Latest issue of Physics Today has an article on satellite altimetry and sea ice thickness:
Satellite altimetry quantifies the alarming thinning of Arctic sea ice
A dramatic 57% loss in the volume of perennial ice between 2004 and 2008 may have set the stage for ice-free summers within the next 30 years.
The Arctic Ocean’s floating sea-ice cover waxes and wanes with the seasons. The icecap grows in the fall when the hours of sunlight shorten and intense cold sets in. When long summer days return, ice floes melt or are driven by wind and ocean currents into the North Atlantic Ocean. A quarter century ago, the coverage ranged from about 7 million to 16 million km2 between late summer and the following March.
Since 1978, when satellites began routinely monitoring the Arctic, the extent covered by perennial ice—that which survives the summer melt—has declined by close to 10% per decade, at least until 2007. In September of that year, the summer ice extent plummeted to a record low 4.2 million km2, down 23% from a previous record low in 2005. The perennial ice lost in those two years alone covered an area almost twice the size of Texas. (For a broader perspective on changes in the Arctic, see reference 1 and the article by Josefino Comiso and Claire Parkinson in PHYSICS TODAY, August 2004, page 38.)
The trend, no doubt, reflects the Arctic’s response to the warming of Earth’s climate. And the recent acceleration is worrying. The Arctic is particularly sensitive thanks to the ice albedo–ocean feedback at work there. For example, a drop in ice cover increases the absorption of solar radiation in the ocean, warms the water, prolongs the melting, and reduces the ice cover yet further. As figure 1 shows, the impact of those changes on the Arctic’s human inhabitants can be severe.