From the Guardian Environment Network, Dirk Notz, Friday 9 September 2011
“Well, it’s not really good timing to write about global warming when the summer feels cold and rainy,” a journalist told me last week. Hence, at least here in Germany, there hasn’t been much reporting about the recent evolution of Arctic sea ice – despite the fact that Arctic sea ice extent in July, for example, was the lowest ever recorded for that month throughout the entire satellite record. Sea-ice extent in August was also extremely low, second only to August 2007 (Fig. 1). Whether or not we’re in for a new September record, the next weeks will show.
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.
More (Click Here)
The following three graphs are from my own calulations. Figure 5 shows the daily extent, Figure 6 shows the daily change in extent (averaged over the prior week) and Figure 7 compares the recent extents as a percentage of the 2003 – 2010 extent for the same day of the year. The data for this analysis were downloaded from “IJIS” (the IARC-JAXA Information System) reporting the AMSR-E sea ice extent on a daily basis since 2002.