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Global warming cloud feedback debate heats up

09.12.2010 19:16 Age: 5 yrs
Category: Climate change
By: Leon Clifford

Cloud feedback erupted as the new front line in the debate over climate change as a paper published in US journal Science was immediately challenged in Cancun by a leading sceptical climate scientist.

Texas A&M University climate scientist Andy Dessler's Science paper criticises the idea that cloud feedback will prevent global warming. It follows on from a paper which suggested that clouds may have an overall cooling effect that was published in August and co-authored by climate scientist and acknowledged global warming sceptic Roy Spencer of the University of Alabama.

Cyberspace lit up as the embargo on the Science paper was lifted on Thursday 9 December at 2pm Eastern Standard Time with rival commentaries simultaneously posted by Dessler, on the Real Climate website, and by Roy Spencer, on his own blogsite, as he issued a statement on the subject at the Cancun climate change conference.

Cloud feedback is one of the key areas of uncertainty identified by climate scientists working with the Intergovernmental Panel on Climate Change (IPCC) and is a key issue cited by climate change sceptics, so this debate is significant. Clouds regulate the amount of energy that goes into and out of the Earth's climate system.  Clouds reflect sunlight back to space and so have a cooling effect.  They also trap infrared radiation emitted from the surface and keep it from escaping into the space and in this way they have a warming effect. Changes in clouds can affect the climate and the question over cloud feedback is whether or not the affect of clouds is to amplify or counter global warming - whether they exhibit positive warming or negative cooling feedback. 

Three takeaways

“There are three takeaways from my paper,” Andy Dessler told Reporting Climate Science .Com. “First the observational evidence of global cloud feedbacks is likely positive. Second the magnitude of the feedback measured in the observations is a good match with the computer models. And third some climate sceptics have put cloud feedbacks as a magical process that will save us from climate change and the evidence here is that it won't save us.”

Dessler's paper is called "A Determination of the Cloud Feedback from Climate Variations over the Past Decade".  The conclusion to the paper includes the following: "For the problem of long-term climate change, what we really want to determine is the cloud feedback in response to long-term climate change. Unfortunately, it may be decades before a direct measurement is possible. In the meantime, observing shorter-term climate variations and comparing those observations to climate models may be the best we can do. This is what I have done in this paper. My analysis suggests that the short-term cloud feedback is likely positive and that climate models as a group are doing a reasonable job of simulating this feedback, providing some indication that models successfully simulate the response of clouds to climate variations. However, owing to the apparent time-scale dependence of the cloud feedback and the uncertainty in the observed short-term cloud feedback, we cannot use this analysis to reduce the present range of equilibrium climate sensitivity of 2.0 to 4.5 K."

Spencer completely disagrees with the findings of Dessler's research. "We demonstrated that the satellite data Dessler analyzed are actually showing negative cloud feedback, not positive feedback," he wrote in a statement released on Thursday 9 December from the Cancun climate change conference in Mexico.

“What is the new evidence of positive cloud feedback that Dessler has published? Well, actually it isn't new. It's basically the same evidence we published in the "Journal of Geophysical Research" earlier this year,” Spencer said in his statement. “Yet we came to a very different conclusion, which was that the only clear evidence of feedback we found in the data was of strongly negative cloud feedback.”

ENSO and clouds

Dessler explained that he had looked at the interaction between clouds and the El Nino Southern Oscillation (ENSO) Pacific Ocean warming and cooling cycle. “In this case the ENSO cycle is a proxy for climate change.  We know that clouds do not cause the ENSO so we looked for any changes in the clouds as a response to the ENSO. And we found that the response was positive feedback.”

Spencer believes that there is a cause and effect issue at work and that clouds actually have a cooling effect that interacts with the ENSO. “Dessler's paper claims to show that cloud feedback is both positive and generally supportive of the cloud feedback assumptions exhibited by the IPCC's computerized climate models. This would, in turn, support the IPCC¹s claim that anthropogenic global warming will become an increasingly serious problem in the future. Unfortunately, the central evidence contained in the paper is weak at best, and seriously misleading at worst. It uses flawed logic to ignore recent advancements we have made in identifying cloud feedback,” Spencer wrote in his statement.

Dessler believes that his work reduces the scope for doubt about clouds. “When you look at what we know about climate change you can see that we know a lot but that clouds were one of the areas of uncertainty. My paper has not settled the issue completely but it has substantially reduced the opportunity for climate sceptics to point at this issue. At some point people are just going to have to accept that if you put more carbon dioxide into the atmosphere then we will increase the temperature,” he said.

Party line

"Dessler's claim (and the IPCC party line)," according to Spencer in his written statement, "is that cloud changes are caused by temperature changes and not the other way around. Causation only occurs in one direction, not the other. In their interpretation, if one observes a warmer year being accompanied by fewer clouds, then that is evidence of positive cloud feedback. Why? Becauseif warming causes fewer clouds, that would let in more sunlight which then amplifies the warming. That is positive cloud feedback in a nutshell."

Spencer continues: "But what if the warming was caused by fewer clouds, rather than the fewer clouds being caused by warming? In other words, what if previous researchers have simply mixed up cause and effect when estimating cloud feedbacks?"

In his posting on the Real Climate website, Dessler is keen to point out what he regards are the key issues.  "In my new paper, I calculate the energy trapped by clouds and observe how it varies as the climate warms and cools during El Nino-Southern Oscillation (ENSO) cycles. I find that, as the climate warms, clouds trap an additional 0.54±0.74W/m2 for every degree of warming. Thus, the cloud feedback is likely positive, but I cannot rule out a slight negative feedback. It is important to note that while a slight negative feedback cannot be ruled out, the data do not support a negative feedback large enough to substantially cancel the well-established positive feedbacks," he wrote.

"Not correct"

Spencer takes issue with what he claims is Dessler's incorrect characterisation of his position that clouds cause El Nino stating "that is not what we claimed, nor is it a necessary condition for our interpretation to be correct."

El Nino and La Nina represent a temporary change in the way the coupled atmospheric-ocean circulation system operates, according to Spencer.  "Any change in the atmospheric circulation can cause a change in cloud cover, which can in turn cause a change in ocean temperatures. We even showed this behavior for the major La Nina cooling event of 2007-08 in our paper," he states.  Adding that it "doesn¹t mean that ³clouds cause El Nino, as Dessler suggests we are claiming, which would be too simplistic and misleading of a statement."

In his Real Climate posting Dessler states that Spencer is "far out of the mainstream" in his ideas about ENSO and adds that "the burden of proof is (for) Dr. Spencer to show that his theory of causality during ENSO is correct. He is, at present, far from meeting that burden. And until Dr. Spencer satisfies this burden, I don’t think anyone can take his criticisms seriously."

Spencer's statement concludes: "Clouds are complicated beasts, and climate researchers ignore that complexity at their peril".

“There has been a cordial disagreement,” said a spokesman for Spencer.


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

Click here.



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