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.
“Some computer models show periods of great sensitivity where the Arctic ice system collapses suddenly, and that trend may occur a bit earlier; that’s the best guess, but exactly when it’s hard to say,” he told the BBC News website.
Dr Scambos co-authored the latest study, published in the journal Geophysical Research Letters, with other scientists from NSIDC and from the National Center for Atmospheric Research (NCAR), also in Boulder, Colorado.
They also calculate that about half, if not more, of the warming observed since 1979 originates in humanity’s emissions of greenhouse gases.