The recent decreases in winter ice cover are clear evidence that Arctic pack ice will continue on its trajectory of rapid decline, says Francis. In a paper published in Geophysical Research Letters, Francis and colleague Elias Hunter found that the rising average wintertime surface sea temperature of the Barents Sea – up 3°C since 1980 – is likely driven by increasing greenhouse gases, which in turn are melting more ice. Francis and Hunter used satellite information dating back 26 years to perform their study.

Scientists have known for some time that the extent of perpetual summer icecover in the Arctic has been shrinking, but until the past few years, that the average amount of winter ice has been relatively steady. The winter ice amount is important because if it begins to decrease, scientists believe that indicates that enough extra heat from the sun is being absorbed in summer in new open water areas so that the ice grows less in winter and is more easily melted the following summer, leading to even less summer ice.

The record-breaking ice loss this year is further, dramatic evidence that this process is underway. While satellites can see the retreat of recent winter ice, no one knew until now what was actually driving the ice back. Francis said she and Hunter were surprised when they discovered that warming ocean temperatures – and not atmospheric effects – were the main source of retreating winter ice, and that the warming is linked to general rising temperatures of the Atlantic Ocean via the Gulf Stream, which brings Atlantic water into the Barents Sea.

“In the Barents Sea, I expected more influence from atmospheric heating; but it [the retreat of the ice edge] seems to be governed almost entirely by warming from the ocean,” Francis said.

Should the warming trend continue – and all indications are that it will – there would be considerable economic and political implications. “Fishing, shipping, oil exploration will all be easier to do in the Arctic if there is less ice around for a shorter time,” Francis said.

Francis and Hunter were in for another surprise in the Bering Sea, between Alaska and Siberia. That sea is virtually cut off from the Pacific Ocean by the Aleutian Islands. The researchers expected the ice edge there to be pushed around by northerly and southerly winds, but that wasn’t the case. Instead, it was the strength or weakness of the Aleutian Low – a semi-permanent storm with predominantly easterly winds in much of the Bering Sea – that determined the ice edge.

In years when the low was weak – when the east wind didn’t blow as hard – the ice edge crept further south. In years when the east winds blew hard, the ice edge retreated northwards. The strength of the Aleutian Low oscillates in cycles lasting 10 to 20 years, Francis said, and right now, appears to be in a weak cycle. That means that the ice edge in the Bering Sea, not exposed to the world’s ocean system like its Barents Sea counterpart, has not retreated as much. Computer models predict, however, that the Aleutian Low will strengthen as the global climate system adapts to increasing greenhouse gases.