Sir

The dramatic loss of sea-ice cover over the Arctic this summer was widely reported, for example in your News story 'Arctic melt opens Northwest passage' (Nature 449, 267; doi:10.1038/449267b 2007), and frequently attributed to global warming. Although the gradual decline in sea-ice extent during the past four decades is in line with that expected from global warming, it is very unlikely that the loss of sea-ice cover this year is explicable solely in terms of temperature change.

Changing wind patterns are an important influence on the distribution of sea ice. Throughout summer 2007, exceptional pressure and wind patterns persisted over the Arctic Ocean. The observed migration of ice cover, from the Siberian and Beaufort seas northwards and eastwards into the Arctic Basin, was in line with the expected response to the anomalous winds. These Arctic wind anomalies were part of a global-scale pattern of highly unusual circulation this summer, the causes of which are as yet unclear.

The growing La Niña in the East Pacific undoubtedly had a major influence globally, and there is some evidence from past events that La Niña predisposes the circulation towards the type of exceptional patterns seen this summer.