To put it another way, an area the size of Alaska, America’s biggest state, was open water two years ago, but is again now covered by ice.
The most widely used measurements of Arctic ice extent are the daily satellite readings issued by the US National Snow and Ice Data Center, which is co-funded by Nasa. These reveal that – while the long-term trend still shows a decline – last Monday, August 25, the area of the Arctic Ocean with at least 15 per cent ice cover was 5.62 million square kilometres.
This was the highest level recorded on that date since 2006 (see graph, right), and represents an increase of 1.71 million square kilometres over the past two years – an impressive 43 per cent.
Other figures from the Danish Meteorological Institute suggest that the growth has been even more dramatic. Using a different measure, the area with at least 30 per cent ice cover, these reveal a 63 per cent rise – from 2.7 million to 4.4 million square kilometres.
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The satellite images published here are taken from a further authoritative source, the University of Illinois’s Cryosphere project.
They show that as well as becoming more extensive, the ice has grown more concentrated, with the purple areas – denoting regions where the ice pack is most dense – increasing markedly.
Crucially, the ice is also thicker, and therefore more resilient to future melting. Professor Andrew Shepherd, of Leeds University, an expert in climate satellite monitoring, said yesterday: ‘It is clear from the measurements we have collected that the Arctic sea ice has experienced a significant recovery in thickness over the past year.
‘It seems that an unusually cool summer in 2013 allowed more ice to survive through to last winter. This means that the Arctic sea ice pack is thicker and stronger than usual, and this should be taken into account when making predictions of its future extent.’