Global Climate

Assessing the sensitivity of planetary albedo to snow and sea ice

On a climatological global-mean basis, a large portion of upwelling solar photons at the top-of-atmosphere are reflected by the atmosphere rather than the surface. This is also largely true even in cryosphere regions, where the surface is highly reflective of solar radiation. This dominance of the atmosphere stems from two effects: First, incoming solar photons at the-top-of-atmosphere are partially absorbed and reflected back to space by the atmosphere, reducing the number of photons reaching the surface; Second, solar photons initially reflected by the surface are partially absorbed and reflected back to the surface by the atmosphere,  and thus only a small portion of them actually reach the top of atmosphere. These both can be thought of as a damping effect of the atmosphere on the surface contribution to planetary albedo.

In contrast to the climatological case, the surface dominates the variability in planetary albedo on seasonal and interannual time scales in cryosphere regions. The surface accounts for about 75% of the change in climatological planetary albedo from one season to another with similar zenith angle and more than 50% of its interannual variability at nearly all times of year, especially during seasons with extensive snow and sea ice extent (see Fig 1).
These results may have important implications for future climate change. Satellites have observed a retreat of NH snow cover and Arctic sea ice associated with a large-scale warming in the NH. This trend may continue in the coming decades, as a response of the climate system to anthropogenic radiative forcing such as increases in greenhouse gas concentration. Assuming cloud fields do not change much in a future climate, our results showing the sensitivity of planetary albedo in cryosphere regions to snow and ice variability imply that a reduction in snow and ice will lead to a significant increase in net incoming solar radiation and thus result in more warming. This supports the idea of a postive surface albedo feedback.

Note that ISCCP flux data set is used to contruct Figs 1 and 2.

download the publication describing these results in more detail.