The study was led by Ramanathan’s Scripps Institution of Oceanography colleague Joel Norris, though Ramanathan said he was not involved in the work and didn’t know about it until shortly before publication. But Ramanathan said that the study basically confirms that there’s nothing to prevent the world from reaching the high levels of warming that have long been feared — except for our own swift policy actions, that is.
“My reaction was, my goodness,” Ramanathan said. “Maybe the 4 to 5 degree warming, certainly we were all wishing there was some certainty that would make it go away. So I consider the findings of this paper, the data shows major reorganization of the cloud system.”
This matters because clouds are fundamental regulators of how much solar radiation makes it to the Earth’s surface (rather than being reflected back to space by white cloud tops), and how much infrared or “longwave” radiation escapes back to space once again.
The new study used satellite records going back to 1983 to show that cloud patterns have shifted on the Earth as cloud belts have pushed poleward, expanding what are called subtropical dry zones between about 20 and 30 degrees latitude in both hemispheres.
The shift is expected to exacerbate climate change, as moving clouds toward the poles means they reflect less sunlight back to space — there is less sunlight at the poles than at Equator, so the reflectivity of clouds counts for less there.
The shift has long been predicted by climate models — but is not exactly something that’s easy to measure. Thus the new study presents key observational evidence that it is actually happening.
Ramanathan isn’t the only scientist to embrace the new study and point to its significance.
“This study is a big deal, both generally for climate change science, and also for understanding recent trends in regional hydroclimate,” said Jonathan Overpeck, who co-directs the Institute of the Environment at the University of Arizona in Tucson, by email.
Overpeck added that the findings are particularly bad news for key parts of the U.S. “The agreement between the new cloud observational data and model simulations confirm that storms bringing much needed water to the US West are indeed moving poleward and away from the Southwest,” he said. “The new work thus indicates that the already drought-plagued Southwest, from California to Colorado and New Mexico will likely continue to dry out over coming decades and as long as greenhouse gas emissions to the atmosphere continue. The research highlights that there are winners and losers with global warming, and that the US Southwest is one of the losers when it comes to water generating storms moving north.”
“This work is yet another example of how, as we resolve some of the outstanding uncertainties (in this case, by producing a more reliable estimate of historical changes in cloud), we find additional confirmation of our basic physical understanding of climate change,” added Michael Mann, a climate scientist at Penn State University, by email.
“The authors make a compelling case that in a properly quality-controlled satellite record of cloud distribution, we see robust additional, independent evidence for the predictions of climate models,” Mann continued. “In particular, we see a poleward expansion of the storm tracks, an expansion of the subtropical region of dry descending air, and a vertical increase in the boundary between the troposphere (the part of the atmosphere where weather happens) and the stratosphere (the higher region where jets fly). There features are all predicted by state-of-the-art climate model projections.”
Granted, not every scientist is fully convinced that the new work resolves, in a major way, the cloud-and-climate problem — long noted as a major uncertainty that has bedeviled climate prediction. There have also been some scientists out this week highlighting limitations to the work.
“The paper finds some large scale patterns, consistent in observations and model simulations, that provide hints for potential mechanisms that may be responsible for the cloud changes,” said NASA’s George Tselioudis, an expert on clouds and their changes, by email. “However, the paper itself does not explore such mechanisms and, given the complexity of the interactions between cloud and the atmospheric circulation, more work is needed before definitive conclusions can be drawn for the climate effects of the observed cloud pattern variability.”
For Ramanathan, though, the study means the validation of a key “feedback” that makes higher levels of warming possible. And it works in a multiplicative fashion with other feedbacks, such as a notorious one involving the loss of Arctic sea ice, which, not unlike a cloud, also reflects radiation from space so long as it covers northern oceans.
“I consider this as the most singular of all the things that we have found, because many of us had been thinking the cloud changes might help us out, by having a strong feedback which is going the other way instead of amplifying it,” Ramanathan said.
“The uncertainty is narrowing down,” he continued later. “I used to say, if I made a 50 percent overestimation of the global warming, it was due to the clouds. But we are running out of that excuse now.”
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