They found that global warming from human emissions of greenhouse gases has increased the odds of the hottest events across more than 80 percent of the surface area of the globe for which observations were available. "Our results suggest that the world isn't quite at the point where every record hot event has a detectable human fingerprint, but we are getting close," Diffenbaugh said. For the driest and wettest events, the authors found that human influence on the atmosphere has increased the odds across approximately half of the area that has reliable observations. "Precipitation is inherently noisier than temperature, so we expect the signal to be less clear," Diffenbaugh said. "One of the clearest signals that we do see is an increase in the odds of extreme dry events in the tropics. This is also where we see the biggest increase in the odds of protracted hot events - a combination that poses real risks for vulnerable communities and ecosystems." The Stanford research team, which includes a number of former students and postdocs who have graduated to positions at other universities, has been developing their extreme event framework in recent years, focusing on individual events such as the 2012-2017 California drought and the catastrophic flooding in northern India in June 2013.

In the new study, a major goal was to test the ability of their framework to evaluate events in multiple regions of the world, and to extend beyond extreme temperature and precipitation, which have been the emphasis of most event attribution studies. Test cases One high-profile test case was Arctic sea ice, which has declined by around 40 percent during the summer season over the past three decades. When the team applied their framework to the record-low Arctic sea ice cover observed in September 2012, they found overwhelming statistical evidence that global warming contributed to the severity and probability of the 2012 sea ice measurements. "The trend in the Arctic has been really steep, and our results show that it would have been extremely unlikely to achieve the record-low sea ice extent without global warming," Diffenbaugh said.

Another strength of a multi-pronged approach, the team says, is that it can be used to study not only the weather conditions at the surface, but also the meteorological "ingredients" that contribute to rare events. "For example, we found that the atmospheric pressure pattern that occurred over Russia during the 2010 heat wave has become more likely in recent decades, and that global warming has contributed to those odds," said co-author Daniel Horton, an assistant professor at Northwestern University in Evanston, Illinois, and a former postdoc in Diffenbaugh's lab who has led research on the influence of atmospheric pressure patterns on surface temperature extremes.

"If the odds of an individual ingredient are changing - like the pressure patterns that lead to heat waves - that puts a thumb on the scales for the extreme event." Diffenbaugh sees the demand for rigorous, quantitative event attribution growing in the coming years. "When you look at the historical data, there's no question that global warming is happening and that extremes are increasing in many areas of the world," he said. "People make a lot of decisions - short term and long term - that depend on the weather, so it makes sense that they want to know whether global warming is making record-breaking events more likely. As scientists, we want to make sure that they have accurate, objective, transparent information to work with when they make those decisions."

Source: ANI