Flood risk 'doubled' by greenhouse pollution
SCIENTISTS have shown for the first time that human activity has made extreme rainfall and floods around the globe worse in recent decades.
Increases in greenhouse gas emissions have also been linked, in a separate study, to a specific flood event – a devastating inundation in Britain 10 years ago.
"We found that emissions substantially increased the odds of floods occurring in ... the record wet autumn of 2000, with a likely increase in odds of about a doubling or more," said Pardeep Pall, a researcher at the University of Oxford and the lead author of the study.
It was not possible, however, to extrapolate from this event to other ones, such as the recent Pakistan or Queensland floods, and blame global warming for their severity, the scientists said.
"It takes a long time to determine whether human influence on the climate system was a factor in any particular event," said Francis Zwiers, director of the Climate Research Division of Environment Canada in Toronto.
Dr Zwiers and his team examined daily records from 6000 weather stations in the northern hemisphere and found that between 1951 and 1999 the intensity of extreme rains and floods increased by 7 per cent.
They compared these observed changes to those predicted by climate model simulations, and identified similar patterns of intensification.
"Our research provides the first scientific evidence that human-induced greenhouse gas increases have contributed to the observed intensification of heavy precipitation events over large parts of the northern hemisphere," said a team member, Xuebin Zhang.
Although they had not studied rainfall data from the southern hemisphere, the same effect was likely to have occurred there, Dr Zwiers said.
A global change in atmospheric composition is thought to be the cause.
"Warmer air contains more moisture and leads to more extreme precipitation," he said.
Both studies are published in the journal Nature.
Autumn 2000 was the wettest in England and Wales since record-keeping began in 1766, and the floods damaged nearly 10,000 properties, with estimated losses of £1.3 billion.
Dr Pall's team generated thousands of climate model simulations of the autumn, under both realistic conditions and in a "parallel world" where no greenhouse gases had been emitted in the 20th century.
The team found that flooding risk was "substantially increased" by these emissions, although the exact magnitude of the contribution remains uncertain.
Dr Allen said scientists were developing systems they hoped would be able to quickly establish the likelihood of a particular weather event having been influenced by climate change.
This would be important for proper distribution of government resources for adaptation to climate change.
It would also makes the risks of a warming world more real to people, because extreme weather events are so damaging.