Alaskan tundra thaws in warming world
Arctic puddles point to melting ice, but their effect is hard to predict.
Parts of Alaska that have been frozen for more than 3,000 years are now puddled with water, according to a team of scientists working in this northernmost US state.
Large parts of northern territories such as Alaska and Siberia contain permafrost: soil that stays frozen all year round. The layer of soil above permafrost may freeze and thaw with the seasons, allowing vegetation to grow. But the ground below remains permanently rock-hard with ice.
In recent years, scientists have grown concerned that rising temperatures in Arctic regions might melt this ice, potentially releasing greenhouse gases and affecting the local ecology. Temperature records show that Alaska has warmed much more than the global average over recent years, notes Colin Prentice, a specialist in the interactions between climate and ecosystems, based at the University of Bristol, UK.
So far there has been little clear indication of serious permafrost melting. Last year, Russian scientists announced that Siberia is becoming peppered with new lakes, but their report was largely anecdotal.
University of Bristol
Their comparison of aerial photographs from 1945, 1982 and 2001 shows that the number of waterlogged pits in 600,000-square-metre region on the Beaufort coastal plain has grown by a factor of about 74 in the past two decades.
Their field observations in the same area reveal how these pits are formed. Large wedges of ice, which sit in the permafrost like nails with their tops some 30 centimetres beneath the surface, melt in the heat of a warm summer. This causes the surrounding permafrost to thaw and the ground to slump, creating a small pond.
It's the pits
Jorgensen and colleagues conclude that anomalously warm summers between 1989 and 1998 are mostly responsible for the changes they see. They estimate that potentially up to a third of Arctic lowland tundra could become similarly affected in coming years.
No one really knows what the consequences of this might be. "It's one of the jokers in the pack," says Prentice.
If melting of permafrost leads to the creation of waterlogged ground, as the Alaskan researchers have seen, this could increase the amount of methane the soil releases, owing to increased activity of anaerobic microbes.
If the soil instead dries out, then it might decompose, releasing carbon dioxide into the air. "Either way you'll see an increased release of greenhouse gases," says Prentice.
This means that permafrost melting should produce a positive feedback on climate, amplifying the warming that causes it.
But Prentice says we shouldn't panic yet. He points out that during the last warm period between ice ages, large areas of what is now permafrost must have thawed, and yet there is no sign, in the trapped air within ancient polar ice sheets, of a consequent surge in atmospheric carbon dioxide or methane.
"I'm a little bit sceptical about some of the more alarming predictions," says Prentice.
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- Jorgensen M. T., et al. Geophys. Res. Lett. 33 L02503 (2006).