A new report from the Lamont-Doherty Earth Observatory at Columbia University attributes the warming to a shift in the jet stream that brought higher temperatures to much more northerly latitudes than normal last year.
The warmer air and melting ice magnify each other in a feedback loop called Arctic amplification.
In the same way that white clothing feels cooler than black on a sunny summer day, white ice reflects sunlight, keeping temperatures at the surface low. When the ice melts, dark sea water and dark land are exposed. These absorb sunlight rather than reflecting it, thus raising temperatures and causing more melting. The cycle continues, and even speeds up, as the dark areas grow larger and white ice disappears.
This is one reason the Arctic is heating up faster than other parts of the globe.
A similar feedback loop has been taking place over the Arctic Ocean, with the increasing loss of sea ice each summer. This has warmed the ocean waters.
Many scientists believe that the warmer Arctic air has also affected the shape of the jet stream, causing it to be more wavy, with troughs dipping farther south and ridges looping farther north. That shift delivered last winter's extra cold temperatures over central North America, and brought warm air over Greenland.
The jet stream is the boundary between cold polar air and warm tropical air, a river of fast moving air that circles the globe like the soft brim of a floppy hat. When the temperature difference between northern cold and tropical warmth is large, the waves in the jet stream tend to smooth out more, but when the Arctic warms up and that difference shrinks, the stream becomes more wavy.
We don't know whether this change in shape of the jet stream is due to human-induced climate change at this point, because records on its shape don't go back far enough, but the effect is there.
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A third feedback loop is caused by air pollution from the south settling on the ice, making it darker and causing it to melt faster.
None of this is good news for the Greenland ice sheet — the second largest in the world. Although it will take centuries for it all to melt, the accelerated melting that is already taking place is having far-reaching effects.
Glacial runoff is cold, fresh water that floats on top of salty ocean water. Its presence can affect ocean currents, such as the Gulf Stream, as well as fish habitats. That, in turn, affects the birds that feed on the fish.
In other words, everything in the North is affected by climate change, and all systems in the air, on land and in the seas are connected. These feedback loops collectively press harder on the accelerator pedal, speeding up the rate of warming — which only increases the urgency for us to take action to slow things down.