Shrinking ice and snow cover in the Northern Hemisphere is reflecting ever less sunshine back into space in a previously underestimated mechanism that could add to global warming, a study showed.

Satellite data indicated that Arctic sea ice, glaciers, winter snow and Greenland’s ice were bouncing less energy back to space from 1979 to 2008. The dwindling white sunshade exposes ground or water, both of which are darker and absorb more heat.

The study estimated that ice and snow in the Northern Hemisphere were now reflecting on average 3.3 watts per square meter of solar energy back to the upper atmosphere, a reduction of 0.45 watt per square meter since the late 1970s.