The unpublished IEA study found carbon emissions from burning fossil fuels had dropped significantly this year — further than in any year in the past four decades. Falling industrial output is largely responsible for the plunge in emissions, but other factors also played a role, including shelving plans for new coal-fired power stations because of falling demand and lack of financing.

The fall will exceed the drop in the 1981 recession that followed a crisis in the oil markets, according to the results of study published in the Financial Times newspaper. “We have a new situation, with the changes in energy demand and the postponement of many energy investments,” said Fatih Birol, the IEA’s chief economist.

“But this only has meaning if we can make use of this unique window of opportunity. (That means) a deal in Copenhagen.”

The December meeting in Copenhagen, under the UN Framework Convention on Climate Change, aims to set down action for tackling heat-trapping carbon emissions beyond 2012, when the current provisions of the Kyoto Protocol run out.

Government policies to cut emissions have also had an impact on emission levels, with the IEA estimating that about a quarter of the reduction is the result of regulation.