AGW Observer

Observations of anthropogenic global warming

New research from last week 19/2011

Posted by Ari Jokimäki on May 16, 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:

Less temperature stress for rice from global warming

Global warming over the period 1961–2008 did not increase high-temperature stress but did reduce low-temperature stress in irrigated rice across China – Sun & Huang (2011) “Climate change is recognized to increase the frequency and severity of extreme temperature events that lead to declining crop yield, but this impact has not been well evaluated in China. We examined the changes in extreme temperature stress over the past five decades by quantifying the indices of temperature stress (TSI) during different growth stages of irrigated rice across mainland China. Our results suggest that the indices of low- or high-temperature stress can be used to explain the year-to-year changes in rice yield. Analysis using the TSI indicated that low-temperature stress (LTS) in the seedling and heading-flowering stages of single rice in northeast China, the seedling stage of early rice and the heading-flowering stage of late rice in the double rice regions has reduced over the period of 1961–2008. No significant trends in LTS were detected during the booting stage. Moreover, global warming did not enhance high-temperature stress (HTS) in the heading-flowering stage over the same period, except in early rice in the mid-lower Yangtze River Valley where the HTS in the 2000s was higher than in previous decades” Wen Sun and Yao Huang, Agricultural and Forest Meteorology, doi:10.1016/j.agrformet.2011.04.009.

Proxy climate reconstructions from peatlands

Development and refinement of proxy-climate indicators from peats – Chambers et al. (2011) “Peat, especially from acidic mires (bogs), is a natural archive of past environmental change. Reconstructions of past climate from bogs commenced in the 19th Century through examination of visible peat stratigraphy, and later formed the basis for a postglacial climatic scheme widely used in Northwest Europe. Nevertheless, misconceptions as to how bogs grow led to a 50-year lacuna in peatclimate study, before the concept of ‘cyclic regeneration’ in bogs was refuted. In recent decades, research using proxy-climate indicators from bogs has burgeoned. A range of proxies for past hydrological change has been developed, as well as use of pollen, bog oaks and pines and other data to reconstruct past temperatures. Most of this proxy-climate research has been carried out in Northern Europe, but peat-based research in parts of Asia and North America has increased, particularly during the last decade, while research has also been conducted in Australia, New Zealand and South America. This paper reviews developments in proxy-climate reconstructions from peatlands; chronicles use of a range of palaeo-proxies such as visible peat stratigraphy, plant macrofossils, peat humification, testate amoebae and non-pollen palynomorphs; and explains the use of wiggle-match radiocarbon dating and relationship to climate shifts. It details other techniques being used increasingly, such as biomarkers, stable isotopes, inorganic geochemistry and estimation of dust flux; and points to new proxies under development. Although explicit protocols have been developed recently for research on ombrotrophic mires, it must be recognised that not all proxies and techniques have universal applicability, owing to differences in species assemblages, mire formation, topographic controls, and geochemical characteristics.” Frank M. Chambers, Robert K. Booth, Francois De Vleeschouwer, Mariusz Lamentowicz, Gael Le Roux, Dmitri Mauquoy, Jonathan E. Nichols and Bas van Geel, Quaternary International, doi:10.1016/j.quaint.2011.04.039.

