The arctic is one of the fastest warming places on earth and is already undergoing large changes due to global warming. One of the most obvious examples of this is the arctic sea ice. This is the frozen covering of ice that sits on top of the arctic sea, stretching from Greenland over the north pole to the top of Russia and around past Alaska and the Candian arctic. Every spring and summer the 24 hr sunlight and warmer temperatures melt the ice back northward and before it refreezes again in autumn and winter.
At the end of the 2012 met season sea ice extent had shrunk to just 3.41 million square kilometers according the US National Snow and Ice Data Center (NSIDC), 18% lower than the previous record in 2007 and 49% lower than the long term average. And unlike 2007, 2012 didn't have close to ideal weather conditions for melting ice either. The figure from NSIDC below shows the area of ice extent at the 2012 minimum (white) compared to the long term average (red line)
So why, with worse weather for melting did 2012 break all melting records? The answer probably lies in the volume of ice left. The volume of arctic ice basically relates to it's thickness, with thicker ice less likely melt in the summer. Since the late 70's, arctic sea ice has lost roughly 3/4 of it's volume, including big losses since the last extent (area of sea covered by 15% ice) record in 2007.You can see this progression on the animated chart below thanks to Tamino. Put simply the ice today is thin and weak and it doesn't take much to melt it.
So where to from here. Well, the bad news is that melting arctic ice causes a positive feedback loop. Ice reflects almost all incoming sunlight back in space, while open water absorbs almost all the sun's energy. So, when more ice melts, there is more open water to absorb the sun's rays, heating up the water and melting more ice. This means that the amount of ice is going to keep decreasing, (especially with continuing increases in arctic temperature due to global warming) with a number of predictions that the arctic might be seasonally ice free within a decade or so. Ice will continue to refreeze in winter, but it will be thin and weak and since there isn't much sun in the arctic that time of year it won't make that much difference to the amount of sunlight reflected whether it is there or not.
The arctic meanwhile is going to become a very different place. The loss of sea ice will accelerate arctic warming and completely disrupt the habitat of many of the species that live on or under the ice. Warming is also changing the arctic land environment as well. More importantly for those of us who don't live there, the arctic also plays an important role in northern hemisphere weather and some scientists have pointed the finger at changes in the arctic for extreme weather events in Europe and North America. For example: allowing cold arctic air to "break out" of the arctic and head south to chill the aforementioned regions. You can also find more about this on Skeptical Science.
Lastly, the massive loss of ice in the arctic demonstrates us just how fast things can change. No-one expected the unprecedented ice loss in 2007 and indeed IPCC model forecasts made before the 2007 melt season were not predicting the loss of summer sea ice until near the end of the century. Yet here we are with the possibility it will be gone by the end of the decade. This should remind us conducting an uncontrolled experiment on the earth's climate is going to have surprises, (and probably not many good ones) and that ecosystems can be much more vulnerable to destruction than we realized. It's too late to stop what's happening in the arctic, that would have required taking proper action of global warming decades ago, and only by taking action to halt global warming can we prevent the kind of breakdown occurring in the arctic from happening elsewhere, such as to the great barrier reef.
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