Northern Hemisphere average annual snow cover has declined in recent decades. This pattern is consistent with warmer global temperatures. Some of the largest declines have been observed in the spring and summer months.

The extent of snow cover and sea ice in the Northern Hemisphere has declined since 1979, coincident with hemispheric warming and indicative of a positive feedback of surface reflectivity on climate. This albedo feedback of snow on land has been quantified from observations at seasonal timescales3–6, and century-scale feedback has been assessed using climate models7–10. However, the total impact of the cryosphere on radiative forcing and albedo feedback has yet to be determined from measurements. Here we assess the influence of the Northern Hemisphere cryosphere on Earth's radiation budget at the top of the atmosphere—termed cryosphere radiative forcing—by synthesizing a variety of remote sensing and field measurements. We estimate mean Northern Hemisphere forcing at -4.6 to -2.2 W m-2 , with a peak in May of -9.0 ± 2.7 W m-2 . We find that cyrospheric cooling declined by 0.45 W m-2 from 1979 to 2008, with nearly equal contributions from changes i