Graph of the Day: September ice melt Bad, But July and August were the Real Story
September 19, 2011
As I noted in a recent post, Arctic Sea ice has not been cooperating with denialist’s insistence on a “recovery”.
A rainy summer might be one reason for an apparent lack of public attention with respect to the ongoing sea-ice loss. Another reason, however, is possibly the fact that we scientists have failed to make sufficiently clear that a major loss of sea ice during the early summer months is climatologically more important than a record minimum in September. This importance of sea-ice evolution during the early summer months is directly related to the role of sea ice as an efficient cooling machine: Because of its high albedo (reflectivity), sea ice reflects most of the incoming sunlight and helps to keep the Arctic cold throughout summer. The relative importance of this cooling is largest when days are long and the input of solar radiation is at its maximum, which happens at the beginning of summer.
If, like this year, sea-ice extent becomes very low already at that time, solar radiation is efficiently absorbed throughout all summer by the unusually large areas of open water within the Arctic Ocean. Hence, rather than being reflected by the sea ice that used to cover these areas, the solar radiation warms the ocean there and thus provides a heat source that can efficiently melt the remaining sea ice from below. In turn, additional areas of open water are formed that lead to even more absorption of solar radiation. This feedback loop, which is often referred to as the ice-albedo feedback, also delays the formation of new sea ice in autumn because of the accompanying surplus in oceanic heat storage.
Measurements from ice buoys show that indeed melting at the bottom of the sea ice has increased significantly in recent years. While field experiments that were carried out in the 20th century showed unambiguously that surface melting used to be the dominant mechanism for the thinning of Arctic sea ice, now in larger and larger areas melting at the underside of the ice is almost equally important. Such melting from below is particularly efficient since the temperature at the ice-ocean interface is fixed by the phase equilibrium that must be maintained there. Hence, any heat provided by the ocean to this interface will lead to thinning of the ice in summer and to slower ice growth in winter. At the surface, the ice temperature is not fixed as long as the ice isn’t melting, and heat input from the atmosphere can in part be compensated for by a change in surface temperature and an accompanying change in outgoing long-wave radiation at the ice surface.