Drought affects climate

Drought-associated changes in climate and their relevance for ecosystem experiments and models – De Boeck & Verbeeck (2011) “Drought periods can have important impacts on plant productivity and ecosystem functioning, but climatic conditions other than the lack of precipitation during droughts have never been quantified and have therefore not been considered explicitly in both experimental and modeling studies. Here, we identify which climatic characteristics deviate from normal during droughts and how these deviations could affect plant responses. Analysis of 609 years of daily data from nine Western European meteorological stations reveals that droughts in the studied region are consistently associated with more sunshine (+45 %), increased mean (+1.6 °C) and maximum (+2.8 °C) air temperatures and vapour pressure deficits that were 51 % higher than under normal conditions. These deviations from normal increase significantly as droughts progress. Using the process-model ORCHIDEE, we simulated droughts consistent with the results of the dataset analysis and compared water and carbon exchange of three different vegetation types during such natural droughts and droughts in which only the precipitation was affected. The comparison revealed contrasting responses: carbon loss was higher under natural drought in grasslands, while increased carbon uptake was found especially in decidious forests. This difference was attributed to better access to water reserves in forest ecosystems which prevented drought stress. This demonstrates that the warmer and sunnier conditions naturally associated with droughts can either improve growth or aggravate drought-related stress, depending on water reserves. As the impacts of including or excluding climatic parameters that correlate with drought are substantial, we propose that both experimental and modeling efforts should take into account other environmental factors than merely precipitation.” De Boeck, H. J. and Verbeeck, H., Biogeosciences, 8, 1121-1130, doi:10.5194/bg-8-1121-2011, 2011. [Full text]

Hydrological cycle changes with climate in Tibetan Plateau

Response of hydrological cycle to recent climate changes in the Tibetan Plateau – Yang et al. (2011) “The Tibetan Plateau (TP) surfaces have been experiencing an overall rapid warming and wetting while wind speed and solar radiation have been declining in the last three decades. This study investigated how climate changes influenced the hydrological cycle on the TP during 1984∼2006. To facilitate the analysis, a land surface model was used to simulate surface water budget at all CMA (China Meteorological Administration) stations on the TP. The simulated results were first validated against observed ground temperature and observation-derived heat flux on the western TP and observed discharge trends on the eastern TP. The response of evaporation and runoff to the climate changes was then analyzed. Major finding are as follows. (1) Surface water balance has been changed in recent decades. Observed precipitation shows insignificant increasing trends in central TP and decreasing trends along the TP periphery while evaporation shows overall increasing trends, leading to decreased discharge at major TP water resource areas (semi-humid and humid zones in the eastern and southern TP). (2) At the annual scale, evaporation is water-limited in dry areas and energy-limited (radiation and air temperature) in wet areas; these constraints can be interpreted by the Budyko-curve. Evaporation in autumns and winters was strongly controlled by soil water storage in summers, weakening the dependence of evaporation on precipitation at seasonal scales. (3) There is a complementary effect between the simulated actual evaporation and potential evaporation, but this complementary relationship may deviate from Bouchet’s hypothesis when vapor pressure deficit (or air temperature) is too low, which suppresses the power of vapor transfer.” Kun Yang, Baisheng Ye, Degang Zhou, Bingyi Wu, Thomas Foken, Jun Qin and Zhaoye Zhou, Climatic Change, DOI: 10.1007/s10584-011-0099-4.

Larsen C ice shelf flow increasing

Acceleration and spatial rheology of Larsen C Ice Shelf, Antarctic Peninsula – Khazendar et al. (2011) “The disintegration of several Antarctic Peninsula ice shelves has focused attention on the state of the Larsen C Ice Shelf. Here, we use satellite observations to map ice shelf speed from the years 2000, 2006 and 2008 and apply inverse modeling to examine the spatial pattern of ice-shelf stiffness. Results show that the northern half of the ice shelf has been accelerating since 2000, speeding up by 15% between 2000 and 2006 alone. The distribution of ice stiffness exhibits large spatial variations that we link to tributary glacier flow and fractures. Our results reveal that ice down-flow from promontories is consistently softer, with the exception of Churchill Peninsula where we infer a stabilizing role for marine ice. We conclude that although Larsen C is not facing imminent collapse, it is undergoing significant change in the form of flow acceleration that is spatially related to thinning and fracture.” Khazendar, A., E. Rignot, and E. Larour (2011), Geophys. Res. Lett., 38, L09502, doi:10.1029/2011GL046775.

