AGW Observer

Observations of anthropogenic global warming

New research from last week 48/2011

Posted by Ari Jokimäki on December 5, 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.

Tropical high clouds react to surface warming as expected from theories

The observed sensitivity of high clouds to mean surface temperature anomalies in the Tropics – Zelinka & Hartmann (2011) “Cloud feedback represents the source of largest diversity in projections of future warming. Observational constraints on both the sign and magnitude of the feedback are limited, since it is unclear how the natural variability that can be observed is related to secular climate change, and analyses have rarely been focused on testable physical theories for how clouds should respond to climate change. In this study we use observations from a suite of satellite instruments to assess the sensitivity of tropical high clouds to interannual tropical mean surface temperature anomalies. We relate cloud changes to a physical governing mechanism that is sensitive to the vertical structure of warming. Specifically, we demonstrate that the mean and interannual variability in both the altitude and fractional coverage of tropical high clouds as measured by CloudSat, MODIS, AIRS, and ISCCP are well-diagnosed by upper tropospheric convergence computed from the mass and energy budget of the clear-sky atmosphere. Observed high clouds rise approximately isothermally in accordance with theory and exhibit an overall reduction in coverage when the Tropics warms, similar to their behavior in global warming simulations. Such cloud changes cause absorbed solar radiation to increase more than does outgoing longwave radiation, resulting in a positive but statistically insignificant net high cloud feedback in response to ENSO. The results suggest that the convergence metric based on simple mass and energy budget constraints may be a powerful tool for understanding observed and modeled high cloud behavior and for evaluating the realism of modeled high cloud changes in response to a variety of forcings.” Zelinka, M. D. and D. L. Hartmann (2011), J. Geophys. Res., doi:10.1029/2011JD016459, in press.

Oxygen content of global ocean is decreasing

Observed decreases in oxygen content of the global ocean – Helm et al. (2011) “Comparing the high-quality oxygen climatology from the World Ocean Circulation Experiment to earlier data we reveal near-global decreases in oxygen levels in the upper ocean between the 1970s and the 1990s. This globally averaged oxygen decrease is -0.93{plus minus}0.23 μmol l-1, which is equivalent to annual oxygen losses of -0.55{plus minus}0.13×1014 mol yr-1 (100-1000 m). The strongest decreases in oxygen occur in the mid-latitudes of both hemispheres, near regions where there is strong water renewal and exchange between the ocean interior and surface waters. Approximately 15% of global oxygen decrease can be explained by a warmer mixed-layer reducing the capacity of water to store oxygen, while the remainder is consistent with an overall decrease in the exchange between surface waters and the ocean interior. Here we suggest that this reduction in water mass renewal rates on a global scale is a consequence of increased stratification caused by warmer surface waters. These observations support climate model simulations of oxygen change under global warming scenarios.” Helm, K. P., N. L. Bindoff, and J. A. Church (2011), Geophys. Res. Lett., doi:10.1029/2011GL049513, in press.

Melting ice reveals artefacts of ancient reindeer hunting in southern Norway

The climatic significance of artefacts related to prehistoric reindeer hunting exposed at melting ice patches in southern Norway – Nesje et al. (2011) “The main aim of this study is to describe consequences of climate change in the mountain region of southern Norway with respect to recently exposed finds of archaeological remains associated with reindeer hunting and trapping at and around ice patches in central southern Norway. In the early years of the twenty-first century, warm summers caused negative glacier mass balance and significant glacier retreat and melting of ice patches in central southern Norway. As a result, prehistoric remains lost and/or left by past reindeer hunters appeared at ice patches in mountain areas of southern Norway. In the warm summer and autumn of 2006 the number of artefact recoveries at ice patches increased significantly because of melting of snow and ice patches and more than 100 objects were recovered in the Oppland county alone. In 2009, detailed multidisciplinary investigations were carried out at the Juvfonne ice patch in Jotunheimen at an elevation of c. 1850 metres. A well-preserved Iron Age hunting station was discovered and in total c. 600 artefacts have been documented at the Juvfonne site alone. Most of the objects were recovered and brought to the Museum of Cultural History at the University of Oslo for conservation, exhibition and storing. Thirteen so called ‘scaring sticks’ recovered from the recently exposed foreland of Juvfonne were radiocarbon dated, yielding ages that group in two separate time intervals, AD 246–534 and AD 804–898 (±1 sigma). By putting the temporal distribution of the radiocarbon-dated artefacts into the context of late-Holocene glacier-size variations in the Jotunheimen and Jostedalsbreen regions, we conclude that the most extensive reindeer hunting and trapping associated with snow/ice patches was related to periods with prevailing warm summers when the reindeer herds gathered on high-altitude, contracted glaciers and ice patches to avoid insect plagues. The ‘freshness’ of the fragile organic finds strongly indicates that at least some of the artefacts were rapidly covered by snow and ice and that they may have been more-or-less continuously covered by snow and ice since they were first buried.” Atle Nesje, Lars Holger Pilø, Espen Finstad, Brit Solli, Vivian Wangen, Rune Strand Ødegård, Ketil Isaksen, Eivind N. Støren, Dag Inge Bakke, Liss M Andreassen, The Holocene November 30, 2011 0959683611425552, doi: 10.1177/0959683611425552.

