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.
The new paper therefore surmises that because of this cloud process, the climate will warm more than previously believed for a doubling of carbon dioxide levels in the atmosphere — by as much as 1.3 degrees Celsius. The paper thus implies that it may be necessary to up the value (or the range) for a critical and long-discussed parameter, known as the climate “sensitivity” for a doubling of CO2, which the U.N.’s Intergovernmental Panel on Climate Change most recently defined as “likely in the range 1.5°C to 4.5°C.” If Tan and her colleagues are proven right, then, the result could be to increase our expectations of future warming.
But that requires them to be proved right — several experts consulted by the Post had differing reactions to the new study.
The research is “a great example of the use of state-of-the-art satellite retrievals to constrain climate model process simulations, and this emerging constraint points towards a positive rather than a negative cloud brightness feedback and towards a higher climate sensitivity,” said George Tselioudis, a NASA climate expert who studies clouds and climate models, by email.
However, Anthony Del Genio, another NASA expert, was a bit more skeptical, noting that satellites may observe one thing on the outside of clouds, but that doesn’t mean what’s happening on the inside of them is the same. He also said that generally, the key processes involved are just very difficult for scientists to observe. “It is easy to say that climate models should fix this problem. But in practice it’s hard to fix because we don’t have the necessary data,” he said by email.
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.”
Still, the new study clearly underscores that, while we don’t know everything about the climate yet, the things we don’t know could just as well harm us as help us.
“Our study suggests that the climate sensitivity range should be shifted upwards,” says Tan. “By how much, we don’t know exactly.”
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