Clouds have bedeviled scientists' efforts to figure out how much warming might result from adding carbon dioxide to the atmosphere. Science is publishing today a new study by Texas A&M atmospheric scientist Andrew Dessler that finds that clouds contribute to future warming. As Science's press summary describes Dessler's results:
On a global scale, clouds presently influence climate in a way that cools the planet. But, they will lose some of that cooling capacity as climate warms, according to a study that supports current ideas about how atmospheric carbon dioxide affects global temperature. Clouds can potentially have both positive and negative feedback effects on climate, and this is responsible for much of our uncertainty about the amount of warming that will be caused by increasing concentrations of carbon dioxide in the atmosphere. It's generally agreed that overall this feedback is positive, with warming being exacerbated as clouds trap larger quantities of outgoing infrared radiation, but so far we have only a general idea of this effect. Andrew Dessler has estimated the actual magnitude of the feedback effect by analyzing ten years of satellite data on the flux of radiation through the top of the atmosphere. He concludes that the feedback effect is indeed positive and of a value that agrees with the canonical range of estimates of how much warming will occur for a doubling of atmospheric carbon dioxide.
This study contradicts the findings of University of Alabama in Huntsville climate researchers Roy Spencer and William Braswell published earlier this year in the Journal of Geophysical Research. Intirguingly Spencer and Brasswell used the same data as Dessler: Spencer stands by their results and responds:
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,…
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. But how can this be? How can two climate researchers using the same dataset come to opposite onclusions?
The answer lies in an issue that challenges researchers in most scientific disciplines – separating cause from effect.
Dessler's claim (and the IPCC party line) 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? Because if warming causes fewer clouds, that would let in more sunlight which then amplifies the warming. That is positive cloud feedback in a nutshell.
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?
What we demonstrated in our JGR paper earlier this year is that when cloud changes cause
temperature changes, it gives the illusion of positive cloud feedback – even if strongly negative cloud feedback is really operating!
I cannot overemphasize the importance of that last statement.
We used essentially the same satellite dataset Dessler uses, but we analyzed those data with
something called 'phase space analysis.' Phase space analysis allows us to "see" behaviors in the climate system that would not be apparent with traditional methods of data analysis. It is like using an MRI to spot a type of tumor that X-rays cannot see.
What we showed was basically a new diagnostic capability that can, to some extent, separate
cause from effect. This is a fundamental advance.
he Dessler paper is like someone publishing medical research that claims the tumors do not
exist because they still do not show up on our latest X-ray equipment … even though the new
MRI technology shows that they do exist!
We even replicated the behavior seen in the satellite data that was analyzed with phase space
analysis — our MRI for the climate system – by using a simple forcing-feedback climate model containing a negative feedback component. We demonstrated that the satellite data Dessler analyzed are actually showing negative cloud feedback, not positive feedback.
Let's hope that further research will some day soon resolve this crticial modeling issue.