Global warming is to blame for most extreme hot days and almost a fifth of heavy downpours, according to a scientific study released on Monday.
“Already today 75 percent of the moderate hot extremes and about 18 percent of the moderate precipitation extremes occurring worldwide are attributable to warming,” Reuters quoted scientists at the Swiss university ETH Zurich as saying in the study.
The rest were due to natural swings in the weather, the study said.
“Moderate” extremes are the type expected in any place once in 1,000 days. In Britain, for example, that is 33.2 degrees Celsius in southeast England or 27 degrees further north in Edinburgh, the news agency said, citing the UK Met Office Hadley Centre.
The scientists, Erich Fischer and Reto Knutti, noted that a United Nations study last year found that it was at least 95 percent probable that most warming since the mid-20th century was man-made.
The study, published in the journal Nature Climate Change, was the first to give global percentages for how warming affects extreme heat and downpours.
“We were surprised,” Fischer said of the findings, especially by the high impact of global warming on heat extremes.
Last year was the warmest since records began in the 19th century, according to the UN’s World Meteorological Organization. Heavy flooding hit countries including Serbia, Bangladesh, Pakistan, South Africa, Morocco and Brazil.
The scientists compared rising numbers of hot days and downpours in modern computer models with historical records stretching back more than a century. They found that warming was loading the dice in favor of extremes.
“The approach here is reminiscent of medical studies, where it is not possible to attribute a single fatality from lung cancer to smoking,” they said. Even so, smoking raised the overall risks of cancer in the same way as warming increased risks of extremes.
Peter Stott, of the Hadley Centre, said the findings show the impact of global warming in statistical terms.
“The weakness is that you can’t apply this method to individual events,” Stott said.
Global average temperatures have risen 0.85 degrees Celsius above pre-industrial times so far, and further warming would increase risks of extremes, according to the study.
A rise in temperatures to a UN ceiling of 2 degrees Celsius above pre-industrial times would raise the share of heat extremes attributable to warming to 96 percent and the share of extreme precipitation events to 40 percent, it said.
Government representatives will meet at a UN summit in Paris in December to agree a global plan beyond 2020 to limit climate change.
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