Climate record shows several abrupt cooling events during early Holocene in North America

Abrupt cooling repeatedly punctuated early-Holocene climate in eastern North America – Hou et al. (2011) “Climate proxy records and general circulation models suggest that Atlantic Meridional Overturning Circulation (AMOC) plays a key role for global climate changes. Paleoceanographic data document multiple episodes of prominent AMOC weakening during the early Holocene. However, proxy records at adjacent continents have not been demonstrated to fully capture the climate responses to multiple AMOC variation due to temporal resolution and/or the proxy sensitivity. Here we present decadal- to multidecadal-resolution hydrogen isotopic records of aquatic biomarkers from Blood Pond, Massachusetts during the early Holocene. Our data reveal a full series of prominent and abrupt cooling events centered on 10.6, 10.2, 9.5, 9.2, 8.8 and 8.4 ka. These abrupt climatic reversals coincide with key intervals of weakened AMOC, suggesting an apparent relationship between AMOC oscillations and the abrupt continental climate changes in northeastern North America. The noticeable connection implies that the AMOC variation did play an important role in the abrupt climate changes during the early Holocene. Our data also suggest that northeastern North America may experience significant climatic variations should the predicted major disturbance of AMOC occur in the coming century as a result of anthropogenic greenhouse gas emissions.” Juzhi Hou, Yongsong Huang, Bryan N Shuman, W Wyatt Oswald, David R Foster, The Holocene November 30, 2011 0959683611427329, doi: 10.1177/0959683611427329.

Why AMOC had already slowed down when Heinrich events started?

Heinrich event 1: an example of dynamical ice-sheet reaction to oceanic changes – Álvarez-Solas et al. (2011) “Heinrich events, identified as enhanced ice-rafted detritus (IRD) in North Atlantic deep sea sediments (Heinrich, 1988; Hemming, 2004) have classically been attributed to Laurentide ice-sheet (LIS) instabilities (MacAyeal, 1993; Calov et al., 2002; Hulbe et al., 2004) and assumed to lead to important disruptions of the Atlantic meridional overturning circulation (AMOC) and North Atlantic deep water (NADW) formation. However, recent paleoclimate data have revealed that most of these events probably occurred after the AMOC had already slowed down or/and NADW largely collapsed, within about a thousand years (Hall et al., 2006; Hemming, 2004; Jonkers et al., 2010; Roche et al., 2004), implying that the initial AMOC reduction could not have been caused by the Heinrich events themselves. Here we propose an alternative driving mechanism, specifically for Heinrich event 1 (H1; 18 to 15 ka BP), by which North Atlantic ocean circulation changes are found to have strong impacts on LIS dynamics. By combining simulations with a coupled climate model and a three-dimensional ice sheet model, our study illustrates how reduced NADW and AMOC weakening lead to a subsurface warming in the Nordic and Labrador Seas resulting in rapid melting of the Hudson Strait and Labrador ice shelves. Lack of buttressing by the ice shelves implies a substantial ice-stream acceleration, enhanced ice-discharge and sea level rise, with peak values 500–1500 yr after the initial AMOC reduction. Our scenario modifies the previous paradigm of H1 by solving the paradox of its occurrence during a cold surface period, and highlights the importance of taking into account the effects of oceanic circulation on ice-sheets dynamics in order to elucidate the triggering mechanism of Heinrich events.” Álvarez-Solas, J., Montoya, M., Ritz, C., Ramstein, G., Charbit, S., Dumas, C., Nisancioglu, K., Dokken, T., and Ganopolski, A., Clim. Past, 7, 1297-1306, doi:10.5194/cp-7-1297-2011, 2011. [Full text]

