Climate scientist Andrew Dessler recently got a chance to apply his interest in communicating science to a personal subject – his own research aimed at reducing uncertainty about clouds’ role in an increasingly human-warmed climate.
The NASA-funded study by the Texas A&M University faculty member was published in December in Science Magazine, a prestigious journal of the American Association for the Advancement of Science.
A separate article about the study in the same magazine’s “News of the Week” section reported that the Dessler’s findings had lent “more confidence” to the expectation that clouds would amplify climate warming caused by human emissions of greenhouse gases.
Most climate scientists – “the overwhelming majority,” Science reported – agree with computer models’ projection that clouds will boost manmade warming. But some researchers argue it is likely clouds will offset that average planetary temperature rise.
Dessler had opportunities to talk about his research in a couple of online formats – a podcast interview that Science posted on its website and a short video that A&M posted. In a press release with quotes from the researcher, the university called his Science paper “a breakthrough study.”
Dessler told Texas Climate News that he believes the study and another recent paper on clouds by researchers at the University of Hawaii at Manoa will have the combined effect of reducing scientific uncertainty about clouds’ role in a human-shaped climate, which he said is “one of the last bastions of uncertainty in climate science.”
He added, “While the problem is not yet solved, I see opportunities for legitimate climate skepticism rapidly vanishing.”
The continuing uncertainty about clouds – fueling much of the uncertainty about how sensitive the climate is to human influence – is related to clouds’ competing effects on temperature. They cool the earth’s climate by reflecting the sun’s radiation. They also warm it by trapping radiation.
There is a net cooling effect now – or as scientists often express it, the “cloud feedback” is negative. Dessler’s study addressed the question of whether it will continue to be negative in a climate that is warming because of greenhouse-gas emissions, or will be positive, adding to that human-propelled warming trend.
The research involved analysis of data collected by an instrument on NASA’s Terra satellite, which measures solar energy entering and leaving the planet’s climate system. The study period, 2000-10, included warming effects of an El Niño weather pattern and cooling effects of a La Niña pattern.
He wrote in the Science paper that his calculations indicated it was “likely” that the cloud feedback was positive during those years. “A small negative feedback is possible,” he added, “but one large enough to cancel the climate’s positive feedbacks is not supported by these observations.”
In the paper’s concluding passage, he wrote:
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.
In the Science podcast, aimed more at a general audience, he explained those findings this way:
Well, the first conclusion is that I’ve evaluated the cloud feedback and measured it using observations, and what I’ve shown is that it is likely positive, meaning that clouds are going to amplify the warming we get from carbon dioxide. And the second important conclusion is that if you then compare what I get from the observations of climate models, the climate models are actually doing quite a good job, taken as a group, and that gives me confidence that the warming of several degrees Celsius predicted by climate models over the next century is probably right, I don’t see any reason to doubt that.
We’ve known what’s in the models for a long time, but we really haven’t had observations with which to test the models, and that’s really, I think, the advance that my paper is. This is really the first time that we’ve been able to test the models in a really global average sense that includes all the clouds over the entire globe and really try to see if the models are doing a good job. And it looks like they are.
Dessler also wrote a blog post about his study for RealClimate, a website that features posts by climate scientists and provides a forum for often-technical discussions about climate-change research.
In his post, Dessler discussed criticism of the study by Roy Spencer, a research scientist at the University of Alabama in Huntsville, who studies clouds and is skeptical about manmade climate change, and provided a link to an email exchange between the two. In the comments section below his RealClimate post, Dessler participated in the dialogue with responses to some of the points and questions by readers.
News coverage of Dessler’s Science paper ranged from an article in A&M’s hometown daily newspaper, The Bryan-College Station Eagle, and the Houston Chronicle to articles by Time, USA Today and ClimateWire (posted on the New York Times website).
– Bill Dawson