Earlier, scientists typically avoided linking individual weather events to climate change, citing the challenges of teasing apart human influence from the natural variability of the weather.
But that is changing. "Over the past decade, there's been an explosion of research, to the point that we are seeing results released within a few weeks of a major event," said Diffenbaugh, who is also the Kimmelman Family Senior Fellow at the Stanford Woods Institute for the Environment. In a new study, published in Proceedings of the National Academy of Sciences, Diffenbaugh and a group of current and former Stanford colleagues outline a four-step "framework" for testing whether global warming has contributed to record-setting weather events.
The new paper is the latest in a burgeoning field of climate science called "extreme event attribution," which combines statistical analyses of climate observations with increasingly powerful computer models to study the influence of climate change on individual extreme weather events. Climate change fingerprints In order to avoid inappropriately attributing an event to climate change, the authors began with the assumption that global warming had played no role, and then used statistical analyses to test whether that assumption was valid. "Our approach is very conservative," Diffenbaugh said. "It's like the presumption of innocence in our legal system: The default is that the weather event was just bad luck, and a really high burden of proof is required to assign blame to global warming." The authors applied their framework to the hottest, wettest and driest events that have occurred in different areas of the world.
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."