NASA has released satellite measurements of the thickness of Arctic sea ice, comparing 2004 to 2008. From The Guardian:
The Earth is going thin on top. A new study has revealed that the Arctic Ocean's permanent blanket of ice around the North Pole has thinned by more than 40% since 2004.
Scientists said the rapid loss was "remarkable" and could force experts to reassess how quickly the Arctic ice in the summer may disappear completely. They blame the loss on global warming, which has driven temperatures in the Arctic to record highs and summer ice extent to recent lows.
...Ron Kwok, senior research scientist at Nasa's Jet Propulsion Laboratory in California, said: "Even in years when the overall extent of sea ice remains stable or grows slightly, the thickness and volume of the ice cover is continuing to decline, making the ice more vulnerable to continued shrinkage."