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Human-induced global warming may have started much earlier than generally acknowledged — and could even have saved the planet from experiencing a new ice age — according to a report in the journal Climatic Change.

William Ruddiman of the University of Virginia, United States, says that climate models predict that the Earth should have experienced an ice age by now. The Earth is currently going through one of its longest periods between ice ages — known as an inter-glacial. Previous inter-glacials have tended to last about 10,000 years. The present one has been going for 14,000 years.

Ruddiman argues that this may be partly due to the activities of farmers who began releasing carbon into the atmosphere by clearing the forests of Europe, China and India about 8,000 years ago.

Global warming is caused by increases in heat-trapping 'greenhouse gases' such as carbon dioxide and methane. The most recent rise in carbon dioxide has been caused by the burning of fossil fuels.

According to Ruddiman, however, changes in human activities thousands of years ago led to an increase in the concentration of methane in the atmosphere. "Irrigation was first used to grow rice in Southeast Asia about 5,000 years ago," he says. ""Because flooded rice fields are a major source of methane, irrigation [was] probably the main factor behind the anomalous methane increase that began at that time."