiga and permafrost behave as a coupled eco-climate system across a broad boreal zone of Siberia. However, neither field-based observations nor modeling experiments have clarified the synthesized dynamics of this system. Here, using a new dynamic vegetation model coupled with a permafrost model, we reveal the processes of interaction between the taiga and permafrost. The model demonstrates that under the present climate conditions in eastern Siberia, larch trees maintain permafrost by controlling the seasonal thawing of permafrost, which in turn maintains the taiga by providing sufficient water to the larch trees. The experiment without permafrost processes showed that larch would decrease in biomass and be replaced by a dominance of pine and other species that suffer drier hydroclimatic conditions. In the coupled system, fire not only plays a destructive role in the forest, but also, in some cases, preserves larch domination in forests. Climate warming sensitivity experiments show that this coupled system cannot be maintained under warming of about 2 °C or more. Under such conditions, a forest with typical boreal tree species (dark conifer and deciduous species) would become dominant, decoupled from the permafrost processes. This study thus suggests that future global warming could drastically alter the larch-dominated taiga–permafrost coupled system in Siberia, with associated changes of WEC processes and feedback to climate.” Ningning Zhang, Tetsuzo Yasunari and Takeshi Ohta, 2011 Environ. Res. Lett. 6 024003 doi: 10.1088/1748-9326/6/2/024003.

Ecosystem in the surface of glaciers

Alpine debris-covered glaciers as a habitat for plant life – Caccianiga et al. (2011) “Debris-covered glaciers represent a significant, increasing fraction of glaciers and can host plant life on their surface. The goal of this work was to evaluate the suitability of supraglacial debris as a habitat for plant life and to discuss its ecological and biogeographic role. The research was carried out on the Miage Glacier (Mont Blanc massif, Western Alps, Italy). Vegetation cover was sampled using a regular sampling grid, recording plant species and number of individuals in 71 plots. Detailed glaciological parameters (surface temperature, debris thickness, glacier surface velocity) were recorded or derived from published data. Relationships between vegetation and environmental variables were assessed through Generalized Linear Models, Principal Components Analysis and Canonical Correspondence Analysis. The glacier surface hosted a high biodiversity, with 40 vascular plant species, including trees and shrubs. Plant cover was arranged along an altitude/glacier velocity gradient, whilst debris thickness as low as 10 cm could sustain plant growth on moving ice. Glacier velocity was the main physical factor affecting vegetation cover, probably through its influence on debris stability. The observed species assemblage is comparable with those of subalpine glacier forelands, but with the addition of high-altitude species. Debris-covered glaciers can provide a relatively favourable habitat for plant life wherever the glacier surface is sufficiently stable, acting as a refugium of high-altitude taxa below their altitudinal limits. Glaciers may behave as a dispersal vector for alpine plant species, which could have been important both during glacial periods and during warm stages of the Holocene.” Marco Caccianiga, Carlo Andreis, Guglielmina Diolaiuti, Carlo D’Agata, Claudia Mihalcea, Claudio Smiraglia, The Holocene April 18, 2011, 0959683611400219, doi: 10.1177/0959683611400219.

Hockey stick from water temperature in Norway

A 2000 year record of Atlantic Water temperature variability from the Malangen Fjord, northeastern North Atlantic – Hald et al. (2011) “A high-resolution sedimentary record from the subarctic Malangen fjord in northern Norway, northeastern North Atlantic has been investigated in order to reconstruct variations in influx of Atlantic Water for the last 2000 years. The fjord provides a regional oceanographic climatic signal reflecting changes in the North Atlantic heat flux at this latitude because of its deep sill and the relatively narrow adjoining continental shelf. The reconstructions are based on oxygen and carbon isotopic studies of benthic foraminifera from a high accumulation basin in the Malangen fjord, providing subdecadal time resolution. A comparison between instrumental measurements of bottom water temperatures at the core location and the reconstructed temperatures from benthic foraminiferal δ18O for the same time period demonstrates that the stable isotope values reflect the bottom water temperatures very well. The reconstructed temperature record shows an overall decline in temperature of c. 1°C from c. 40 BC to AD 1350. This cooling trend is assumed to be driven by an orbital forced reduction in insolation. Superimposed on the general cooling trend are several periods of warmer or colder temperatures. The long-term fluctuations in the Malangen fjord are concurrent with fluctuations of Atlantic Water in the northern North Atlantic. Although they are not directly comparable, comparisons of atmospheric temperatures and marine records, indicate a close coupling between the climate systems. After AD 1800 the record shows an unprecedented warming within the last 2000 years.” M. Hald, G. R. Salomonsen, K. Husum, L. J. Wilson, The Holocene April 18, 2011, 0959683611400457, doi: 10.1177/0959683611400457.

AMO’s role significant in Northern Hemisphere’s climate

Atlantic Multidecadal Oscillation and Northern Hemisphere’s climate variability – Wyatt et al. (2011) “Proxy and instrumental records reflect a quasi-cyclic 50–80-year climate signal across the Northern Hemisphere, with particular presence in the North Atlantic. Modeling studies rationalize this variability in terms of intrinsic dynamics of the Atlantic Meridional Overturning Circulation influencing distribution of sea-surface-temperature anomalies in the Atlantic Ocean; hence the name Atlantic Multidecadal Oscillation (AMO). By analyzing a lagged covariance structure of a network of climate indices, this study details the AMO-signal propagation throughout the Northern Hemisphere via a sequence of atmospheric and lagged oceanic teleconnections, which the authors term the “stadium wave”. Initial changes in the North Atlantic temperature anomaly associated with AMO culminate in an oppositely signed hemispheric signal about 30 years later. Furthermore, shorter-term, interannual-to-interdecadal climate variability alters character according to polarity of the stadium-wave-induced prevailing hemispheric climate regime. Ongoing research suggests mutual interaction between shorter-term variability and the stadium wave, with indication of ensuing modifications of multidecadal variability within the Atlantic sector. Results presented here support the hypothesis that AMO plays a significant role in hemispheric and, by inference, global climate variability, with implications for climate-change attribution and prediction.” Marcia Glaze Wyatt, Sergey Kravtsov and Anastasios A. Tsonis, Climate Dynamics, DOI: 10.1007/s00382-011-1071-8.

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