Some of the weather extremes bedeviling people around the world have become far more likely because of human-induced global warming, researchers reported on Tuesday. Yet they ruled it out as a cause of last year’s devastating floods in Thailand, one of the most striking weather events of recent years.
A new study found that global warming made the severe heat wave that afflicted Texas last year 20 times as likely as it would have been in the 1960s. The extremely warm temperatures in Britain last November were 62 times as likely because of global warming, it said.
The findings, especially the specific numbers attached to some extreme events, represent an increased effort by scientists to respond to a public clamor for information about what is happening to the earth’s climate. Studies seeking to discern any human influence on weather extremes have usually taken years, but in this case, researchers around the world managed to study six events from 2011 and publish the results in six months.
Some of the researchers acknowledged that given the haste of the work, the conclusions must be regarded as tentative.
“This is hot new science,” said Philip W. Mote, director of the Climate Change Research Institute at Oregon State University, who led the research on the Texas heat wave and drought. “It’s controversial. People are trying different methods of figuring out how much the odds may have shifted because of what we have put into the atmosphere.”
The general conclusion of the new research is that many of the extremes being witnessed worldwide are consistent with what scientists expect on a warming planet. Heat waves, in particular, are probably being worsened by global warming, the scientists said. They also cited an intensification of the water cycle, reflected in an increase in both droughts and heavy downpours.
The study on extreme weather was released along with a broader report on the state of the world’s climate. Both are to be published soon in the Bulletin of the American Meteorological Society. The broad report found no surcease of the climate trends that have led to widespread concern about the future.
The Arctic continued to warm more rapidly than the planet as a whole in 2011, scientists reported, and sea ice in the Arctic was at its second-lowest level in the historical record. In 2010, rains were so heavy that the sea level actually dropped as storms moved billions of gallons of water onto land, they said, but by late 2011 the water had returned to the sea, which resumed a relentless long-term rise.
So far this year in the United States, fewer weather disasters seem to be unfolding than in 2011. But it is still turning out to be a remarkable year, with wildfires, floods, storms that knocked out electrical power for millions and sizzling heat waves in March and June.
Globally, the new research makes clear that some of the recent weather damage resulted not from an increased likelihood of extremes, but from changes in human exposure and vulnerability. The 2011 floods in Thailand are a prime example.
An analysis by Dutch and British scientists found that the amount of rain falling in Thailand last year, while heavy, was not particularly unusual by historical standards, and that “climate change cannot be shown to have played any role in this event.”
More important, the researchers said, was rapid development in parts of Thailand. Farm fields have given way to factories in the floodplains of major rivers, helping to set the stage for the disaster.
In the new report, researchers in Oregon and Britain found that natural climate variability played a big role in setting the stage for the heat wave in Texas. The weather in 2011 was heavily influenced by a weather pattern called La Niña, which has effects worldwide, including making drought in the American Southwest more likely.
But even taking that into account, the researchers found, the overall warming of the planet since the 1960s made it about 20 times as likely that such a heat wave would occur in Texas in a La Niña year.
Martin P. Hoerling, a meteorologist with the National Oceanic and Atmospheric Administration who was not involved in the new study but is conducting his own research on the Texas disaster, agreed that human-induced global warming had probably made the odds of record-setting heat somewhat more likely. But he said his research showed that the rainfall deficits were unrelated to global warming.
He said he was skeptical about several aspects of the new paper, including the claim of a 20-fold increase in likelihood.
More broadly, he said he was worried that the newly published studies had been done so hastily that the conclusions may not stand the test of time. “We need to think carefully about what kind of questions we can credibly pursue with this sort of rapid turnaround,” Dr. Hoerling said.