Last year's heatwave, which claimed an estimated 20,000 lives across Europe, could have been a scorching taste of summers to come, Swiss scientists warn.
By Alex Kirby
BBC News Online environment correspondent
Researchers believe the blistering temperatures may have been a result of variability caused by climate change.
In the online edition of the journal Nature the scientists say only global warming could have caused the event.
They believe greenhouse gases are not only raising the overall temperature, but increasing weather variability too.
Christoph Schar and colleagues at the Swiss Federal Institute of Technology, Zurich, say the 2003 heatwave was extremely unusual, even with the steady rise in global average temperatures since 1850.
'Every other year'
They write: "The European summer climate might experience a profound increase in year-to-year variability in response to greenhouse forcing.
"Such an increase might be able to explain the unusual European summer of 2003, and would strongly affect the incidence of heatwaves and droughts in the future."
The scientists used computer-driven weather models to see whether climate variability (the difference between weather extremes) would increase in line with average temperature.
One simulation suggested that, towards the end of this century, every second summer in Europe could be as hot and dry as last year's.
Dr Schar says the 2003 summer is the only evidence of increased variability he has seen so far. Others say they have seen no evidence at all of variability.
But Professor John Christy, director of the Earth System Science Center at the University of Alabama, Huntsville, US, told Nature's Science Update website: "I am not an alarmist - I deal with real data, and the data don't show this kind of effect."