About 7,000 cubic kilometres of ice remained at the end of last summer. By contrast, in 2004 there was 13,000 cubic km of sea ice in the Arctic.

"Very soon we may experience the iconic moment when, one day in the summer, we look at satellite images and see no sea ice coverage in the Arctic, just open water," said Dr. Seymour Laxon, with London's Centre for Polar Observation and Modelling.

The consequences of losing Arctic ice coverage, even for only part of the year, could be profound. The ice caps reflect sunlight back into space, without them the region will heat up even more than at present. As a result, ocean temperatures will rise and methane deposits could melt, evaporate and bubble into the atmosphere. Methane is a particularly powerful greenhouse gas and rising levels in the atmosphere are likely to accelerate global warming.

"Our greatest concern is that loss of Arctic sea ice creates a grave threat of passing two other tipping points -- the potential instability of the Greenland ice sheet and methane hydrates. These latter two tipping points would have consequences that are practically irreversible on time scales of relevance to humanity," said NASA's top climate scientist, James Hansen.