The relentless, weather-gone-crazy type of heat that has blistered the United States, Canada and other parts of the world in recent years is so rare it can't be anything but man-made global warming, according to a new statistical analysis from a top American scientist.

The research by a man often called the "godfather of global warming" says that, from the 1950s through the 1980s, the likelihood of such sweltering temperatures occurring was rarer than 1 in 300. Now, the odds are closer to 1 in 10, according to the study by James Hansen. The NASA scientist says that statistically, what's happening is not random or normal, but pure and simple climate change.

James E. Hansen, head of the NASA Goddard Institute for Space Studies, says global warming is no longer 'some scientific theory.' (Mary Altaffer/Associated Press)

"This is not some scientific theory," Hansen told The Associated Press in an interview. "We are now experiencing scientific fact."

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 the remaining climate change skeptics.

Several climate scientists are praising his new work.

In a blunt departure from most climate research, Hansen's study — based on statistics, not the more typical climate modelling — blames three heat waves purely on global warming:

• The 2011 drought that devastated Texas and Oklahoma.
• 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 and Central Canada this year. But Hansen believes this is simply another prime example of global warming at its worst.

Simple mathematics

The new research makes the case for the severity of global warming in a different way than most scientific studies and uses simple mathematics instead of relying on complex climate models or an understanding of atmospheric physics. It also doesn't 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's impossible to say whether single events are caused by global warming, because of the influence of natural weather variability. However, that position has been shifting in recent months, as other studies 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. He wrote an op-ed piece that appeared online Friday in the Washington Post.

"There is still time to act and avoid a worsening climate, but we are wasting precious time," he wrote.

The science in Hansen's study is excellent "and reframes the question," said Andrew Weaver, a climate scientist at the University of Victoria in British Columbia 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 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 heat waves in some regions and heavy rainfall in others.

Another upcoming study by Kevin Trenberth, climate analysis chief at the U.S. National Centre 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 communicate the problem.

Disagreement likely

But there is bound to be continued disagreement. Previous studies had been unable to link the two, and one by the U.S. 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 in a statement. But he also said it is true that scientists can't 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."

Skeptical scientist John Christy of the University of Alabama at Huntsville said Hansen shouldn't have compared recent years to the 1950s-1980s time period because he said that was a quiet time for extremes. But Derek Arndt, director of climate monitoring for the U.S. government's National Climatic Data Center, said that range is a fair one and often used because it is the "golden era" for good statistics.

Granger Morgan, head of engineering and public policy at Carnegie Mellon University in Pittsburgh, Penn., called Hansen's study "an important next step in what I expect will be a growing set of statistically based arguments."

In a landmark 1988 study, Hansen predicted that if greenhouse gas emissions continue, which they have, Washington, D.C., would have about nine days each year of 35 C or warmer in the decade of the 2010s. So far this year, with about four more weeks of summer, the city has had 23 days with the temperature reaching at least 35 C.