AGW Observer

Observations of anthropogenic global warming

New research from last week 18/2011

Posted by Ari Jokimäki on May 9, 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:

Evidence for strengthening of tropical hydrological cycle

Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data – Zhou et al. (2011) “Scores of modeling studies have shown that increasing greenhouse gases in the atmosphere impact the global hydrologic cycle; however, disagreements on regional scales are large, and thus the simulated trends of such impacts, even for regions as large as the tropics, remain uncertain. The present investigation attempts to examine such trends in the observations using satellite data products comprising Global Precipitation Climatology Project precipitation and International Satellite Cloud Climatology Project cloud and radiation. Specifically, evolving trends of the tropical hydrological cycle over the last 20–30 years were identified and analyzed. The results show (1) intensification of tropical precipitation in the rising regions of the Walker and Hadley circulations and weakening over the sinking regions of the associated overturning circulation; (2) poleward shift of the subtropical dry zones (up to 2° decade−1 in June-July-August (JJA) in the Northern Hemisphere and 0.3–0.7° decade−1 in June-July-August and September-October-November in the Southern Hemisphere) consistent with an overall broadening of the Hadley circulation; and (3) significant poleward migration (0.9–1.7° decade−1) of cloud boundaries of Hadley cell and plausible narrowing of the high cloudiness in the Intertropical Convergence Zone region in some seasons. These results support findings of some of the previous studies that showed strengthening of the tropical hydrological cycle and expansion of the Hadley cell that are potentially related to the recent global warming trends.” Zhou, Y. P., K.-M. Xu, Y. C. Sud, and A. K. Betts (2011), J. Geophys. Res., 116, D09101, doi:10.1029/2010JD015197. [Full text]

More research on Forbush decrease effects

Forbush decreases, solar irradiance variations, and anomalous cloud changes – Laken et al. (2011) “Changes in the galactic cosmic ray (GCR) flux due to variations in solar activity may provide an indirect connection between the Sun’s and the Earth’s climates. Epoch superpositional (composite) analyses of high-magnitude GCR fluctuations, known as Forbush decrease (FD) events, have been widely used to test this hypothesis, with varied results. This work provides new information regarding the interpretation of this approach, suggesting that FD events do not isolate the impacts of GCR variations from those of solar irradiance changes. On average, irradiance changes of ∼0.4 W m−2 outside the atmosphere occur around 2 days in advance of FD-associated GCR decreases. Using this 2 day gap to separate the effects of irradiance variations from GCR variations on cloud cover, we demonstrate small, but statistically significant, anomalous cloud changes occurring only over areas of the Antarctic plateau in association with the irradiance changes, which previous workers had attributed to GCR variations. Further analysis of the sample shows that these cloud anomalies occurred primarily during polar darkness, precluding the possibility of a causal link to a direct total solar irradiance effect. This work suggests that previous FD-based studies may have ineffectively isolated the impacts of GCR variations on the Earth’s atmosphere.” Laken, B., D. Kniveton, and A. Wolfendale (2011), J. Geophys. Res., 116, D09201, doi:10.1029/2010JD014900.

First signature of rebound in Antarctic ozone

Rebound of Antarctic ozone – Salby et al. (2011) “Restrictions on CFCs have led to a gradual decline of Equivalent Effective Stratospheric Chlorine (EESC). A rebound of Antarctic ozone, however, has remained elusive, masked by large interannual changes that dominate its current evolution. A positive response of ozone is not expected to emerge for at least 1–2 decades, possibly not for half a century. We show that interannual changes of the Antarctic ozone hole are accounted for almost perfectly by changes in dynamical forcing of the stratosphere. The close relationship enables dynamically-induced changes of ozone to be removed, unmasking the climate signal associated with CFCs. The component independent of dynamically-induced changes exhibits a clear upward trend over the last decade – the first signature of a rebound in Antarctic ozone. It enables ozone to be tracked relative to CFCs and other changes of climate.” Salby, M., E. Titova, and L. Deschamps (2011), Geophys. Res. Lett., 38, L09702, doi:10.1029/2011GL047266.

