Much has been written in recent weeks about the causes behind the unusually cold, snowy winters in Europe and large parts of North America over the past two years. The immediate cause is clear, as explained in an excellent article by Justin Gillis of The New York Times. As Gillis wrote, "A pattern of atmospheric circulation that tends to keep frigid air penned in the Arctic has weakened during the past two winters, allowing big tongues of cold air to descend far to the south, while masses of warmer air have moved north." These topsy-turvy shifts were vividly depicted last week in a map created by NASA, which shows that temperatures in mid-January were as much as 30 degrees F above normal in some parts of the Arctic, while much of the U.S. has been beset with temperatures far below normal and loads of snow.
The big question is whether this oddball weather is related to the profound warming of the Arctic and the steady decline in the extent and thickness of Arctic Ocean sea ice. Gillis' balanced account says the jury is still out on that question, although experts cited in his story make a convincing case that this weather shift is related to global warming.
What has been missing from this discusson is that similar climate shifts, caused by changing wind patterns, have affected parts of Antarctica in recent decades. Considering that Antarctica is home to 90 percent of the ice on Earth - the Arctic looks like Copacabana by comparison - what's happening in the Far South is worth noting.
But first the Far North. As Gillis explains, some scientists believe that the tight vortex of cold air that normally keeps the Arctic's frigid temperatures from spilling too far south is changing. Traditionally, the strong air pressure difference between frigid Arctic regions and warmer temperate zones has created a tight, whirling stream of cold air that generally confines polar air to polar regions. But, some scientists theorize, as the Arctic has rapidly warmed - in part because the loss of sea ice is causing the dark surface of the Arctic Ocean to absorb more heat - that temperature and pressure differential has weakened. That, in turn, has allowed cold air to stream south and warmer air to flow north. "The effect," writes Gillis, "is sometimes compared to leaving a refrigerator door open, with cold air flooding the kitchen even as warm air enters the refrigerator."
Antarctica has also been experiencing changes in its polar vortex in recent decades, which have caused some parts of the continent to warm precipitously, while others have gotten colder. As I explain in Fraser's Penguins, as air and ocean temperatures have warmed in the temperate zones north of Antarctica, that has increased the temperature and pressure difference between those temperate zones and ice-bound Antarctica. That, in turn, seems to have kicked into high gear a climatic pattern called the Southern Anular Mode, or SAM, which has strengthened the westerly winds around Antarctica, pulling in warmer, moister air from the north. As a result, the 900-mile Antarctic Peninsula has experienced precipitous warming, with winter temperature in the Palmer Station region - where I spent five months - increasing by 11 degrees F in the last 50 years and summer temperatures rising 5 degrees F. As in the Arctic, sea ice along the western Antarctic Peninsula is in sharp decline and now blankets the Southern Ocean nearly three fewer months a year than in 1979.
In contrast with the Arctic, Antarctica's polar vortex has actually strengthened, in part because the destruction of the ozone layer above the South Pole has led to a steep drop in temperatures high above the pole. (Ozone traps heat.) That, scientists theorize, has created a greater contrast between super-cold polar tempeatures and temperatures to the north, which has tightened the polar vortex and altered the continent's wind patterns. Those shifts have led to the rapid rise of temperatures along the Antarctic Peninsula while causing more cold air to be dragged off the polar plateau and onto the western Ross Sea, where temperatures have been falling and sea ice growing.
Comparisons between the two poles are difficult, in large measure because the two ends of the Earth are so different. The Arctic is (or was) a frozen ocean surrounded by continental land masses, which serve to moderate the Arctic's climate. The Antarctic is a continent of ice, three miles deep in places, surrounded by the Southern Ocean, which acts as a vast insulating barrier that has long kept warmer temperatures from the north away from the conintent. That insulating, moat-like effect may be changing somewhat now, just as climatic patterns in the Arctic may also be changing. This, unfortunately, is the new normal, with rising global temperatures worldwide starting to disrupt the relatively stable climate of the past 12,000 years, a period that has fostered the rise of human civilization.