Arctic sea ice is continuing its seemingly interminable decline, and it looks like the loss could be contributing to the recent spate of cold winters over northern Europe and North America.
Researchers are still unsure about how important sea-ice loss is to winter weather. The fluctuating weather that Europe and the Americas have experienced since December last year is a reminder that many different factors are at play.
Jiping Liu at the Georgia Institute of Technology in Atlanta crunched data on ice and weather from the last three decades and found a link between the extent of Arctic sea ice and cold, snowy winters in the regions immediately to the south.
"The study adds weight to growing belief that Arctic sea ice is driving an increase of cold winters," says Adam Scaife of the UK Met Office. His team has also found evidence to support such a link.
Open water
It is well established that the dramatic loss of winter sea ice in the Arctic has caused it to feel global warming more sharply than the rest of the planet. Less ice means more exposed water, which ? being darker than ice ? absorbs more solar energy. To compound things, with less ice to insulate the warmer ocean from the air above, more of that energy is released back into the atmosphere as heat.
The net result is that Arctic air is significantly warmer than before. Some years, winter temperatures have reached 4 ?C above average ? an enormous difference.
Things get a bit messier when figuring out how this affects the weather further south. When the Arctic winter is warmer ? and there's less of a contrast between temperatures at the pole and in the tropics ? the jet stream tends to creep south and lose some of its strength. This is known as a "negative Arctic oscillation". During these negative phases, the jet stream blows warm weather in over the Mediterranean and allows cold, dry Arctic air to rush in over the northern continents.
Solar cycles
But while Liu's research strengthens the case for a strong link between sea ice and the negative Arctic oscillations that encourage extreme winters, Scaife and others point out that it is just one factor among many.
A case in point, this winter started out quite mild. The Arctic oscillation only flipped into a negative phase in mid-January, triggering cold spells that killed hundreds of people in eastern Europe and Asia. This suggests other factors are at play.
James Overland of the Pacific Marine Environmental Laboratory in Seattle believes the weather we have seen in the past few months is largely down to natural variations. "There are other factors besides sea-ice anomalies that influence winter snowfall," agrees Liu. They include the El Ni?o/La Ni?a cycle and the roughly-decadal variations in the solar activity. El Ni?o and solar minima both bring cold winters.
Balancing act
The big challenge now is to integrate everything we know about the different factors into one big model, to figure out how they all play out together. "If it were down to the sun alone, we would be heading from colder winters to milder ones on average," says Scaife ? because solar activity is on the rise right now.
But Liu's results show that decrease in Arctic ice cover will have the opposite effect. How the two factors will balance out in the short term is impossible to say.
In the long term, solar activity will continue to cycle up and down, but we expect ice to keep melting. "If you're looking 30 years ahead, when Arctic will be closer to 80 per cent ice-free rather than 30 or 40 per cent. We can expect to see colder winters," says Overland.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1114910109
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