The relentless heat that has blistered the United States and other parts of the world in recent years is so rare that it cannot be anything but man-made global warming, according to an analysis by a top U.S. government scientist.
The research says that the likelihood of such temperatures occurring from the 1950s through the 1980s was rarer than 1 in 300.
Now, the odds are closer to 1 in 10, according to the study by National Aeronautics and Space Administration scientist James Hansen.
He says that, statistically, what is happening is not random or normal, but pure and simple climate change.
"This is not some scientific theory. We are now experiencing scientific fact," Hansen told The Associated Press in an interview.
Hansen is a scientist at NASA's Goddard Institute for Space Studies in New York and a professor at Columbia University. He is also a strident activist who has called for government action to curb greenhouse gases for years. While his study was published online Saturday in the Proceedings of the National Academy of Science, it is unlikely to sway opinion among remaining skeptics.
However, several climate scientists praised the new work.
In a blunt departure from most climate research, Hansen's study - based on statistics, not the more typical climate modelling - blames these three heat waves purely on global warming:
- Last year's devastating Texas-Oklahoma drought.
- The 2010 heat waves 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 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 math 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.
The increase in the chance of extreme heat, drought and heavy downpours in certain regions is so huge that scientists should stop hemming and hawing, Hansen said. "This is happening often enough, over a big enough area that people can see it happening," he 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.
However, that position has shifted in recent months, as other studies too have concluded climate change is happening right before our eyes.
Hansen hopes his new study will shift people's thinking about climate change and goad governments into action.
The science in Hansen's study is excellent "and reframes the question," said Andrew Weaver, a climate scientist at the University of Victoria, 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," Weaver said.
For years, scientists have run complex computer models using combinations of various factors to see how likely a weather event would happen with and without global warming.
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 heat waves in some regions and heavy rainfall in others.
Another upcoming study by Kevin Trenberth, climate analysis chief at the National Center for Atmospheric Research, links the 2010 Russian heat wave to global warming by looking at the underlying weather that caused the heat wave. He called Hansen's paper an important one that helps to communicate the problem.
However, 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.
