Clouds cool Earth by reflecting shortwave light from the Sun away from the planet and insulate it by blocking longwave radiation from escaping into space. Currently, on a global scale, clouds reflect and release more heat than they trap. This keeps Earth cooler than it would be otherwise. However, anthropogenic greenhouse gases impact atmospheric temperatures, and the way clouds respond can either amplify or dampen these changes. This so-called cloud feedback has been the main source of uncertainty in global climate change simulations for decades.
Scientists have explored cloud feedback in the tropics, but less is known about how clouds respond to climate change closer to the poles. Now Wall and Hartmann have shown that small-scale thermodynamic processes in clouds of mixed liquid and ice composition drive shortwave cloud feedback at high latitudes.
Global climate models such as the third and fifth phases of the Coupled Model Intercomparison Project (CMIP3 and CMIP5) predict that as temperatures rise, dimmer clouds will trap more heat in the subtropics of the Southern Hemisphere, whereas brighter, more reflective clouds will dominate farther south at higher latitudes. A similar pattern is predicted for the Northern Hemisphere, with brighter clouds toward the North Pole.
The researchers used global climate models to tease out the mechanism behind this predicted pattern. Specifically, they set out to test the hypothesis that as the Southern Hemisphere’s polar jet stream and associated storm clouds migrate closer to the South Pole, they could contribute to brighter high-latitude clouds.
They ran experiments with three different global climate models. The study was simplified by using an aquaplanet configuration, which assumes Earth is covered entirely by water.
In all three models, they found that poleward movement of the jet stream does not cause brighter clouds at high latitudes. Instead, changing the freezing temperature in the models had a larger effect toward the poles, suggesting that small-scale thermodynamic processes inside mixed liquid- and ice-phase clouds drive changes in cloud feedback at high latitudes. A better understanding of these processes would make climate models more accurate. (Journal of Advances in Modeling Earth Systems (JAMES), doi:10.1002/2015MS000520, 2015)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), How climate change impacts clouds’ ability to cool Earth, Eos, 97, doi:10.1029/2016EO044301. Published on 29 January 2016.