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A series of emails between Andy Dessler of Texas A&M University and Roy Spencer of the University of Alabama in Huntsville were published on Spencer's blog on 31 December 2010.
The subject of cloud feedback is one of the most hotly contested issues in climate science. The emails follow on from some sharp exchanges between the two on rival web postings in the wake of the publication of a paper by Dessler in Science in December which came to very different conclusion to an earlier paper published in the Journal of Geophysical Research in August and co-authored by Spencer. Both papers rely on essentially the same CERES satellite data to come to opposite conclusions.
Science paper author Andy Dessler, of Texas A&M University, told Reporting Climate Science .Com at the time that his result "substantially reduces" scope for scepticism on climate change while Spencer questioned the timing of the publication of Dessler's paper which coincided with the end of the Cancun climate change conference - Spencer has subsequently apologised for this comment.
Superficially, the debate is about whether clouds amplify global warming (through so called positive feedback) or whether they act to mitigate it in certain circumstances by contributing to a cooling effect as temperatures rise (negative feedback). But in reality it is more subtle as Dessler believes increases in surface temperature cuase clouds to trap more energy which in turn leads to higher temperatures while Spencer believes that it is the clouds that actually cause a "substantial portion" of surface temperature changes. In other words, the two scientists have a completely different view of cause and effect linking clouds and surface temperature.
In the email exchange Dessler describes his research paper's findings as follows: “I showed that the energy trapped by clouds increases as the surface temperature increases, and concluded that there is a positive cloud feedback acting.” Dessler describes how Spencer objected to this, saying that clouds are actually causing the surface temperature change, which implied that Dessler must have put cause and effect back to front. Dessler goes on to state that the evidence around the El Nino Southern Oscillation (ENSO) does not agree with Spencer's view: “My response to this is that the temperature variations over the last 10 years are primarily driven by ENSO, and we know that ENSO is not caused by clouds. This is the crux of our disagreement."
Spencer states that the evidence that clouds cause a substantial portion of the temperature changes during the ten-year period of the CERES data used in his paper is twofold. Firstly, that “the temperature changes tend to lag the radiative flux changes, something that is revealed by “connecting the dots” in the scatterplots of radiative flux-vs-temperature”. And secondly that “this lagged behavior strongly decorrelates the temperature-versus-radiative flux variations (as is seen in Andy’s, and virtually all previously published, scatter plots of this type)”. He points out that this poorly-correlated behaviour is consistent with the short-term behaviour of "most if not all" of the climate models used by the Intergovernmental Panel on Climate Change (IPCC) in the fourth assessment report, and was mimicked by a simple forcing-feedback model described in Spencer's paper.
Spencer believes that Dessler's feedback does not lag and is “closer to simultaneous”, which would lead to strongly correlated data. Furthermore, he believes that the decorrelated data connected with the “forcing” process in which clouds cause the surface temperature increase may be mistaken for evidence of positive feedback. “All I know is that the “forcing” so strongly decorrelates that data that doing linear regression to get a feedback estimate is going to result in a regression slope approaching zero, which is then commonly misinterpreted as strongly positive feedback,” he states.
Spencer describes the satellite data as suggesting that the climate system meanders “through varying states of radiative imbalance, with the temperature changes always trying to play catch-up with the radiative flux changes, …but then the atmospheric circulation causes another change in cloudiness, and the temperature then has to slowly respond to that, too”. In other words, radiative equilibrium “is never actually reached”.
Towards the end of the email exchange, Dessler challenges Spencer to quantify the percentage of surface temperature change caused by clouds; Spencer's response was that “just what percentage of all of the variability is due to “forcing” versus “feedback” is still an open question”.
See our previous story Gobal warming cloud feedback debate heats up
Roy Spencer's blog site here.
Andy Dessler's posting on Real Climate here.
"A Determination of the Cloud Feedback from Climate Variations over the Past Decade" by A. E. Dessle published in SCIENCE VOL 330 10 DECEMBER 2010
Click here for Science.
"On the diagnosis of radiative feedback in the presence of unknown radiative forcing" by Roy W. Spencer and William D. Braswell published in JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, D16109, doi:10.1029/2009JD013371, 201
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