Possible climate transitions from breakup of stratocumulus decks under greenhouse warming
Abstract
Stratocumulus clouds cover 20% of the low-latitude oceans and are especially prevalent in the subtropics. They cool the Earth by shading large portions of its surface from sunlight. However, as their dynamical scales are too small to be resolvable in global climate models, predictions of their response to greenhouse warming have remained uncertain. Here we report how stratocumulus decks respond to greenhouse warming in large-eddy simulations that explicitly resolve cloud dynamics in a representative subtropical region. In the simulations, stratocumulus decks become unstable and break up into scattered clouds when CO2 levels rise above 1,200 ppm. In addition to the warming from rising CO2 levels, this instability triggers a surface warming of about 8 K globally and 10 K in the subtropics. Once the stratocumulus decks have broken up, they only re-form once CO2concentrations drop substantially below the level at which the instability first occurred. Climate transitions that arise from this instability may have contributed importantly to hothouse climates and abrupt climate changes in the geological past. Such transitions to a much warmer climate may also occur in the future if CO2 levels continue to rise.
These simulations may not be perfect … but they're much, much better than anything else we have.
As rumors of the Earth Machine have spread, the project has drawn a mix of support, envy, and skepticism. A new approach like that is desperately needed, says Trude Storelvmo, an atmospheric scientist at the University of Oslo. "This is a very welcome and innovative idea." She adds that it could bolster the case for expanded observations of clouds—necessary because NASA's current cloud satellites have worked nearly a decade longer than planned.
In contrast, Amy Clement, a cloud scientist at the University of Miami in Florida, laments the focus on building more complex models. "As a result, in my opinion, we are losing a lot of our ability to gain fundamental understanding of the climate system." However, she adds, given Schneider's acumen as a climate scientist, the model might lead to such understanding. Bretherton, meanwhile, likes the group's ambitions but questions whether a new model is needed to realize them. "We already have too many climate models in the United States," he says. "It divides our resources and makes scientific progress slower."
Other people think the project is discounting rewards that will come when existing models are pushed to run globally at higher resolutions. Much of the climate science community in Europe, for example, is invested in a proposal called Extreme Earth, which would push models to a resolution of 1 kilometer per cell. Although such code would require a network of supercomputers and wouldn't run as long as traditional models, it would also eliminate many parameters that Schneider is seeking to improve with AI, replacing them with physics. "I'm so frustrated with the idea of parameterizing these things," says Bjorn Stevens, a climate scientist at the Max Planck Institute for Meteorology in Hamburg, Germany. "What I find more exciting is getting rid of those rules of thumb."
There's also a big assumption baked into the Earth Machine: that the cloud problem can even be solved, adds Joel Norris, a cloud scientist at the Scripps Institution of Oceanography in San Diego, California. Perhaps any sort of parameterization, even one tuned by AI, cannot crack clouds to a meaningful degree. "It may be the case you can't reduce the uncertainty," Norris says. Some satellite observations essential to rendering clouds, such as the exact location of water vapor in the lower atmosphere, simply don't exist. And Schneider's team could be shocked when it sees how apparently unconnected parts of the model go awry when clouds are tweaked, Held adds. "There's just a lot of connections."
Schneider's team is aware of all those concerns and shares many of them. But the members are ambitious and have grown impatient waiting for a breakthrough. They've lived with human-driven climate change, and its dogged uncertainties, as a reality for their entire adult lives. It's time for the clouds to lift.
