By Richard Black Environment correspondent, BBC News website
Arctic ice is melting faster than computer models of climate calculate, according to a group of US researchers.
Since 1979, the Arctic has been losing summer ice at about 9% per decade, but models on average produce a melting rate less than half that figure.
The scientists suggest forecasts from the Intergovernmental Panel on Climate Change (IPCC) may be too cautious.
The latest observations indicate that Arctic summers could be ice-free by the middle of the century.
"Somewhere in the second half of the century, it would happen," said Ted Scambos of the National Snow and Ice Data Center (NSIDC) at the University of Colorado.
The fact that all models show ice loss over the observed period and all project large ice losses into the future is a very strong message Marika Holland
"Some computer models show periods of great sensitivity ..full text