Unusual meteorological conditions were blamed for the extremely hot, dry summer. But Peter Stott, of the Hadley Centre for Climate Prediction and Research in England, said human activity, particularly greenhouse gas emissions, at least doubled the risk of the unusual event.

"We are responsible for increasing significantly the risk of such heatwaves, largely through greenhouse gas emissions" Stott told Reuters.

"If we carry on as usual with emissions, our predictions indicate that every other year will be as hot as 2003 by the middle of the century," he added.

Stott and his colleagues studied climate change throughout the 20th century. They suspect human influence probably started altering the climate as far back as the 18th century.

"But it has only been in the last 50 years that the temperature has really started to accelerate as a result of greenhouse gas emissions," Stott said.

The scientists set out to determine the chances of having a European heatwave like last year's and whether the odds have changed.

Using climate models, they compared what the weather would have probably been like without any human influences, with simulations that included the impact of greenhouse gas emissions.

"It showed there was a significant observed warming in Europe that was associated with increasing greenhouse gas emissions," said Stott, who reported the findings in the journal Nature.

"We saw that there was a much greater risk of heatwaves now than there used to be in the pre-industrial climate."

Stott and his colleagues estimated that as much as three-quarters of the current risk of a heatwave is due to human influence on climate.

In a commentary in the journal, Swiss and German scientists described the research as a breakthrough because it is the first successful attempt to detect man-made influence on a specific extreme climate event.