The relentless, weather-gone-crazy heat that has blistered the United States and other parts of the world in recent years is so rare it cannot be anything but man-made global warming, a top Nasa scientist says.
The research, by a man often called the "godfather of global warming" says the likelihood of such temperatures occurring from the 1950s through the 1980s was rarer than one in 300. Now the odds are closer to 1 in 10, according to the study by Professor James Hansen.
He says that statistically what is happening is not random or normal, but simply climate change.
"This is not some scientific theory. We are now experiencing scientific fact," Prof Hansen said.
Prof Hansen is a scientist at Nasa's Goddard Institute for Space Studies in New York and a professor at Columbia University. But he is also a strident activist seeking government action to curb greenhouse gases.
But his study, published online in the Proceedings of the National Academy of Science, is unlikely to sway opinion among the remaining climate change sceptics, although several scientists have praised the new work.
In a blunt departure from most climate research, Prof Hansen's study - based on statistics, not the more typical climate modelling - blames these three heatwaves purely on global warming:
:: Last year's devastating Texas-Oklahoma drought;
:: The 2010 heatwaves in Russia and the Middle East, which led to thousands of deaths;
:: The 2003 European heat wave blamed for tens of thousands of deaths, especially among the elderly in France.
The analysis was written before the current drought and record-breaking temperatures that have seared much of the United States this year. But Prof Hansen believes this too is another prime example of global warming at its worst.
The new research makes the case for the severity of global warming in a different way than most scientific studies and uses simple maths instead of relying on complex climate models or an understanding of atmospheric physics.
It also does not bother with the usual caveats about individual weather events having numerous causes.
"This is happening often enough, over a big enough area that people can see it happening," Prof Hansen said.
Scientists have generally responded that it is impossible to say whether single events are caused by global warming because of the influence of natural weather variability.
But that position has been shifting in recent months, as other studies too have concluded climate change is happening right before our eyes.
Prof Hansen hopes his new study will shift people's thinking about climate change and goad governments into action. He wrote an opinion piece that appeared online in the Washington Post on Friday. saying: "There is still time to act and avoid a worsening climate, but we are wasting precious time."
The science in Prof Hansen's study is excellent "and reframes the question", said Prof Andrew Weaver, a climate scientist at the University of Victoria in British Columbia, Canada, who was a member of the Nobel Prize-winning international panel of climate scientists that issued a series of reports on global warming.
"Rather than say, 'Is this because of climate change?' That's the wrong question. What you can say is, 'How likely is this to have occurred with the absence of global warming?' It's so extraordinarily unlikely that it has to be due to global warming," Prof Weaver said.
For years scientists have run complex computer models using combinations of various factors to see how likely a weather event would happen without global warming and with it.
About 25 different aspects of climate change have been formally attributed to man-made greenhouse gases in dozens of formal studies. But these are generally broad and non-specific, such as more heatwaves in some regions and heavy rainfall in others.
Another upcoming study by Kevin Trenberth, climate analysis chief at the National Centre for Atmospheric Research, links the 2010 Russian heatwave to global warming by looking at the underlying weather that caused it. He called Prof Hansen's paper an important one that helped communicate the problem.
But previous studies had been unable to link the two, and one by the National Oceanic and Atmospheric Administration concluded that the Russian drought, which also led to devastating wildfires, was not related to global warming.
White House science adviser John Holdren praised the paper's findings but also said it was true that scientists cannot blame single events on global warming.
"This work, which finds that extremely hot summers are over 10 times more common than they used to be, reinforces many other lines of evidence showing that climate change is occurring and that it is harmful," he said.
Prof Hansen says he hopes his study will spur action including a tax on the burning of fossil fuels, which emit carbon dioxide, a key greenhouse gas, but others doubt it.
Science policy expert Roger Pielke of the University of Colorado said in an email: "Hansen is pursuing a deeply-flawed model of policy change, one that will prove ineffectual and with its most lasting consequence a further politicisation of climate science (if that is possible!)."
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