Yet another confirmation of increased Greenland ice sheet melt

Melting trends over the Greenland ice sheet (1958–2009) from spaceborne microwave data and regional climate models – Fettweis et al. (2011) “To study near-surface melt changes over the Greenland ice sheet (GrIS) since 1979, melt extent estimates from two regional climate models were compared with those obtained from spaceborne microwave brightness temperatures using two different remote sensing algorithms. The results from the two models were consistent with those obtained with the remote sensing algorithms at both daily and yearly time scales, encouraging the use of the models for analyzing melting trends before the satellite era (1958–1979), when forcing data is available. Differences between satellite-derived and model-simulated results still occur and are used here to identify (i) biases in the snow models (notably in the albedo parametrization, in the thickness of a snow layer, in the maximum liquid water content within the snowpack and in the snowfall impacting the bare ice appearance in summer) and (ii) limitations in the use of passive microwave data for snowmelt detection at the edge of the ice sheet due to mixed pixel effect (e.g., tundra or rock nearby the ice sheet). The results from models and spaceborne microwave sensors confirm a significant (p-value = 0.01) increase in GrIS surface melting since 1979. The melt extent recorded over the last years (1998, 2003, 2005 and 2007) is unprecedented in the last 50 yr with the cumulated melt area in the 2000′s being, on the average, twice that of the 1980′s.” Fettweis, X., Tedesco, M., van den Broeke, M., and Ettema, J., The Cryosphere, 5, 359-375, doi:10.5194/tc-5-359-2011, 2011.

Storm track cloudiness shifts poleward -> positive cloud feedback

Changes in extratropical storm track cloudiness 1983–2008: observational support for a poleward shift – Bender et al. (2011) “Climate model simulations suggest that the extratropical storm tracks will shift poleward as a consequence of global warming. In this study the northern and southern hemisphere storm tracks over the Pacific and Atlantic ocean basins are studied using observational data, primarily from the International Satellite Cloud Climatology Project, ISCCP. Potential shifts in the storm tracks are examined using the observed cloud structures as proxies for cyclone activity. Different data analysis methods are employed, with the objective to address difficulties and uncertainties in using ISCCP data for regional trend analysis. In particular, three data filtering techniques are explored; excluding specific problematic regions from the analysis, regressing out a spurious viewing geometry effect, and excluding specific cloud types from the analysis. These adjustments all, to varying degree, moderate the cloud trends in the original data but leave the qualitative aspects of those trends largely unaffected. Therefore, our analysis suggests that ISCCP data can be used to interpret regional trends in cloudiness, provided that data and instrumental artefacts are recognized and accounted for. The variation in magnitude between trends emerging from application of different data correction methods, allows us to estimate possible ranges for the observational changes. It is found that the storm tracks, here represented by the extent of the midlatitude-centered band of maximum cloud cover over the studied ocean basins, experience a poleward shift as well as a narrowing over the 25 year period covered by ISCCP. The observed magnitudes of these effects are larger than in current generation climate models (CMIP3). The magnitude of the shift is particularly large in the northern hemisphere Atlantic. This is also the one of the four regions in which imperfect data primarily prevents us from drawing firm conclusions. The shifted path and reduced extent of the storm track cloudiness is accompanied by a regional reduction in total cloud cover. This decrease in cloudiness can primarily be ascribed to low level clouds, whereas the upper level cloud fraction actually increases, according to ISCCP. Independent satellite observations of radiative fluxes at the top of the atmosphere are consistent with the changes in total cloud cover. The shift in cloudiness is also supported by a shift in central position of the mid-troposphere meridional temperature gradient. We do not find support for aerosols playing a significant role in the satellite observed changes in cloudiness. The observed changes in storm track cloudiness can be related to local cloud-induced changes in radiative forcing, using ERBE and CERES radiative fluxes. The shortwave and the longwave components are found to act together, leading to a positive (warming) net radiative effect in response to the cloud changes in the storm track regions, indicative of positive cloud feedback. Among the CMIP3 models that simulate poleward shifts in all four storm track areas, all but one show decreasing cloud amount on a global mean scale in response to increased CO2 forcing, further consistent with positive cloud feedback. Models with low equilibrium climate sensitivity to a lesser extent than higher-sensitivity models simulate a poleward shift of the storm tracks.” Frida A-M. Bender, V. Ramanathan and George Tselioudis, Climate Dynamics, DOI: 10.1007/s00382-011-1065-6.

Role of celebrities in climate change communication

Sources, media, and modes of climate change communication: the role of celebrities – Anderson (2011) “This article reviews existing research on the portrayal of climate change within the print media, paying particular attention to the increasing role that celebrities have come to play within popular culture. While this is certainly not a new development, celebrities are increasingly appearing as key voices within the climate change debate, providing a powerful news hook and potential mobilizing agent. Early coverage of climate change was dominated by scientific sources, but as the debate became more institutionalized and politicized a wider variety of competing sources entered the news arena. Yet media prominence is not necessarily a reliable indicator of influence. How issues are framed is of crucial importance and celebrity interventions can be a double-edged sword.” Alison Anderson, Wiley Interdisciplinary Reviews: Climate Change, DOI: 10.1002/wcc.119.

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One Response to “New research from last week 18/2011”

  1. Kevin C said

    Here’s an interesting one on aerosols, from statisticians rather than climatologists:

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