On a cloudless winter night, it's more likely for the temperatures plunge below freezing than when there's a nice blanket of clouds that can keep the day’s heat from executing a nocturnal escape. That might be obvious, but the effect is stronger than you might have thought. Strong enough, it seems, to melt Greenland’s ice sheets.
Clouds work like those silver space blankets they give to disaster victims and marathon runners—they either keep heat in, or they reflect sunlight back into space. A new study from the University of Wisconsin says that clouds are playing a larger part in the melting of Greenland’s ice than was previously thought. It’s a big deal, because there’s a lot of ice there—the meltwater is expected to account for one third of the planet’s rising sea levels.
Two NASA satellites, CloudSat and CALIPSO, launched during the last decade, have changed scientists’ understanding of clouds, and how they affect climate. "Once you know what the clouds look like," says study author and professor Tristan L’Ecuyer, "you know how much sunlight they’re going to reflect and how much heat from Earth’s surface they’re going to keep in."
The study reveals that—contrary to previous thinking—the increased meltwater runoff is down to blankets of clouds preventing the melted ice from re-freezing, instead of increasing melting directly. "Clouds enhance meltwater runoff by about one-third relative to clear skies," says the paper.
Greenland’s snowpack is already reflective of radiated heat, so this radiation bounces between the snow and the clouds. Some of this causes the ice to melt, while the insulating clouds keep the temperatures too high for refreezing to occur. This additional meltwater then drains away instead, contributing to rising sea levels.
What can be done? Not much. L’Ecuyer’s team has built complex computer models which include the data from CloudSat and CALIPSO, to help us better predict the effects of clouds on ice sheets in the future. These models won’t help us halt or reverse the effects, but they will allow us to "better predict future contributions of the Greenland ice sheet to global sea level rise," says L’Ecuyer.