Asked by: Keith Hudson, Birmingham
Ice ages are controlled by cyclic changes in the Earth's orbit and orientation, and calculations suggest another one should have begun several thousand years ago. In 2005, a team led by Professor William Ruddiman of the University of Virginia suggested that man-made global warming might be holding back the next big freeze. They argued that ancient agricultural practices, deforestation and biomass-burning may have boosted levels of carbon dioxide and methane, and thus cancelled out the cooling produced by the astronomical cycles.
Evidence for the idea has continued to accumulate. In December 2008, an international team of climate experts presented an analysis of air trapped in ice cores, which reveals the composition of the atmosphere over thousands of years. The results show that both carbon dioxide and methane began increasing around 5,000 to 8,000 years ago. This is in line with the historical origins of large-scale agriculture in Asia and extensive deforestation in Europe - and thus adds weight to the idea that human activity may indeed be holding off the next ice age.