In recent years we've seen a large loss of sea ice in Arctic and a small increase in the Antarctic. This is a well established fact, but sometimes we are asked 'why don't they both decrease, if global warming is global?'. This is a reasonable question, and one explanation for this was found in an interesting analysis by Chylek et. al. in GRL, 'Twentieth century bipolar seesaw of the Arctic and Antarctic surface air temperatures': (http://www.agu.org/journals/gl/gl1008/2010GL042793/).
They show that over the 20th century when the Antarctic was a little warmer, the Arctic was a little cooler, and vice-versa, and explain this by suggesting that the Atlantic ocean couples the Antarctic and Arctic regions, with ocean currents sometimes sending warm water northwards away from the Antarctic (so cooling the Antarctic) through the equator and on to the Arctic (so warming the Arctic), and sometimes this not happening quite as much, making the Arctic a little cooler, and the Antarctic a little warmer.
This imposes a strong signal of warming and cooling on top of the signal from CO2 driven global warming, so we shouldn't be surprised if major changes driven by temperatures, such as the extent of summer sea ice cover, appear at one of the poles without showing up immediately at the other.
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