One proposal, investigated by Axel Schweiger and colleagues (Geophys. Res. Lett. 35, L10503; 2008), suggests that a persistent high-pressure system lingering over the Arctic region from June to August 2007 may have led to unusually low cloud cover that could have exacerbated normal summer melt and contributed to the record low. However, as is so often the case in climate studies, the story turns out to be more complex.
Satellites recorded sufficiently low cloud cover over the Arctic that summer to warrant the conditions to be considered unusual, but the blue skies did not coincide with the areas where the ice cover vanished. Instead, the summer sunshine spread across the northernmost Arctic Ocean, where sea ice was left intact even in 2007. Ice thicknesses decreased significantly in these northernmost regions in response to the sunny weather, although direct sunlight is not very effective at warming bright surfaces like sea ice — as anyone knows who has worn a black T-shirt and white trousers in the sun. But these ice losses did not count towards the record low because they were losses in thickness rather than ice-covered area.
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