A new study shows it's now possible for scientists to confidently measure the influence of climate change on some extreme weather events, such as heat waves.
In this first-ever assessment of the emerging field of climate science attribution, researchers from the National Academies of Sciences, Engineering and Medicine are setting the record straight on how well scientists can currently tease out the climate-change fingerprints of different types of extreme weather.
In 2015, the United States was hit by myriad extreme weather events: record snowfall in the Northeast; flooding in North Carolina, Texas and Oklahoma; an ongoing drought in California. And America wasn't alone. India experienced a heat wave that killed more than 2,300 people, and the strongest ever tropical cyclone was observed in the Pacific Ocean last year.
Following these events, climate scientists were repeatedly asked the same question by politicians, the public, even the media: Was that event caused by climate change?
"In the past, a typical scientist's response to questions about climate change's role in any given weather event was, 'We cannot attribute any single event to climate change,'" scientists on the National Academies' Committee on Extreme Weather Events and Climate Change wrote in a report published Friday. "The science has advanced to the point that this is no longer true as an unqualified statement."
So when does such a noncommittal answer no longer hold up?
According to the report, scientists have the most confidence in their ability to discern climate change impacts on temperature extremes—whether they are warmer than usual, or not as cold as usual. For these two types of weather events, scientists have a lot of historical data, a strong understanding of how climate change influences the physics of them, and climate models that can accurately replicate past events.
This doesn't mean every heat wave should be assumed to be strongly tied to climate change, clarified J. Marshall Shepherd, a study author and climate researcher at the University of Georgia. Nor does it mean climate change is always influencing extreme weather events in the same way, making them stronger or weaker, more frequent or less frequent, Shepherd told InsideClimate News. It just means that scientists now have the tools to figure out how climate change is impacting notable weather events.
This isn't true for all extreme weather. The study explains that researchers have only moderate confidence in their ability to pin down the link for droughts and extreme rainfall; wildfire is another example of an event that is harder to attribute to climate change, since so many human factors may be involved.
And scientists have even less confidence in their ability to conduct such an analysis for snowstorms, tropical cyclones and extratropical cyclones.
Finally, the report clarified that scientists are currently unable to run such studies for isolated thunderstorms because there's too little data and too many questions about their link to climate change.
Climate Change Is Impacting the Weather. Period.
According to Shepherd, it's time to stop asking whether events are caused by climate change.
"We can essentially lay it to rest, put the tombstone on it," Shepherd said during a press conference following the report's release.
Scientists are in agreement that "climate change is impacting the weather," Martin Hoerling, a scientist at the National Oceanic and Atmospheric Administration, told InsideClimate News.
What's more important—and the focus of this study—is figuring out by how much and in what way climate change is influencing these events, Hoerling said. Specifically, whether climate change influences the frequency and/or the intensity of such events.
The study did not assess the climate influence of any particular event, such as the ongoing drought in California, or grade the findings of past attribution studies.
The 163-page report did review the approaches of attribution studies in the past. Those studies relied primarily on historical data, or on climate models, or some combination of the two. The most reliable studies used both observational data and modeling, the study said.
A key goal of this work is to be able to predict extreme weather events, David Titley, chair of the committee that conducted the study, said at the press conference. "If we could predict them and verify them...it's pretty hard to argue these things aren't happening" or that climate change isn't influencing them, said Titley, director of the Center for Solutions to Weather and Climate Risk at Penn State University.
More important, he said, is how extreme weather prediction can be used for risk assessment. "If we can help people understand how those risks are changing and why those risks are changing, that is a very powerful tool in a toolbox" for city managers, emergency planners and company owners, Titley said.
Sheila V Kumar contributed to this report.