Now that we're at page thirty, I thought I'd try to add some new controversy into the topic:
Prehistoric humans may have pushed climate change
14:20 07 July 2010 by Anil Ananthaswamy
Humans were fiddling with climate thousands of years before the onset of agriculture - albeit unwittingly. At least, that is if we played a part in the extinction of woolly mammoths.
Until recently, anthropogenic climate change was deemed to begin with the burning of fossil fuels during the industrial revolution. Then in 2003, William Ruddiman, a palaeoclimatologist at the University of Virginia, suggested the advent of agriculture 8000 years ago ramped up levels of the greenhouse gas methane in the atmosphere, warming the world by about 0.8 °C. Now it seems we were toying with climate about 15,000 years ago.
Woolly mammoths roamed over much of Eurasia and North America from the mid-to-late Pleistocene about 300,000 years ago until numbers began to decline 15,000 years ago, before the beginning of the Holocene. The transition to the warmer Holocene is characterised by a dramatic change in the type of vegetation, from the open steppe tundra favoured by the cold-adapted mammoths to an increase in tree cover.
A previous study had shown that when elephants and other large animals are excluded from a patch of African savannah, tree cover increases by 9 per cent over 36 years. So Christopher Doughty of the Carnegie Institution of Science in Stanford, California, and colleagues wondered whether the extinction of Pleistocene megafauna like the mammoth could be behind the Holocene's shift in vegetation.
Last year, researchers at the University of Wisconsin-Madison studied fossil pollen and spores of a dung fungus found in sediment cores drilled from a North American lakebed and established that the decline in megafauna populations preceded the change in vegetation.
To quantify the effect of changing vegetation on climate, Doughty's team focused on Siberia and Beringia, the region that once formed the land bridge between eastern Siberia and Alaska. Previously collected pollen records show there was a rapid growth in dwarf trees of the genus Betula in this area around 15,000 years ago. The researchers plugged this information into a computer model to find out the effect on the climate of increasing tree cover and diminishing grassland and found that it led to a global temperature increase of about 0.1 °C (Geophysical Research Letters, DOI: 10.1029/2010gl043985).
"There is a strong connection between when humans arrived, when mammoths went extinct and when you see this big increase in vegetation," says Doughty. "They overlap almost exactly."
If humans played a role in the extinction of the mammoths, then they had a hand in the climate change that followed. "I see it as humans' first big impact on the planet," says Doughty.