Water-related stresses from climate change threaten endangered plant species

Climate change threatens endangered plant species by stronger and interacting water-related stresses – Bartholomeus et al. (2011) “Atmospheric CO2-concentration, temperature and rainfall variability are all expected to increase in the near future. The resulting increased dynamics of soil moisture contents, together with increased plant physiological demands for both oxygen and water, will lead to an increased occurrence of wet and dry extremes of plant stresses, i.e. of oxygen and drought stress, respectively, alone and in interaction. The use of indirect environmental variables of previous studies and their focus on one stress at a time has hampered understanding the causal impact of climate change on plant species composition through changes in abiotic site conditions. Here, we use process-based simulations of oxygen and drought stress and show that both stresses will increase (on average with ca. 20% at sites where both stresses occur) in a warmer and more variable future (2050) climate (applying a national downscaled version of IPCC scenarios). These stresses will increasingly coincide, i.e. both stresses will occur more often (but not at the same time) within the same vegetation plot. We further show that particularly this increased coincidence of water-related stresses will negatively affect the future occurrence of currently endangered plant species (a reduction of 16%), while such a decrease is not apparent for common species. Individual stresses did not affect the occurrence of endangered plant species. Consequently, the species that are already threatened under the current climate, will suffer most from climate change.” Bartholomeus, R. P., F. Witte, P. M. van Bodegom, J.C. Van Dam, and R. Aerts (2011), J. Geophys. Res., doi:10.1029/2011JG001693, in press.

Drought-induced tree mortality has increased in Canada’s boreal forests

A drought-induced pervasive increase in tree mortality across Canada’s boreal forests – Peng et al. (2011) “Drought-induced tree mortality is expected to increase worldwide under projected future climate changes. The Canadian boreal forests, which occupy about 30% of the boreal forests worldwide and 77% of Canada’s total forested land, play a critical role in the albedo of Earth’s surface and in its global carbon budget. Many of the previously reported regional-scale impacts of drought on tree mortality have affected low- and middle-latitude tropical regions and the temperate forests of the western United States, but no study has examined high-latitude boreal regions with multiple species at a regional scale using long-term forest permanent sampling plots. Here, we estimated tree mortality in natural stands throughout Canada’s boreal forests using data from the permanent sampling plots and statistical models. We found that tree mortality rates increased by an overall average of 4.7% yr−1 from 1963 to 2008, with higher mortality rate increases in western regions than in eastern regions (about 4.9 and 1.9% yr−1, respectively). The water stress created by regional drought may be the dominant contributor to these widespread increases in tree mortality rates across tree species, sizes, elevations, longitudes and latitudes. Western Canada seems to have been more sensitive to drought than eastern Canada.” Changhui Peng, Zhihai Ma, Xiangdong Lei, Qiuan Zhu, Huai Chen, Weifeng Wang, Shirong Liu, Weizhong Li, Xiuqin Fang & Xiaolu Zhou, Nature Climate Change 1,467–471(2011)doi:10.1038/nclimate1293.

Snowfall to total winter precipitation ratio has decreased in Poland

Long-term variability of occurrence of precipitation forms in winter in Kraków, Poland – Twardosz et al. (2011) “The paper discusses long-term change in snowfall, rainfall and mixed precipitation viewed in conjunction with air temperature and North Atlantic Oscillation (NAO) in winter (December–February). In the study of contemporary climate change and its effect on the hydrological cycle it is useful to focus on winter precipitation forms. A 146-year secular observation series from Kraków, spanning the period 1863–2008, was used to extract data on the number of days with precipitation and on precipitation amount broken down by form. Statistically significant trends were found in total and mixed precipitation, but not in snowfall and rainfall. The climate warming effect has contributed to a material decrease in the snowfall to total winter precipitation ratio during the second half of the 20th c. The highest impact of air temperature was found in the wintertime variation in number of days with snowfall while the NAO had a significant influence on the frequency and amount of both rainfall and snowfall.” Robert Twardosz, Ewa Łupikasza, Tadeusz Niedźwiedź and Adam Walanus, Climatic Change, DOI: 10.1007/s10584-011-0352-x. [Full text]

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