To estimate how much additional area the birch might have covered, they started with the way modern-day elephants affect their environment by eating plants and uprooting trees. If mammoths had effects on vegetation similar to those of modern elephants , then the fall of mammoths would have allowed birch trees to spread over several centuries, expanding from very few trees to covering about one-quarter of Siberia and Beringiathe land bridge between Asia and Alaska. In those places where there was dense vegetation to start with and where mammoths had lived, the main reason for the spread of birch trees was the demise of mammoths, the model suggests.
Another study, published last year, shows that "the mammoths went extinct, and that was followed by a drastic change in the vegetation," rather than the other way around, Doughty says. "With the extinction of this keystone species, it would have some impact on the ecology and vegetationand vegetation has a large impact on climate."
Doughty and colleagues then used a climate simulation to estimate that this spread of birch trees would have warmed the whole planet more than 0.1 degrees Celsius (0.18 degrees Fahrenheit) over the course of several centuries. (In comparison, the planet has warmed about six times more during the past 150 years, largely because of people's greenhouse gas emissions.)
Only some portionabout one-quarterof the spread of the birch trees would have been due to the mammoth extinctions, the researchers estimate. Natural climate change would have been responsi
|Contact: Maria-Jose Vinas|
American Geophysical Union