When a ship carrying scientists and adventure tourists became stuck in ice in the Antarctic late last month, climate change sceptics had a field day. On social media sites, they pointed out that a group whose journey was meant to highlight the effects of global warming was trapped by a substance that was supposed to be melting.
“Global warming idiots out of danger,” one noted when the ship’s 52 passengers were taken to safety by helicopter after more than a week on the ice.
The episode had little connection to climate change — shifting winds had caused loose pack ice to jam against the ship — and this was far from the first time that a ship had been trapped, even in the Antarctic summer.
But sea ice cover in the Antarctic is changing, and scientists see the influence of climate change, although they say natural climate variability may be at work, too. “The truth is, we don’t fully understand what’s going on,” said Ted Maksym, a researcher at the Woods Hole Oceanographic Institution.
Unlike the Arctic, where sharp declines in recent decades in the ice that floats on sea surfaces have been linked to warming, sea ice in the Antarctic has increased, scientists who study the region say. Averaged over the entire Antarctic coast, the increase is slight — about 1 per cent a decade. At the same time, larger increases and decreases are being seen on certain parts of the continent.
“We’re constantly struggling against that statement, that Antarctic ice is increasing,” said Sharon E Stammerjohn, a scientist at the Institute of Arctic and Alpine Research at the University of Colorado. “It misses key changes that are happening. And there are really strong climate signals in those changes.”
Most of the sea ice changes are occurring in an area covering about a third of the Antarctic coast, from the Ross Sea to the Bellingshausen Sea and the Antarctic Peninsula, said Paul Holland, a researcher with the British Antarctic Survey. Areas around the Ross Sea have seen large increases in ice, while in the Bellingshausen and along the peninsula, ice cover has declined sharply.
Researchers agree that the changes in those seas are related to north-south winds that circulate clockwise around a stationary zone of increasingly lower-pressure air. That brings warmer air from the north into the Bellingshausen Sea and peninsula, pushing ice against the coast and melting some of it, and colder air from the south into the Ross Sea, which spreads the ice away from the coast and creates more of it. But why that low-pressure air is getting lower is still a subject of debate.
Scientists say that increases in greenhouse gas concentration in the atmosphere, and depletion of atmospheric ozone, have changed temperature gradients from the tropics to the poles, which affects atmospheric circulation.
The consensus now is that there is a net loss of ice from Antarctica’s ice sheets and glaciers, and it is the melting of this ice, rather than any loss of sea ice, that concerns scientists who study sea-level increases.