Sep 28, 2012

The Arctic melts, Antarctic ice is getting thicker, but …

There are many and varied reasons to explain the comings and goings of the Antarctic ice cap, writes Michael D. Lemonick. What might surprise is the ice is getting thicker.

The sea ice that covers the Arctic Ocean has plummeted to its lowest level on record -- but down at the other end of the world, the sea ice surrounding Antarctica has swelled. That’s no surprise, considering that winter is just ending in the southern hemisphere -- but what may be surprising is that the overall extent of Antarctic ice has grown by about 1% per decade, on average, since satellite records began a little over 30 years ago. You might reasonably suspect that all the fuss about disappearing Arctic sea ice is overblown, then, given the growth of ice down south. But you’d be wrong, for all sorts of reasons. The first is that the 1% growth per decade in the Antarctic pales next to the much faster 15.5% drop per decade in the Arctic. They aren’t even in the same ballpark. Not only that: while the sea ice bordering Antarctica has been growing slightly, the massive ice sheets that sit directly atop the frozen continent are shrinking, at an accelerating rate, with worrisome implications for global sea level rise. The disparity is even more dramatic when you realise that most of the sea ice surrounding Antarctica drifts away during the summer to melt in warmer waters, and reforms anew in the winter. The Antarctic sea ice cover is nearly all first-year ice, which is typically one to two metres thick. In the Arctic, by contrast, the ice is hemmed in by Canada, Alaska, Russia and Greenland. It mostly can’t drift away, so whatever is left behind at the end of summer gets even thicker the following winter. That multi-year ice, which can be up to five metres thick, which is much harder to melt, dominated the Arctic Ocean when satellites first went into orbit in 1979. If you look at the volume of ice rather than just the area it covers, the disparity between the Arctic ice loss and Antarctic ice gain is just that much more impressive. Still, if the planet is warming, how can the sea ice be expanding in the waters surrounding Antarctica in the first place? Keeping in mind that it isn’t expanding by much, scientists offer several possible explanations. One is that there’s been more precipitation in recent decades (which itself could well be due to global warming). That puts a cap of relatively fresh water atop the denser, saltier water below, and in winter, when that top layer cools, it stays on top rather than mixing with the warmer water underneath, thus encouraging the growth of ice. Another factor may be the ozone hole that opens up at this time every year over the South Pole. Ozone loss tends to cool the upper atmosphere -- an effect that percolates down to the surface. Still another factor is purely natural climate variation, which is still happening even though man-made global warming has a growing influence on every aspect of the Earth’s climate system with every passing decade. In any case, climate scientists have long expected that the Arctic would warm up faster than the Antarctic. After all, the former is an ocean surrounded by land, while the latter is land surrounded by ocean. Wind patterns, weather systems and ocean currents behave differently at the two poles. And because the coldest part of the Antarctic is land, the ice there has been able to accumulate into a giant ice cube the size of a continent and up to 3.2 kilometres thick -- which tends to hold back local warming considerably. By the second half of the century, however, climatologists say that the human warming signal will become more apparent, and Antarctic sea ice will begin to follow its Arctic cousin in a downward spiral. That, in turn, could speed up melting of the all-important Antarctic land ice, thereby raising global sea levels. *This article was originally published at Climate Central

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