For each degree Celsius rise in ocean surface temperature, the ratio of cirrus cloud area to cumulus cloud area over the ocean dropped 17-27 percent. The authors propose that higher ocean surface temperatures directly cause the decline in cirrus clouds by changing the dynamics of cloud formation and rainfall. As these cumulus convective clouds grow taller, cloud water droplets collide and combine into raindrops and fall out of the cloud or continue to rise until they freeze into ice crystals and form cirrus clouds. More of the cloud droplets form raindrops and fewer are left in the cloud to form ice crystals. As a result, the area of cirrus cloud is reduced.
Clouds play a critical and complicated role in regulating the temperature of the Earth.
Thick, bright, watery clouds like cumulus shield the atmosphere from incoming solar radiation by reflecting much of it back into space. Thin, icy cirrus clouds are poor sunshields but very efficient insulators that trap energy rising from the Earth's warmed surface. A decrease in cirrus cloud area would have a cooling effect by allowing more heat energy, or infrared radiation, to leave the planet. The researchers estimate that this effect could cut by two-thirds the projected increase in global temperatures initiated by a doubling of carbon dioxide in the atmosphere. The American Meteorological Society is the nation's leading professional society for scientists in the atmospheric, oceanic, and related sciences.