Emissions of greenhouse gases have more than doubled the risk of European heatwaves similar to last year's, according to a study by UK scientists.
By Richard Black
BBC environment correspondent
In 2003, temperatures across western Europe soared by several degrees Celsius above normal - and five degrees in the case of Switzerland.
It is thought that the unusually hot summer caused tens of thousands of excess deaths.
Details of the study appear in the latest issue of the journal Nature.
Oxford University and UK Meteorological Office researchers behind the new study say it may soon be possible to hold nations and companies responsible for such events.
"This study suggests a way in which one might be able to link greenhouse gas emissions to actual harm," Oxford's Professor Myles Allen told the BBC.
He co-authored two pieces in Nature - the scientific analysis which calculated the risk, and, in an unusual collaboration with barrister Richard Lord from Brick Court Chambers in London, a commentary looking at how the law might view this science.
"I'm not suggesting we have done the whole thing yet, to the satisfaction of a judge and jury, but we are showing how a method could be applied in this direction," he said.
With other forms of environmental damage, it is relatively straightforward to assign responsibility and so determine who is liable for reparations.
Climate change is much more complex in that it is a global phenomenon, and has many causes.
This study is one of the first attempts to link rising levels of greenhouse gases to specific weather events.
Last year's European summer appears to have been the warmest for five hundred years; and by running computer models of climate, these researchers calculated that greenhouse gases from human activities have more than doubled the chances of such heatwaves occurring.
Professor Allen believes this approach could one day allow individuals harmed by climate change to seek compensation - and makes an analogy with how the courts deal with cigarette-smoking.
"People have always got lung cancer, before they started smoking," he said, "but obviously smoking significantly increases the risk of lung cancer; and on those grounds courts have, in a number of jurisdictions, decided that smoking was therefore an effective cause.
"Now, a lot depends on how much the court would want the factor in question to have increased the risk before they were prepared to intervene.
"But the interesting result coming out of this paper is that we are seeing human contributions to risk of a half, three-quarters or so, which is the kind of substantial increase in risk which starts to get the courts interested."
Already, a number of legal cases have been filed citing damage from climate change.
Just last week a delegation of conservationists and lawyers filed a petition with Unesco (United Nations Educational, Scientific and Cultural Organisation) asking it to rule that governments must cut back greenhouse gas emissions in order to conform with their legal obligations under the World Heritage Convention.
These scientists believe that more sophisticated computer models of climate will soon make it possible to assign blame for environmental harm stemming directly from increasing temperatures.
But calculating the contribution of greenhouse gas emissions to other extreme weather events, such as hurricanes, will be more difficult.
It looks like being a hot issue for Europeans - Nature's scientific paper suggests many more summers like 2003 are on the way.
"We estimate that the risk is increasing all the time as a result of the warming of the climate," Dr Peter Stott, another author on the paper, told the BBC.
"In fact, our predictions say that if we carry on without serious attempts to reduce greenhouse gas emissions, then we could be experiencing a summer like the one we had in 2003 in Europe every other year."