A leading climate professor says that new evidence which further reinforces the connection between global warming and extreme rainfall is "extremely important" in setting out a methodology which could one day be used to sue energy companies for climate damage.
Two studies were published in the journal Nature on 16 February, using complex computer simulation models to assess the contribution of planet-warming gases to intense precipitation events.
One paper found that greenhouse gas emissions substantially increased the risks of the autumn 2000 floods in the UK.
In nine out of ten cases, their model saw the flood risk increased by more than 20%, and in two thirds of cases, the increase was over 90%.
Professor Carlo Jaeger of the Potsdam University for Climate Impact Research, who compiled a study linking carbon emissions to the 2003 heatwave in Europe, told EURACTIV that the methodology used in the study had "dramatic consequences," even if it was still evolving.
"It is extremely important that we have a method which enables us to assess fractions of probability, to assess how strongly the dice was loaded because in court, that's actually how liability issues are settled," he said.
"If you have an accident, a court [might] ask: 'What was the contribution of the manufacturing supplier or whatever to this accident?' Damages are then allocated by such proportions."
Autumn 2000 deluge in the UK
The autumn 2000 deluge, which flooded more than 10,000 homes, took place amid the wettest British autumn since records began in 1766 – and caused £1.3 billion of insurance losses.
If climate change could be conclusively shown to have enhanced the probability of flooding by 20%, then the same proportion of the damages could theoretically be recouped in damages from companies that had contributed to it.
"Whether this will ever happen, whether this has a chance of success, is another story," Professor Jaeger admitted. The computer systems involved in the study depended heavily on seasonal forecasting which is not always reliable, he said.
"But these methods are evolving and I have no doubt that we will be able to establish fractions of attributable risk and allocate fractions of damage," he added.
The methodology used by the University of Oxford research team who produced the report involved running thousands of computer simulations comparing the atmosphere as it was with a projection of what it would have been without anthropogenic greenhouse gas emissions.
The impact of the subsequent rainfall projections on British river basins was then assessed.
Over one billion hours of computer time were donated by 300,000 members of the public to help the research through the climateprediction.net project.
Causal link difficult to make
While no scientist can yet definitively claim to have discovered a causal link between any single weather event and the human emission of planet-warming gases, a warmer atmosphere is known to hold more water because of accelerated evaporation processes.
One formula holds that for every degree of warming near the Earth's surface, there will be 6-7% more moisture in the air.
Another research paper in the same issue of the 'Nature' journal went further, claiming to have proved a link between greenhouse gas emissions and the increase in northern hemispheric floods.
The climate projections of researchers from the University of Victoria in Canada were found to mirror actual rain gauge measurements so closely that the results could not be explained by natural variability.
If anything, the report said, "the impacts of future changes in extreme precipitation may be underestimated" because models tend to underestimate observed warming-related floods.
Professor Jaeger cautioned that "it is not very wise to always cry wolf and create very dramatic images". But he noted two immediate consequences of the study.
"First, it is very good to plant trees. There is something to be said for large scale reforestation efforts in the Mediterranean and other areas. Secondly, we must give more space to our rivers. Not doing so dramatically increases the probability of floods even without climate change."
It was something we should have learned already, "and by the way, rivers look more beautiful if you give them more space," he concluded.