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

The extent of snow cover1 and sea ice2 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 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. Climate feedback mechanisms are often characterized in terms of their influence on top-of-atmosphere (TOA) net energy flux (F) with changing surface temperature7–10, highlighting the utility of understanding the direct impact of evolving Earth System components on F . Extending a previous analysis of seasonal snow radiative effect3, we define cryosphere radiative forcing (CrRF) as the instantaneous perturbation to Earth’s TOA energy balance induced by the presence of surface cryospheric components. CrRF is thus directly analogous to cloud radiative forcing, a commonly used diagnostic in climate model and remote sensing analyses. This study focuses entirely on the short-wave (solar spectrum) component of Northern Hemisphere CrRF. Long-wave CrRF may be non-negligible in regions where snow or ice alters surface emissivity3, and long-wave feedbacks associated with cryosphere evolution can also be large11. CrRF is influenced by several factors, including the surface albedo change induced by snow or ice, which depends on snow/ice albedo and characteristics of the snow-free surface (especially vegetation), and local insolation and atmospheric state (primarily cloudiness), which determine the propagation of surface albedo changes to TOA flux8,12. Here, we quantify plausible CrRF ranges by combining observations of snow and sea-ice cover fraction (Sx , where x indicates snow or sea ice), surface