The retreat of Arctic sea ice acts as an accelerator for climate change, since the bright ice surfaces reflect much more sunlight than the darker surfaces of open water. Ice reflects up to 90% of solar energy back to space while water absorbs a large amount of energy and warms both itself and the overlying air layers.
If warming leads to retreating ice, then this effect causes additional warming and the ice melts even faster: a classical positive feedback reaction. Of course this would also work the other way around: If lower temperatures cause an expansion of snow and ice covered areas, this would lead to an additional cooling.
The ability of surfaces to reflect radiation is expressed by the albedo, a figure that quantifies the amount of reflected radiation in percent.
Scientists of the University of California in San Diego could now, by satellite measuring, verify that the albedo north of the 60. degree of latitude is falling and that this is related to the retreat of arctic sea ice. The result of the measurements is that the albedo decreased from 0,52 to 0,48 between 1979 and 2011. Instead of 52% now only 48% of the solar radiation in the Arctic is reflected. This corresponds to an additional average input of solar energy of ca. 6,4 W/m² since 1979. Averaged over the globe this corresponds to an additional energy input of 0,21 W/m², which is 25% of the amount attributed to the increase of CO2 in the same period.
These figures are significantly higher than those expected before by models and estimates.
Another result of the measuring is the fact that the albedo also decreased in areas which were covered with ice all over the year. One explanation for this is an increasing occurrence of melt water lakes on the ice which again absorb more solar energy and cause additional warming.
Melting fjord ice, Liefdefjord.
Sources: Spiegel Online Wissenschaft, Proceedings of the National Academy of Sciences of the United States of America (PNAS)