It has been a bad fortnight for climate change news, from the heat-induced devastation of coral reefs to warnings that sea levels could rise much faster than we thought. This unfortunate pattern continues with the announcement that we have been miscalculating the feedback between global warming and cloud formation, suggesting most climate projections have been too optimistic.
Clouds are among the hardest aspects to incorporate into models of how the atmosphere will respond to inputs such as increased carbon dioxide. This is because not all clouds behave alike. We know that the warming induced by carbon dioxide emissions will cause the atmosphere to hold more water vapor, leading to more clouds. However, with high clouds warming the planet and lower-altitude clouds having a cooling effect, it is hard to know which will dominate.
Yale University graduate student Ivy Tan has looked at clouds from both sides now and her conclusions, published in Science, are disturbing. Sadly, it turns out they won't block the Sun nearly as much as we would like.
Clouds are formed from ice crystals and supercooled liquid droplets, and the balance between these components is important. “For a fixed amount of cloud water, spherical liquid droplets tend to be smaller in size and also to outnumber ice crystals, because ice nuclei are relatively scarce in Earth’s atmosphere in comparison to cloud condensation nuclei,” Tan noted in the paper. “As a consequence, clouds that consist of a higher fraction of liquid are optically thicker and hence more reflective of sunlight.”
Tan pointed to previous work that showed certain clouds contain fewer crystals and more droplets than previously realized. The crucial question is how this will change in a world where the atmosphere contains both more carbon dioxide and more heat.
Using 79 months of data from NASA's Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite observations, Tan concluded that models have over-estimated the current rate of ice crystal growth. When she ran what she sees as more realistic processes through climate models, the results were terrifying: A model that previously predicted 4.0°C (7.2°F) of warming now produced 5.0 to 5.3°C (9.0 to 9.5°F).
“An initial doubling of CO2 concentrations causes the entire troposphere to deepen,” the paper reported. “This in turn increases the altitude at which a particular temperature is reached, with consequent effects on the crystal/droplet ratio.”
The impact on cloud formation is greatest over the Southern Ocean, the paper noted. However, additional heat captured there will make its way around the entire globe.
The paper expresses confidence that “should the low [ice formation] bias be eliminated” from global climate models, the estimation of temperatures for a particular concentration of carbon dioxide will increase. Whether the extra warming will be as large as Tan's initial estimates remains to be seen; the paper acknowledges that other models may be affected by different amounts.
However, even if the additional warming turns out to be only half as large as Tan's model shows, our problems just got bigger.