Only some portion — about one-quarter — of the spread of the birch trees would have been due to the mammoth extinctions, the researchers estimate. Natural climate change would have been responsible for the rest of the expansion of birch trees. Nonetheless, this suggests that when hunters helped finish off the mammoth, they could have caused some global warming.
In Siberia, Doughty says, "about 0.2 degrees C (0.36 degrees F) of regional warming is the part that is likely due to humans."
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