China’s ecosystems have been global warming sink

Net exchanges of CO2, CH4, and N2O between China’s terrestrial ecosystems and the atmosphere and their contributions to global climate warming – Tian et al. (2011) “China’s terrestrial ecosystems have been recognized as an atmospheric CO2 sink; however, it is uncertain whether this sink can alleviate global warming given the fluxes of CH4 and N2O. In this study, we used a process-based ecosystem model driven by multiple environmental factors to examine the net warming potential resulting from net exchanges of CO2, CH4, and N2O between China’s terrestrial ecosystems and the atmosphere during 1961–2005. In the past 45 years, China’s terrestrial ecosystems were found to sequestrate CO2 at a rate of 179.3 Tg C yr-1 with a 95% confidence range of (62.0 Tg C yr-1, 264.9 Tg C yr-1) while emitting CH4 and N2O at rates of 8.3 Tg C yr-1 with a 95% confidence range of (3.3 Tg C yr-1, 12.4 Tg C yr-1) and 0.6 Tg N yr-1 with a 95% confidence range of (0.2 Tg N yr-1, 1.1 Tg N yr-1), respectively. When translated into global warming potential, it is highly possible that China’s terrestrial ecosystems mitigated global climate warming at a rate of 96.9 Tg CO2eq yr-1 (1 Tg = 1012 g), substantially varying from a source of 766.8 Tg CO2eq yr-1 in 1997 to a sink of 705.2 Tg CO2eq yr-1 in 2002. The southeast and northeast of China slightly contributed to global climate warming; while the northwest, north, and southwest of China imposed cooling effects on the climate system. Paddy land, followed by natural wetland and dry cropland, was the largest contributor to national warming potential; forest, followed by woodland and grassland, played the most significant role in alleviating climate warming. Our simulated results indicate that CH4 and N2O emissions offset approximately 84.8% of terrestrial CO2 sink in China during 1961–2005. This study suggests that the relieving effects of China’s terrestrial ecosystems on climate warming through sequestering CO2 might be gradually offset by increasing N2O emission, in combination with CH4 emission.” Tian, H., X. Xu, C. Lu, M. Liu, W. Ren, G. Chen, J. Melillo, and J. Liu (2011), J. Geophys. Res., 116, G02011, doi:10.1029/2010JG001393.

Cereal harvest dates reveal Chech temperatures

Cereal harvest dates in the Czech Republic between 1501 and 2008 as a proxy for March–June temperature reconstruction – Možný et al. (2011) “Cereal crop harvests reflect the weather patterns of the period immediately preceding them, and thus the dates at which they begin may be used as a source of proxy data on regional climate. Using systematic phenological observations in the Czech Lands (now known as the Czech Republic) after 1845, together with exploration of further surviving documentary evidence (chronicles, diaries, financial accounts etc.), it has proved possible to create series of winter wheat harvest dates for the period 1501–2008. Employing linear regression, the harvesting dates of the main cereal species (wheat, rye, barley, oats) were first converted to winter wheat harvest days and then normalised to the same altitude above sea level. The next step consisted of using series of winter wheat harvest dates to reconstruct mean March–June temperatures in the Czech Republic, applying standard palaeoclimatological methods. Series reconstructed by linear regression explain 70% of temperature variability. A profound cold period corresponding with late winter wheat harvests was noted between 1659 and 1705. In contrast, warm periods (i.e. early winter wheat harvests) were found for the periods of 1517–1542, 1788–1834 and 1946–2008. The period after 1951 is the warmest of all throughout the entire 1501–2008 period. Comparisons with other European temperature reconstructions derived from documentary sources (including grape harvest dates), tree-rings and instrumental data reveal generally close agreement, with significant correlations. Lower correlations around A.D. 1650 and 1750 may be partly related to deterioration of socio-economic conditions in the Czech Lands resulting from prolonged wars. The results obtained demonstrate that it is possible to use widely-available cereal harvest data for climate analysis and also that such data constitute an independent proxy data series for the region of Central Europe crucial to further studies of the potential impact of climatic variability and climate change on agriculture.” Martin Možný, Rudolf Brázdil, Petr Dobrovolný and Mirek Trnka, Climatic Change, DOI: 10.1007/s10584-011-0075-z.

