July 15, 2010

Woolly Mammoth Hunters Contributed To Climate Change

According to a study of prehistoric climate change, ancient hunters who stalked the world's last woolly mammoths likely helped warm the Earth's far northern latitudes thousands of years before humans started to burn fossil fuels.

Researchers at the Carnegie Institution for Science decided that the demise of the leaf-chomping woolly mammoths contributed to a proliferation of dwarf birch trees in and around the Arctic, darkening a largely barren, reflective landscape and accelerating a rise in temperatures across the polar north.

The study found that the northward march of vegetation affected the climate because of the "albedo effect," in which replacement of white snow and ice with darker land surfaces absorbs more sunlight and creates a self-repeating warming cycle.

The end of the last Ice Age was already under way when the extinction of wooly mammoths began because it was marked by a worldwide rise in temperatures and the dramatic retreat of glaciers that once covered much of the Northern Hemispheres.

However, the latest findings suggest that human activity played a role in altering Earth's climate long before mankind began burning coal and oil for energy.

Chris Field, director of the Carnegie Institution's Department of Global Ecology and a co-author of the study, said on Tuesday that if mammoth hunters helped hasten Arctic warming, that would potentially be the first such human impact on climate.

He said that with the advent of agriculture about 7,000 years ago at more southern latitudes, humans are believed to have modified the climate through deforestation and cultivation of new plants.

The earlier climate consequences of declining mammoth populations were extremely subtle.

The study found that flourishing plant life as the vegetarian beast were disappearing about 15,000 years ago helped warm the Arctic and boreal regions in what is now Siberia and North America by 32 degrees Fahrenheit over a period of several centuries.

According to the Intergovernmental Panel on Climate Change, ancient human-caused warming was tiny compared to modern-day warming, in which the Earth's temperature rose about 1.33 degrees F since the start of the 20th century, with temperatures rising at least twice as fast in the Arctic.

The scientists said that the research attributes about a fourth of the Arctic's vegetation-driven warming to the decline of the woolly mammoth.  If human hunters helped kill off the beasts, then they bear some responsibility for warming the climate.

"We're not saying this was a big effect," Field said. "The point of the paper isn't that this is a big effect. But it's a human effect."

The study analyzed pollen records in sediments of lakes in Alaska, Siberia and Canada's Yukon Territory.  Scientists were able to reconstruct forest growth in what was once a woolly mammoth habitat.

The researchers also analyzed behavior of African elephants as they knock down trees to dine on the leaves that they prefer to less-nutritious grasses.

Field said that the Earth already was warming at the time the mammoths were disappearing, but there is evidence that dramatic growth of vegetation in the far North followed the large animals' demise rather than preceded it.

"What we tried to do was say how much of the tree increase was due to the extinction of mammoths," he told Reuters.

However, he said it was not possible to quantify how much of the extinction was because of human hunting.  He said that whether hunters ultimately pushed mammoths back over the brink remains a mystery.

If humans did kill off the mammoths, "I'm sure they didn't have anything but a very local picture of what they were doing," Field said.

The findings are scheduled to be published in the journal Geophysical Research Letters.


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