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The icehouse effect
01 June 2002
From New Scientist Print Edition.
Fred Pearce

Enlarge Figure 1

The waters around Antarctica are filling up with ice. Despite recent well-publicised collapses in the ice shelves, there are over 200,000 more square kilometres of the white stuff floating in the Southern Ocean than 20 years ago.

This doesn't mean the climate change sceptics are right though. Strangely, the phenomenon is actually further proof of global warming. The ice build-up is probably caused mainly by greater snowfall, which in turn is a result of higher humidity. And that's because warmer temperatures have increased evaporation, says Jay Zwally of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

His findings, based on 20 years of satellite images, contradict predictions that up to half of Antarctica's sea ice could disappear within the next century. "I think Antarctic sea ice will continue to increase over the next decade, at least," Zwally told New Scientist. "After that maybe, maybe not."

But the findings are not completely unexpected. Suki Manabe of Princeton University suggested in 1992 that extra snowfall could cause Antarctic ice to grow faster than rising temperatures could melt it.

However, the growth is patchy. In some areas, melting does exceed the build-up of ice (see Map). Around the Antarctic peninsula, which stretches towards the tip of South America, a fifth of the sea ice disappeared between 1979 and 1998. And in March this year the melting hit the headlines when the peninsula's giant Larsen ice shelf collapsed and broke into icebergs.

"The Antarctic peninsula break-up is clearly associated with warming there," says Zwally. But this month's collapse of an ice shelf in the Ross Sea on the other side of the continent is not, because that shelf has a natural cycle of growth and collapse. Overall, the Ross Sea has 13 per cent more ice than in 1979.

Nonetheless, if the rest of the region follows the strong warming trend evident around the Antarctic peninsula, then we can still expect serious melting to get under way as global warming gathers pace.

Other Antarctic researchers back these conclusions. "Snowfall makes a significant contribution to Antarctic sea ice," says John King of the British Antarctic Survey in Cambridge. "Also, adding fresh water makes the ocean more stable and reduces the transfer of heat from the deep ocean. This makes it easier for the water to freeze."

The good news is that, if it continues, the growth in sea ice will slow global warming very slightly. The extra ice will reflect more solar energy back into space than the dark seawater it covers.

The implications for wildlife in the Southern Ocean are not clear. "A lot of bio-productivity is associated with the mixture of open water and sea ice near the outer boundary of the ice pack," says Zwally. "That long perimeter will survive whether the ice increases or decreases."

The icing up of the Antarctic is in sharp contrast to the Arctic, where up to 40 per cent of the summer ice has disappeared in the past 50 years. And the spring snowmelt in the Alaskan Arctic is eight days earlier than in the 1960s, it was reported last week. Robert Stone of the National Oceanic and Atmospheric Administration in Boulder, Colorado, says this is due not only to warmer temperatures, but also to reduced snowfall caused by changes in atmospheric circulation.

From issue 2345 of New Scientist magazine, 01 June 2002, page 6

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