Ancient man saved the world from a new Ice Age. That is the startling conclusion of climate researchers who say man-made global warming is not a modern phenomenon and has been going on for thousands of years.
Prehistoric farmers who slashed down trees and laid out the first rice paddies and wheatfields triggered major alterations to levels of greenhouse gases such as methane and carbon dioxide in the atmosphere, they say.
As a result, global temperatures - which were slowly falling around 8,000 years ago - began to rise. 'Current temperatures would be well on the way toward typical glacial temperatures, had it not been for the greenhouse gas contributions from early farming practices,' says Professor William Ruddiman of Virginia University.
The theory, based on studies of carbon dioxide and methane samples taken from Antarctic ice cores, is highly controversial - a point acknowledged by Ruddiman. 'Global warming sceptics could cite my work as evidence that human-generated greenhouse gases played a beneficial role for several thousand years by keeping the Earth's climate more hospitable than it would otherwise have been,' he states in the current issue of Scientific American.
'However, others might counter that, if so few humans with relatively primitive technologies were able to alter the course of climate so significantly, then we have reason to be concerned about the current rise of greenhouse gases to unparalleled concentrations at unprecedented rates.'
Elaborating on his theory, Ruddiman said: 'Rice paddies flooded by irrigation generate methane for the same reason that natural wetlands do - vegetation decomposes in the stagnant water. Methane is also released as farmers burn grasslands,' Ruddiman points out.
Similarly, the cutting down of forests had a major effect. 'Whether the fallen trees were burnt or left to rot, their carbon would soon have been oxidised and ended up in the atmosphere as carbon dioxide.'
Computer models of the climate made by scientists at the University of Wisconsin-Madison suggest this rise in carbon dioxide and methane would have had a profound effect on Earth: without man's intervention, our planet would be 2C cooler than it is now, and spreading ice caps and glaciers would affect much of the world.
The idea that ancient farming may have had an impact on Earth's climate was given a cautious welcome by Professor Paul Valdes, an expert on ancient climate change based at Bristol University.
'This is a very interesting idea,' he told The Observer. 'However, there are other good alternative explanations to explain the fluctuations that we see in temperature and greenhouse gas levels at this time. For example, other gases interact with methane and carbon dioxide in the atmosphere and changes in levels of these could account for these increases in greenhouse gases.'