The Weekend Wonk: Andy Dessler on Clouds and Climate Uncertainties
April 9, 2016
New paper on clouds and climate sensitivity making waves. If true, this study suggests that clouds will make the climate more, not less, sensitive to climate change.
Reaction I’m hearing from trusted sources is: worthwhile topic, proceed cautiously, more work needed.
Video interview above, Andrew Dessler, who was not involved in the current study, nevertheless knows his clouds.
For a long time, there’s been one key part of the Earth system that, just maybe, could help us out a little bit with our global warming problem: Clouds.
Clouds are central to the climate because their white surfaces reflect sunlight back to space, keeping the planet cooler than it would be otherwise. But they also trap infrared or heat radiation and prevent it from escaping the Earth (among many other relevant effects). So if a changing climate also changes clouds — which scientists definitely expect to happen — that could be very important, and there have been prominent suggestions that this could all play out in such a way as to slow down climate change.
Clouds are “the biggest unknown in terms of the Earth’s radiation budget right now. They’ve been identified to be the largest source of uncertainty,” says Ivy Tan, a geoscientist at Yale who studies them. But uncertainty can cut both ways, and Tan is the lead author of a new study in Science suggesting that changes in clouds won’t actually protect us as much as we might have thought from the consequences of atmospheric carbon dioxide — which, in turn, means the Earth could warm more than otherwise expected.
Tan and her colleagues focused on a particular type of clouds called “mixed-phased,” which are comprised of both ice crystals and also some supercooled liquid water. Mixed-phased clouds are very common across the Earth, especially in cold and temperate regions, occurring, not surprisingly, higher in the air in mid-latitude regions and closer to the ground as you near the poles.
The reason they’re so important to climate is not just their abundance but their composition — the liquid parts of the cloud are better at deflecting sunlight away from the Earth, and liquid parts of these clouds are expected to increase, not surprisingly, as the climate warms. And that ought to be a negative feedback that makes global warming somewhat less bad than it would be otherwise.
“The more liquid you have in your cloud, the more reflective of shortwave radiation, or sunlight, it is,” Tan explains. “It’s a jucier cloud, it’s going to be thicker, it’s denser, so it’s going to reflect more sunlight back out to space than a cloud with ice would. The ice clouds are thinner, wispier, and more transparent to sunlight.”
But if you change the ratio of water to ice, you also change the strength of the feedback. And based on recent satellite observations, Tan and her colleagues — from Yale and Lawrence Livermore National Laboratory — argue that water has a higher prevalence in these clouds than many climate models assume or allow for. Their water content is “severely underestimated on a global scale” in such models, they write.
If the satellite observations are right, that means that as the climate warms, there will be less ice in these clouds to convert to liquid — and thus, less sunlight that will be reflected away from the Earth, leaving behind a warmer Earth.
As the atmosphere warms, clouds become increasingly composed of liquid rather than ice, making them brighter. Because liquid clouds reflect more sunlight back to space than ice clouds, this “cloud phase feedback” acts as a brake on global warming in climate models.
But most models’ clouds contain too much ice that is susceptible to becoming liquid with warming, which makes their stabilizing cloud phase feedback unrealistically strong. Using a state-of-the-art climate model, the researchers modified parameters to bring the relative amounts of liquid and ice in clouds into agreement with clouds observed in nature. Correcting the bias led to a weaker cloud phase feedback and greater warming in response to carbon dioxide.
“We found that the climate sensitivity increased from 4 degrees C in the default model to 5-5.3 degrees C in versions that were modified to bring liquid and ice amounts into closer agreement with observations,” said Yale researcher Ivy Tan, lead author of the paper.
Climate sensitivity refers to the change in global mean surface temperature due to a doubling of carbon dioxide. Climate models predict between 2.1 and 4.7 degrees C (3.75 to 8.5 degrees F) of warming in response to a doubling of carbon dioxide.
“We saw a systematic weakening of the cloud phase feedback and increase in climate sensitivity as we transitioned from model versions that readily convert liquid to ice below freezing to model versions that can maintain liquid down to colder temperatures, as observed in nature,” Tan explained.
These results add to a growing body of evidence that the stabilizing cloud feedback at mid- to high latitudes in climate models is overstated. Moreover, several recent studies have concluded that other important cloud feedbacks also are likely to exacerbate warming rather than dampen it. These include amplifying feedbacks from increases in cloud top altitude and from decreases in the coverage of subtropical low clouds.
“The evidence is piling up against an overall stabilizing cloud feedback,” concluded Zelinka. “Clouds do not seem to want to do us any favors when it comes to limiting global warming.”
“The broader implication of this work is that for the same amount of carbon dioxide in the atmosphere, we’ll see greater global warming than currently predicted,” explains the study’s lead researcher, Trude Storelvmo, a Yale assistant professor of geology and geophysics, in a telephone interview with The Christian Science Monitor, “so for global policy it means more fossil fuels need to stay in the ground.”
While Storelvmo acknowledges that this hardware has been orbiting our planet for a decade, she explains that it takes time to see the patterns emerge and then to determine the studies to be undertaken, on top of the time and computer power it takes to carry out the research.
The actual numbers are up for debate, but there does seem to be fairly broad acceptance in the scientific community as to the fundamentals of the work’s conclusions, as Michael Mann, distinguished professor of atmospheric science at Pennsylvania State University, explains to the Monitor.
“I find the paper reasonably compelling that shortcomings in how certain key cloud processes are treated could well be leading to an overestimation of the ability of cloud feedbacks to ameliorate global warming,” says Dr. Mann in an e-mail interview. “There is indeed other recent work that makes a similar case.”
But he describes the study as more of a ” ‘proof-of-concept’ than a precise estimate of the impact of the effects studied,” pointing out that the authors themselves are hesitant to commit to any definite figures in relation to temperature rises.
It is probably premature, then, to say that the new research means that scientists as a whole will now be concluding that thanks to a better understanding of clouds, the Earth is more sensitive to carbon dioxide than we thought — and will likely warm more than expected, or, at the high range of what’s currently expected. It’s rare that one study has such a sweeping impact.
“Headlines that scream ‘Scientists say sensitivity higher than thought!’ will not be justified,” says Gavin Schmidt, who directs NASA’s Goddard Institute for Space Studies. “This is one extra ingredient that needs to go into the hopper.”
Note, a very senior climate scientist tells me via email, “I think the paper is fine as a first step but it is not the last step, and much more is needed to establish how clouds change as the climate changes.”