Global warming made the floods that devastated England and Wales in the autumn of 2000, costing £3.5bn, between two and three times more likely to happen, new research has found. This is the first time scientists have quantified the role of human-induced climate change in increasing the risk of a serious flood and represents a major development in climate science.

“It shows climate change is acting here and now to load the dice towards more extreme weather,” said Myles Allen of Oxford University, who led the work, which he started after his own home was nearly flooded in 2000. It will also have wider consequences, say experts, by making lawsuits for compensation against energy companies more likely to succeed.

It may also have billion-dollar consequences by determining which countries benefit from the future $100bn-a-year UN adaptation fund which aims to build resilience against the impacts of climate change.

“This is ground-breaking work,” said Professor Bob Watson, chief scientific adviser to the department of the environment and former chair of the Intergovernmental Panel on Climate Change. Until now, he said, scientists could state that global warming was expected to cause more extreme weather, but not that it was to blame for any specific event. “The research shows human-induced climate change is not an issue for the next decades or century: it is an issue facing us today.”

Chris Huhne, secretary of state for energy and climate change, said: “The evidence for human influence on climate is now even more compelling. Climate change is not a distant threat, it is a clear and present danger – and one that we can do something about.”

The work makes lawsuits against major polluters more likely, said barrister Richard Lord QC, an expert on climate litigation at Brick Court Chambers in London: “Showing that the chance of an event occurring has increased by say 100% or 200% gives you a much better chance of showing causation. It gets you around one of the legal obstacles.”

Lord said he could foresee such cases – which are already being attempted in the US – succeeding if international negotiations fail to control greenhouse gas emissions and the damage caused by climate impacts continues to increase.

Between September and November 2000, over 500mm of rain fell in the UK, the wettest autumn since records began in 1766. More than 10,000 homes were flooded and £3.5bn of insurance claims were made. After Allen’s home was nearly flooded, his colleague, Pardeep Pall, suggested using modelling to determine the role of global warming, but the amount of computing time required was formidable. To solve that problem, Allen used his Climateprediction.net project, through which members of the public have donated over one billion hours of PC time to running models.

The team ran models simulating the real weather and, crucially, models of the region’s weather as it might have been in a world without human-produced greenhouse gases. The models were run thousands of times to enable the probability of the extreme floods to be determined in both scenarios, taking 40,000 years of computing time. Global warming was found to have most likely doubled the risk of the 2000 floods, but there is a one in 10 chance that the increased risk was as high as 700% or as low as 20%.

The involvement of those who volunteered computer time was essential, said Allen: “I think it is absolutely fantastic what they do. It’s a great credit to them.” Allen’s team had previously shown that the scorching heatwave that struck Europe in 2003, leading to many thousands of premature deaths, was made four to six times more likely as a result of global warming. But the continent-wide nature of the heatwave meant it could be studied using broad climate models. The flood study required far more detailed modelling. “We did the easy problems first,” said Allen.

The Met Office is now in the process of setting up a service that will use its supercomputers to calculate the role of climate change in exacerbating extreme weather on a regular basis. “In future, it will not be enough for weather services to predict the weather, they will have to explain it,” said Allen, who is careful to note that some weather events may become less common due to climate change. His new WeatheratHome project is using home PCs to examine projected weather in greater detail.

Apart from litigation, the broader implications of being able to link climate change to specific weather events include determining which natural disasters have been heavily influenced by warming and which have not, and which should receive international adaptation funds as a result. “We are going to have to have a rational approach to assess risks and that would be the way to decide the priorities for spending on adaptation,” said Watson. “It will make for a more cost-effective way of using the money.”

Another study, also published in Nature, shows for the first time that the increase in extreme rain, snow and hail predicted as a consequence of global warming is real. The researchers, led by Professor Francis Zwiers at the University of Victoria, Canada, found that increases in intense rainstorms over land in the northern hemisphere, as measured by rain gauges, mirrored the projections from climate models and could not be explained by natural climate variability.

In fact, the models appear to underestimate the increase in intense rain. “The models may be giving us an overly rosy scenario,” said Professor Rowan Sutton, a climate scientist at the University of Reading, who was not involved in the research. The increase in extreme precipitation has a firm foundation in basic physics, the scientists said, as a warmer atmosphere can hold more water.

Overall, Sutton said, the new studies of extreme rain and flooding emphasise that climate change is having a tangible impact now. “It is not just affecting global average temperatures, it is affecting aspects of climate that really matter to people – and to societies and economies – in serious ways.”

Citizen science

More than 300,000 members of the public in at least 100 countries have donated spare PC time to the Climateprediction.net project. Their contributions add up to a total of about one billion hours of processing time, which might have cost $120m if bought commercially.

The colossal computing power is needed to run many repetitions of complex climate models in order to assess the probability of particular changes in climate due to global warming. Climateprediction.net has completed 80,000,000 years of modelling since its launch in 2003.

The group’s new project, WeatheratHome, increases the ability of the modelling to predict weather on a regional scale into the 2020s and 2030s. This is even more computationally intensive but promises information that is more useful on the ground. In particular, the project will examine Europe, southern Africa and the western US.

So far about 60,000 model years have been run, said team leader Myles Allen. “If anyone else wants to sign up, we have plenty of work for them!”

Other distributed computing projects include the astronomy project Einstein@home, whose team discovered a new pulsar in 2010 and published a paper about it in the journal Science. Another is Folding@home, which examines the complex process of protein assembly and has produced more than 70 papers to date.