Clouds will respond to climate change in ways that further heat the planet, a new study suggests.
The research, published yesterday in the journal Science, appears to solve one of of the biggest remaining mysteries in climate science: How well do computer climate models predict the behavior of clouds?
That's important because clouds can work to cool or heat the Earth, depending on the type of cloud and where it sits in the atmosphere. Clouds cool the planet by reflecting incoming radiation from the sun. They heat it by trapping outgoing radiation from the planet's surface. The question scientists have been struggling to answer is which of these two effects will dominate as climate change intensifies.
"Clouds are really, I would say, the biggest uncertainty in understanding how much warming we're going to get in the future," said study author Andrew Dessler, an atmospheric scientist at Texas A&M University. "And up until my paper, all we really had were the models. We had no idea if the models were completely wrong."
Computerized climate models vary widely in their predictions of how clouds will respond to long-term climate change. A few models predict clouds will be neutral players, neither compounding warming nor counteracting it, while others predict clouds will exacerbate warming.
Some climate skeptics have alleged that models "got clouds completely wrong," Dessler said. He believes that his paper, which suggests long-term climate change will create a positive feedback from clouds that produces additional heating of the planet, "shows that models are doing a reasonable job as a group."
A bolt from Cancun
One of those skeptics is Roy Spencer, a climatologist at the University of Alabama, Huntsville. He issued a statement yesterday attacking Dessler's study, calling its "central evidence weak at best, misleading at worst."
Spencer has published a paper arguing that clouds will cool the planet and counteract warming. He drew on that work to argue that Dessler's study confuses the cause and effect of warming by failing to take into account the idea that changes in clouds drive temperature, rather than temperature changes driving cloud behavior.
Dennis Hartmann, a professor of atmospheric sciences at the University of Washington, agreed with Dessler.
"I do think it's very significant that this analysis shows that a strongly negative, short-term cloud feedback is very unlikely, based upon the evidence, and that positive cloud feedback is more likely," said Hartmann, who did not contribute to the new study. "Current climate models vary widely on their assessments of cloud feedback. But if you were forced to draw consensus on what models are saying so far, they're saying that cloud feedback is moderately positive."
The new analysis is based on the first 10 years of data collected by an instrument flying aboard NASA's Terra satellite that monitors how much radiation is entering and leaving Earth's atmosphere. The instrument, known as CERES (short for "Clouds and Earth's Radiant Energy System"), began collecting information in March 2000.
Dessler used the data to determine how the El Niño-Southern Oscillation weather cycle affected the amount of radiation leaving the atmosphere over a 10-year period -- an indirect measurement of cloud behavior and the ensuing climate response.
A 10-year glimpse of cloud behavior
That's not a precise analogue for cloud behavior in response to long-term climate change, he said.
The latter "is really what we care about," Dessler added. "In order to understand how clouds are going to respond to long-term warming, you have to wait until there is long-term warming. That will take decades. Looking at the short-term is the best we can do right now."
Hartmann noted that the warming observed during an El Niño cycle of a year or two is different than the long-term climate change prompted by human activities that produce greenhouse gases like carbon dioxide.
El Niño warms the tropics, whereas climate change driven by greenhouse gases warms the planet up everywhere, Hartmann said. But the scientist said he thought Dessler's approach still amounted to a "useful diagnostic tool" for trying to understand whether climate models' representation of clouds is on the right track.
Meanwhile, Dessler said his next step is aimed at identifying how well individual climate models do predicting cloud behavior, by examining their output for different regions of the globe -- such as land versus ocean, or high latitudes versus low latitudes.
"This is a significantly harder problem, and it's a tougher test of the models," he said. "My hope is that looking at the spatial distribution will allow me to say, 'These models are doing a good job. These models are doing a terrible job.'"
Interfering with skeptics' 'negative impact'?
In his statement attacking Dessler's study, Spencer also said he suspected, but had no proof, "that Dessler was under pressure to get this paper published to blunt the negative impact our work has had on the [Intergovernmental Panel on Climate Change]'s efforts."
Spencer appeared in Cancun accompanied by Marc Morano, founder of Climate Depot, which regularly attacks mainstream climate change science, and Lord Christopher Monckton, a British skeptic who asserted the Kyoto Protocol threatens national sovereignty and individual freedom.
In a response posted yesterday afternoon on the blog "RealClimate," Dessler said his disagreement with Spencer stemmed from their very different views about the cause of the El Niño-Southern Oscillation (ENSO), quoting an e-mail exchange with Spencer.
"My position is the mainstream one, backed up by decades of research," Dessler wrote on the blog. "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)."
He added: "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."