Atmosphere warms, surface winds weaken in tropics

Evidence for a weakening of tropical surface wind extremes in response to atmospheric warming – Gastineau & Soden (2011) “The changes of extreme winds and its links with precipitation are assessed over the past two decades using daily satellite observations and climate model simulations. Both observations and models indicate a decrease in the frequency of the strongest wind events and an increase in the frequency of light wind events in response to a warming of the tropical oceans. The heaviest precipitation events are found to be more frequent when the tropical oceans warm, but the surface winds associated with these extreme rainfall events weaken. These results add further evidence to suggest that the atmospheric circulation becomes less energetic as the climate warms. It further suggests that the enhancement of the extreme precipitation events is mainly a result of increasing atmospheric water vapor and occurs despite a weakening of the large-scale circulation, which acts to diminish the mass convergence toward the precipitating zones.” Gastineau, G., and B. J. Soden (2011), Geophys. Res. Lett., 38, L09706, doi:10.1029/2011GL047138.

Non-clathrate methane pulses during DO-events

Repeated pulses of vertical methane flux recorded in glacial sediments from the southeast Bering Sea – Cook et al. (2011) “There is controversy over the role of marine methane hydrates in atmospheric methane concentrations and climate change during the last glacial period. In this study of two sediment cores from the southeast Bering Sea (700 m and 1467 m water depth), we identify multiple episodes during the last glacial period of intense methane flux reaching the seafloor. Within the uncertainty of the radiocarbon age model, the episodes are contemporaneous in the two cores and have similar timing and duration as Dansgaard-Oeschger events. The episodes are marked by horizons of sediment containing 13C-depleted authigenic carbonate minerals; 13C-depleted archaeal and bacterial lipids, which resemble those found in ANME-1 type anaerobic methane oxidizing microbial consortia; and changes in the abundance and species distribution of benthic foraminifera. The similar timing and isotopic composition of the authigenic carbonates in the two cores is consistent with a region-wide increase in the upward flux of methane bearing fluids. This study is the first observation outside Santa Barbara Basin of pervasive, repeated methane flux in glacial sediments. However, contrary to the “Clathrate Gun Hypothesis” (Kennett et al., 2003), these coring sites are too deep for methane hydrate destabilization to be the cause, implying that a much larger part of the ocean’s sedimentary methane may participate in climate or carbon cycle feedback at millennial timescales. We speculate that pulses of methane in these opal-rich sediments could be caused by the sudden release of overpressure in pore fluids that builds up gradually with silica diagenesis. The release could be triggered by seismic shaking on the Aleutian subduction zone caused by hydrostatic pressure increase associated with sea level rise at the start of interstadials.” Cook, M. S., L. D. Keigwin, D. Birgel, and K.-U. Hinrichs (2011), Paleoceanography, 26, PA2210, doi:10.1029/2010PA001993.

Detecting solar oscillations in global temperature

Empirical Mode Decomposition Applied to Solar Irradiance, Global Temperature, Sunspot Number, and CO2 Concentration Data – Barnhart & Eichinger (2011) “Empirical Mode Decomposition (EMD) is a tool which can decompose and analyze the cyclic components from oscillatory data in the time-domain. When combined with traditional Hilbert spectral analysis, it is similar to spectral tools such as Fourier analysis, wavelet analysis, and generalized time-frequency analysis. However, the EMD method is specifically designed to analyze nonstationary data from nonlinear processes. Fluctuations of total solar irradiance, global temperature, sunspot number, and CO2 concentration are decomposed into their periodic components using the EMD method. The cyclic components of the data are analyzed and compared in the time-domain. An 11-year oscillation in global mean temperature is found and compared with the Schwabe cycle from sunspot and total solar irradiance proxy data. Also, the relative radiative forcing from different periodic components of total solar irradiance and CO2 concentration are empirically estimated.” B.L. Barnhart and W.E. Eichinger, Journal of Atmospheric and Solar-Terrestrial Physics, doi:10.1016/j.jastp.2011.04.012.


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