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Alexandra Witze, Earth in action

Poles apart, the Arctic and Antarctic exhibit very different records for sea ice

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SN Prime | November 5, 2012 | Vol. 2, No. 42

People might think they’re twins, but the North Pole and the South Pole are really more like distant cousins who, at family reunions, can’t believe they are related.

To visit the North Pole, you have to take a helicopter or plane that lands you on an ice runway just a few meters thick. Beneath your feet stretch several miles of water. Better hope the ice doesn’t crack.

To visit the South Pole, you land on another ice runway, but one solidly anchored to the bedrock beneath. This is why the National Science Foundation can build a huge permanent research station here. No chance you could fall through and drown.

Therein lies the contrast between the Earth’s poles: The Arctic is an ocean surrounded by land (Alaska, Canada, Greenland, Scandinavia, Russia). The Antarctic is land surrounded by an ocean (the Southern Ocean). Those stark differences mean that the poles respond very differently to planetary changes.

Take this year’s seasonal fluxes in sea ice. When it’s summer in the Northern Hemisphere, the Arctic’s ice cap melts back and shrinks. Meanwhile, it’s winter in the Southern Hemisphere. That means that sea ice is growing around the fringes of the Antarctic continent, like a ring of hair encircling a bald pate.

Both poles set a record this September. Up north, the Arctic experienced the biggest loss in sea ice since satellites began watching in 1979. Down south, the Antarctic gained the most sea ice seen in that same period. Not surprisingly, some climate skeptics jumped on this observation as seeming evidence that polar changes somehow even each other out.

Nothing could be further from the truth. There are good reasons why sea ice differs between the poles, and growth in the south doesn’t counterbalance loss in the north.

For starters, look at the magnitude of the changes. Antarctica’s winter sea ice maximum is growing, but slowly: Between 1979 and 2012, its average September extent grew by just under 1 percent per decade. Up north, over the same time period, the area covered by sea ice in September has been dropping by 13 percent per decade.

Around Antarctica, the sea ice fringe this year was about 1 million square kilometers above the 1979–2000 average (a time period often chosen to represent the “old normal” of sea ice conditions). In a different but telling measure in the Arctic, this year’s loss between the winter freeze and the summer melt is 11.83 million square kilometers, the largest seasonal loss by far in the satellite record.

Put yet another way, Antarctica is gaining a Connecticut’s worth of ice each year, while the Arctic is losing an Indiana. Why the difference? Because while Arctic changes are dominated by air and ocean temperatures, Antarctic changes are all about winds and ocean currents.

As Arctic sea ice melts, it exposes the darker water beneath. This open expanse absorbs more of the sun’s incoming energy and sets up a feedback loop that melts more ice and then even more.

In contrast, Antarctic ice grows as winds whip fiercely across the Southern Ocean, cooling the sea’s surface below. Westerly winds have grown stronger in recent years, causing the cooling to deepen. So too has each winter’s appearance of the ozone hole; as man-made chlorofluorocarbon chemicals chew away at the protective ozone layer, more cold air from higher altitudes can percolate down to the surface and keep Antarctica chilly.

What’s important about Antarctica, climate-wise, is not its fringe of sea ice but the massive ice sheets that rest atop the continent. Unlike Arctic ice, which is already floating and thus doesn’t add anything to sea level rise when it melts, Antarctic ice sheets contain enough ice to cause the equivalent of many tens of meters of sea level rise, were they all to melt at once. And now parts of those ice sheets are starting to stir.

Along the Antarctic Peninsula, that thin tendril that reaches up toward Chile, temperatures have risen nearly 2 degrees Celsius in the past half-century. Glaciers have been thinning and speeding their race to the sea. If warming continues apace, the peninsula will soon reach temperatures at which ice shelves collapsed around 11,000 years ago, scientists report in the Sept. 6 Nature.

Farther along the Antarctic coast, the thin-ning Pine Island Glacier is another bellwether of southern change. The glacier’s tongue extends off the edge of the continent and over the ocean. British Antarctic Survey researchers and colleagues have found that even as cold winds blow over the glacier, warm ocean water percolates from below the glacier’s tongue, melting and weakening it from underneath.

So while the Antarctic’s sea ice may be growing, the continent’s icy heart is beginning to thaw. Within decades, the south may very well catch up to the north. And even the skeptics will run out of arguments.

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