News from Science Magazine of the imminent release of a new study:

The jury is still out on whether humans wiped out the mammoths. But researchers have found evidence that the disappearance of the woolly giants probably helped to change the climate. If the beasts were indeed hunted to extinction, that means human-driven climate change could have started long ago, the researchers say.

Like modern-day elephants, mammoths were nature’s tree pruners. Their diet included large amounts of leaves and branches from young trees, and they kept the temperate northern lands of North America, Europe, and Asia well trimmed and mostly free of forests. In particular, mammoths feasted in the grasslands that had sprung up in Beringia, the land bridge between Siberia and Alaska that now sits at the bottom of the Bering Sea. But then, starting around 15,000 years ago, mammoth populations in the region plummeted. At about the same time, a genus of birch trees called Betula, native to the northern grasslands, underwent a population explosion.

And the results suggest the expansion of Betula trees actually warmed the earth a bit:

The results, the researchers report in a paper to be published in an upcoming issue of Geophysical Research Letters, suggest that when the mammoths disappeared, the Betula trees expanded across Beringia, forming forests that replaced as much as one-quarter of the grassland. The trees’ leaves, which are darker than grasses, absorbed more solar radiation, and their trunks and branches, which jutted above the snowpack, continued the effect even in winter. The researchers calculated that the mammoths’ disappearance contributed at least 0.1?C to the average warming of the world around 15,000 years ago. Within Beringia, the warming due to the loss of the mammoths was probably closer to 0.2?C, the team concluded.