Cloud science matters to many climatologists almost as much as it does to meteorologists. Depending on clouds’ altitudes and compositions, those vaporous puffs can influence global temperatures enough that climate scientists include them in their predictive models. A new study published in Science reveals, however, that most models underestimate how much cooling the planet receives now from clouds that contain both water droplets and ice crystals. Moreover, the chilling impact of those clouds may diminish as atmospheric greenhouse gas concentrations increase.
Clouds Respond to Carbon Dioxide
Although anyone can observe clouds simply by looking up, these important atmospheric features have proven difficult to study scientifically, especially when it comes to mixed-phase clouds. These clouds, prevalent at higher latitudes, drift across the skies in the upper reaches of the Earth’s troposphere, where the air temperature ranges from about 0°C to −40°C. Unlike clouds in lower, warmer layers of the atmosphere, which contain only liquid water droplets, high-altitude mixed-phase clouds contain water as supercooled liquid droplets mixed with some ice fragments. The ratio of water to ice can vary depending on the cloud—and since the cloud’s behavior depends on a set of complex interactions between melting ice and freezing water, the effects of these interactions can be difficult to predict and study on a large scale.
This unpredictability has important implications for climate change because mixed-phase clouds have been a climate mystery for many years. Clouds can have either a cooling or a warming effect on the planet, depending on their altitude and composition: low-lying, watery clouds block sunlight for a mild cooling effect, whereas higher-altitude icy clouds, including mixed-phase clouds, let in more sunlight. Because scientists weren’t sure of the exact mix of water and ice in mixed-phase clouds, their effect on climate was difficult to determine.
What’s more, unlike already liquid clouds at lower altitudes, mixed-phase clouds’ composition will likely change as their layer of the atmosphere heats up. “The entire troposphere deepens with global warming,” explained Ivy Tan, one of the authors of the new study. As the troposphere creeps upward, the rising warmth should melt some of the ice in clouds. When that happens, “regions with clouds once dominated with ice now predominantly contain liquid,” Tan said.
The shift to more liquid makes the clouds denser, more opaque, and better at reflecting the Sun’s radiation. However, cloud researchers could only estimate how much this transition in cloud composition would act as a brake on climate warming because they had only a general idea of how much of the water in mixed-phase clouds was frozen.
CALIPSO’s Unclouded Vision
To get a better picture of these mixed-phase clouds, the research team, composed of Ivy Tan and Trude Storelvmo of Yale University in New Haven, Conn., and Mark Zelinka of Lawrence Livermore National Laboratory in Livermore, Calif., looked at data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), a NASA satellite that uses lidar to observe clouds from above. By shining a laser down into the clouds, the satellite was able to determine whether any given spot in a cloud was made of water, ice, or air. Its polar orbit allows the satellite to get much more data on cloud composition over the entire planet than ground-based lidar systems, which typically send laser beams up from a fixed point or vehicle. After analyzing more than 6 years’ worth of data, the team of scientists found that mixed-phase clouds were much more liquid, and less icy, than previous estimates had suggested—their study brought the estimated fraction of liquid water in the cloud from 20% to around 60%.
The findings could have a big impact on what we know about Earth’s future: Since mixed-phase clouds have less ice in them to begin with than most scientists originally thought, their potential for becoming denser is also less than previously predicted. This diminished potential would mean that clouds are much less effective as a buffer against global warming than climatologists have assumed.
With all these new data taken into account, Tan said, the team’s climate model yields considerably higher estimates than it previously did for how much global temperature will rise because of greenhouse gas emissions. In a scenario where the concentration of atmospheric carbon dioxide doubles, the team’s model had predicted a global temperature rise of 2.0°C to 4.6°C, which agreed with most climatologists’ expectations. With the revised impact prediction for mixed-phase clouds, the model’s estimates increase by as much as 1.3°C, Tan and her colleagues report.
Another Piece in the Climate Puzzle
The new study has “made a simple and persuasive point demonstrating the importance of the mixed-phase cloud feedback in the Earth system,” Daniel McCoy, an atmospheric scientist at the University of Washington in Seattle, commented in an email. McCoy was not involved in the study.
Some scientists remain unconvinced that the team’s new projections for global temperature increase are on the mark. Emphasizing that mixed-phase clouds are just one element of a dynamic climate system, Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York, N.Y., who was not involved with the study, urged in an email that the new cloud data should be tested with other constraints and other models. “This is one extra ingredient that needs to go into the hopper,” he wrote.
—Elizabeth Deatrick, Writing Intern
Citation: Deatrick, E. (2016), Icy clouds may counter climate warming less than expected, Eos, 97, doi:10.1029/2016EO050169. Published on 13 April 2016.