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
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Sea ice


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Table 2.2 | The assessed remaining carbon budget and its uncertainties.

Shaded blue horizontal bands illustrate the uncertainty in historical temperature increase from the 1850–1900 base period until the 2006–2015 period as estimated from global near-surface air temperatures, which impacts the additional arming until a specific temperature limit like 1.5°C or 2°C relative to the 1850–1900 period. Shaded grey cells indicate values for when historical temperature increase is estimated from a blend of near-surface air temperatures over land and sea ice regions and sea-surface temperatures over oceans.