The Arctic ice in March

The National Snow and Ice Data Centre has just released it's latest monthly report. It said that the maximum ice extent in the Arctic was reached on 31 March this year, which was the latest maximum since satellite records began in 1979. The previous latest date was on March 29, 1999 (two days earlier than the maximum this year).

The Bering and Baltic seas were where ice extent increased, driven mainly by cold weather and winds from the north. Elsewhere it decreased or was near average.

The maximum extent was 15.25 million square kilometers (5.89 million square miles). This was 670,000 square kilometers (260,000 square miles) above the record low maximum extent, which occurred in 2006. Usually there is a decrease in ice during March, but this March the ice extent grew at an average of 13,200 square kilometers (5100 square miles) per day. The linear rate of decline for March over the 1978 to 2010 period is 2.6% per decade as shown in this chart from the National Snow and Ice Data Centre.

The Centre reports that the younger multi-year ice was replenished a bit this winter and the strong negative Arctic Oscillation prevented as much ice from moving out of the Arctic. The report was noncommittal on the expectations for summer ice melt, except to say:

The larger amount of multiyear ice could help more ice to survive the summer melt season. However, this replenishment consists primarily of younger, two- to three-year-old multiyear ice; the oldest, and thickest multiyear ice has continued to decline. Although thickness plays an important role in ice melt, summer ice conditions will also depend strongly on weather patterns through the melt season.

The report also states that there aren't any satellites taking ice thickness measurements across the whole of the Arctic right now:

At the moment there are no Arctic-wide satellite measurements of ice thickness, because of the end of the NASA Ice, Cloud, and Land Elevation Satellite (ICESat) mission last October. NASA has mounted an airborne sensor campaign called IceBridge to fill this observational gap.

If you go to the NSIDC website, there are some good images showing the change in ice thickness over the past twenty years or so, with the decline in older ice overall, and the slight recovery this past year.

Polar extremes

The arctic is warming, a lot! Antarctica as a whole is not warming like the arctic. Why the difference?

The arctic is mostly ocean covered in floating ice. It is warming faster than most other places on earth and the sea ice is rapidly melting.

Antarctica is the driest continent on earth. The climate there has been significantly modified by the ozone hole. It's not warming as fast as the arctic, but the glacial melt is contributing to rising sea levels, and could realistically add more than one metre to sea levels within the next 90 years.

Ozone declined rapidly last century and is only now stabilising. It is expected to largely recover to 1980 levels by the middle of this century.

The depletion of ozone has changed the polar vortex, which extends from the surface to the stratosphere and follows the earth's contour lines at the south pole. The loss of ozone from 1980 onwards strengthened the polar vortex winds by 15%. The winds driven by the ozone hole are shielding eastern Antarctica from the warming that is affecting the rest of the world. However, the winds transfer down to the surface and are carrying warm air to western Antarctica, where glaciers are melting.

Low pressure cells over the Amundsen Sea are forcing up deep water, which is warmer than 1°C. This warmer water is getting in under the Pine Island Glacier and melting it from below.

Of 244 Antarctic Peninsula glaciers, 87% have retreated over the last 50 years.

For a comprehensive and up-to-date reading on the Antarctic, see the Turner et al SCAR report of Antarctic Climate Change and the Environment, which was released in November 2009.

A recent slide presentation (Feb 2010) based largely on the above report can be seen here.