Feedback on Cloud Feedback
Guest article by Andrew Dessler, Real Climate, December 9, 2010
I have a paper in this week’s issue of Science on the cloud feedback that may be of interest to realclimate readers. As you may know, clouds are important regulators of the amount of energy in and out of the climate system. Clouds both reflect sunlight back to space and a trap infrared radiation and keep it from escaping the space. Changes in clouds can therefore have profound impacts on our climate.
A positive cloud feedback loop posits a scenario whereby an initial warming of the planet, caused, for example, by increases in greenhouse gases, causes clouds to trap more energy and lead to further warming. Such a process amplifies the direct heating by greenhouse gases. Models have been long predicted this, but testing the models has proved difficult.
Making the issue even more contentious, some of the more credible skeptics out there (e.g., Lindzen, Spencer) have been arguing that clouds behave quite differently from that predicted by models. In fact, they argue, clouds will stabilize the climate and prevent climate change from occurring (i.e., clouds will provide a negative feedback).
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 ± 0.74 W/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, such as water vapor, as Lindzen and Spencer would argue.
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, such as water vapor, as Lindzen and Spencer would argue.
I have also compared the results to climate models. Taken as a group, the models substantially reproduce the observations. This increases my confidence that the models are accurately simulating the variations of clouds with climate change.
Obviously, climate skeptics are quite upset with my results. Dr. Roy Spencer, for example, has been criticizing my paper on his blog. Dr. Spencer’s argument is, as he wrote in an e-mail to Dr. Richard Kerr of Science:
Andy’s study assumes that all co-variations between clouds and temperature are due to feedback, when in fact they are a mixture of feedback and “internal forcing” (natural cloud fluctuation causing temperature changes).
Now, Andy DOES at least raise this as a possibility, referencing our (Spencer & Braswell) 2010 JGR paper on the subject (his ref. #26). But he then claims that since (1) ENSO is the main source of climate variability during 2000-2010, and since (2) no one has demonstrated that ENSO is in any way caused by cloud changes, that our cause-versus-effect claim does not apply to the 2000-2010 time period.
His second claim is incorrect.
As Fig. 4a in our paper (http://www.drroyspencer.com/wp-content/uploads/Spencer-Braswell-JGR-2010.pdf ) shows, the major 2007-08 La Nina event shows the characteristic looping pattern in temperature-versus-radiative flux data that results from clouds causing temperature changes
In other words, Dr. Spencer is arguing that clouds are causing ENSO cycles, so the direction of causality in my analysis is incorrect and my conclusions are in error.
After reading this, I initiated a cordial and useful exchange of e-mails with Dr. Spencer (you can read the full e-mail exchange here). We ultimately agreed that the fundamental disagreement between us is over what causes ENSO. Short paraphrase:
Spencer: ENSO is caused by clouds. You cannot infer the response of clouds to surface temperature in such a situation.
Dessler: ENSO is not caused by clouds, but is driven by internal dynamics of the ocean-atmosphere system. Clouds may amplify the warming, and that’s the cloud feedback I’m trying to measure.
My position is the mainstream one, backed up by decades of research. This mainstream theory is quite successful at simulating almost all of the aspects of ENSO.
Dr. Spencer, on the other hand, is as far out of the mainstream when it comes to ENSO as he is when it comes to climate change. He is advancing here a completely new and untested theory of ENSO — based on just one figure in one of his papers (and, as I told him in one of our e-mails, there are other interpretations of those data that do not agree with his interpretation).
Thus, the burden of proof is 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.
It’s also worth noting that the picture I’m painting of our disagreement (and backed up by the e-mail exchange linked above) is quite different from the picture provided by Dr. Spencer on his blog. His blog is full of conspiracies and purposeful suppression of the truth. In particular, he accuses me of ignoring his work. But as you can see, I have not ignored it — I have dismissed it because I think it has no merit.
That’s quite different.
I would also like to respond to his accusation that the timing of the paper is somehow connected to the IPCC’s meeting in Cancun. I can assure everyone that no one pressured me in any aspect of the publication of this paper. As Dr. Spencer knows well, authors have no control over when a paper ultimately gets published.
And as far as my interest in influencing the policy debate goes, I’ll just say that I’m in College Station this week, while Dr. Spencer is in Cancun. In fact, Dr. Spencer had a press conference in Cancun — about my paper. I didn’t have a press conference about my paper. Draw your own conclusion.
I hope that this post has explained my work and cleared up exactly what my disagreement with Dr. Spencer is. If interested readers do some basic research on the causes of ENSO, I’m confident they will agree with me that my interpretation of the data is sound.
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