Arctic Warming

Interesting charts will be a regular feature of this blog. Today’s comes from a recent publication by the Arctic Monitoring and Assessment Programme (AMAP), specifically the SWIPA report presented at COP15 in Copenhagen (Dec 2009). There’s a wealth of information there which I will no doubt return to many times in the coming months.

The chart shows the current increase in average surface temperature from a 1951-1980 reference period with respect to latitude. Reading left to right is to travel from the South Pole to the North Pole. The broken blue line represents the 0.7 °C increase in global average temperature.

The deviation in the Arctic is striking, the rate of warming has been more than twice that of the global average. The implications for the Greenland ice sheet, Arctic sea ice and northern permafrost are likely to be equally striking. But why is the Arctic warming so fast?

This Arctic amplification is mainly driven by sea ice. There are three main contributors; ice-albedo feedback, latent heat required to melt ice and the insulating effect of sea ice.

The most well recognised is the ice-albedo feedback. A little warming melts a little polar ice over the Arctic Ocean and land surfaces. Ice has a high albedo, it reflects 60-90% (depending on roughness and snow cover) of incoming solar radiation. As the ice melts, the uncovered land or water has a much lower albedo, reflecting as little as 10% of the incoming energy and absorbing the rest. This increase in absorption amplifies the initial warming effect.

During the Arctic summer the process of melting ice takes approximately half of the absorbed energy without raising temperatures. This energy is the latent heat that must be added to the ice to weaken the molecular bonds but does not raise the temperature. The temperature remains at the melting point. With less ice available to melt, more of the absorbed energy is expressed as temperature increase.

During the winter sea ice acts as an insulator, preventing the relatively warm Arctic Ocean waters releasing energy and warming the atmosphere. Less winter sea ice results in a warmer winter atmosphere.

Regarding the latent heat and ocean water insulation, it seems that surface atmospheric temperatures only tells part of the story. They don’t represent the complete energy picture of the Arctic, just how much energy is in the surface atmosphere.

There may also be other processes relating to ocean heat transport from lower latitudes. Finally the three sea ice processes all cause heating of the lower atmosphere, this is likely to modify prevailing weather systems.