Earth's critical stabiliser on wane

Alex Gough, of the University of Otago department of physics, takes notes as an oceanographic mooring is raised from beneath the sea ice in the Antarctic. Photo by Andy Mahoney.

In "Swings and roundabouts in ice melt, figures show" (ODT 13.10.09) Dr Jock Allison invites climate scientists to present counter-evidence in a user-friendly manner to his opinion that recent changes in sea-ice extent are not particularly alarming from a global-warming point of view.

Sea ice reflects around 80% of the sun's energy that falls on it.

As a result, the ocean underneath it remains cooler.

This is critical for the earth to balance the excess heat collected around the equator and maintain a stable average global temperature.

The end-of-summer sea-ice extent is the value most often used to examine long-term trends in the Arctic.

This is because the sea ice left at the end of summer is typically the thickest and most resilient to future melt and its extent is therefore useful as a measure of the health of the Arctic sea-ice cover.

Also, almost all the sun's energy that reaches the polar regions does so during summer.

Scientists have been able to measure global sea-ice extent since 1978, when the first of a series of satellites was launched.

Since this time, the summertime sea-ice extent in the Arctic has retreated at an average rate of roughly 750,000sq km (almost three times the area of New Zealand) per decade, though there has been variability from year to year and there is a strong indication the rate accelerated in the late 1990s.

Despite increases in both 2008 and 2009, they have still been the second and third-lowest years in the record after 2007.

The ice remaining in the Arctic this year is the youngest and thinnest on record, which makes it look unlikely that this is part of a major recovery.

Arguments against this evidence or its significance often bring attention to the relatively short time period covered by the satellite data.

But before satellites there were observations from ships, aircraft and drifting scientific stations that spanned most of the Arctic Ocean.

Russian sea-ice charts dating back to 1933 indicate that the currently low extent of Arctic sea ice is unprecedented in at least 76 years.

During this time, there were decade-long periods of advance and retreat, but the overall trend has been one of decline.