Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008

The extent of snow cover(1) and sea ice(2) 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 timescales(3-6), and century-scale feedback has been assessed using climate models(7-10). 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 in land snow cover and sea ice. On the basis of these observations, we conclude that the albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1 : 1 W m(-2) K-1, substantially larger than comparable estimates obtained from 18 climate models.