Scientists are learning just how climate change impacts extreme weather events For decades, it was impossible to say that a specific weather event was caused, or even made worse, by climate change. But advanced research methods are changing that.

Scientists are learning just how climate change impacts extreme weather events

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[POST-BROADCAST CORRECTION:This story incorrectly identifies Michael Wehner's place of employment. He is a researcher at Lawrence Berkeley National Laboratory, not Lawrence Livermore National Laboratory.]

JUANA SUMMERS (HOST): A derecho barreled through South Dakota yesterday. A heat wave is lingering over Texas, and wildfires are burning across Alaska. When weather gets extreme, a lot of people wonder and worry about climate change. Michael Wehner is a senior scientist at Lawrence Livermore National Laboratory.

MICHAEL WEHNER (SENIOR SCIENTIST, BERKELEY LAB): People want to know - you know, has climate change affected me? Did climate change flood my house? Did climate change make it so hot that my power went out? Those kinds of questions - and those are good questions.

SUMMERS: For a long time, scientists did not really have answers. But as NPR's Rebecca Hersher reports, that's changing.

REBECCA HERSHER (BYLINE): This is cutting-edge science, and here's how it works. After a flood or a heat wave or some other disaster, scientists sit down and compare what actually happened - like, how hot it got or how much rain fell - to what would have happened if there was no global warming. And to do that, they use really powerful computers, excellent weather satellites and fancy new math. And it's easier to do for some types of weather. Wehner was one of the OG scientists working on this problem.

WEHNER: Well, the heat waves were where we started.

HERSHER: Because heat waves are relatively simple. There aren't a lot of variables - temperature, maybe humidity and wind if you're getting fancy. And since you're comparing the present to the past, before global warming took off, you need good historical records, which there are for temperature - going back to the 1800s - all of which allows scientists to say some pretty bullish things about how climate change is making heat waves worse.

WEHNER: Any heat wave that occurs from now on, the temperature has been increased by climate change.

HERSHER: They can even tell you how much hotter it is.

WEHNER: For garden-variety heat waves, like the hottest day of the year, the hottest day, you know, in every 10 years - in the United States, climate change has increased that heat wave's temperature by between three and five degrees Fahrenheit.

HERSHER: Three to five degrees is a big difference if you really think about it - 85 compared to 90, 95 compared to 100. And, actually, studies have found that the higher you get, the more deadly each additional degree actually is. Last summer, this type of science had its biggest moment yet. There was an extreme heat wave in the Pacific Northwest - 115 degrees in Oregon and Washington, 120 in parts of Canada. And when scientists analyzed it, they found something shocking.

WEHNER: It was virtually impossible without climate change.

HERSHER: Another way to say that - climate change caused the heat wave. Now, that's new territory for most people - the idea that the weather we're living through isn't just worse because of global warming; it is only possible because of global warming. But other types of disasters are harder to tie to climate change, like wildfires. They're some of the hardest.

MEGAN KIRCHMEIER-YOUNG (RESEARCH SCIENTIST, ENVIRONMENT AND CLIMATE CHANGE CANADA): Wildfires are a really great example of how we cannot say if climate change caused a particular wildfire event.

HERSHER: Megan Kirchmeier-Young is a researcher at Environment and Climate Change Canada, and she says it's clear that climate change is making hot, dry conditions more common, which obviously makes wildfires more likely to take off. But then there are all the human influences. For example, a person can start a wildfire, and firefighters can keep it from spreading.

KIRCHMEIER-YOUNG: Any fire has got so many factors going on, and only some of them are really closely related to the climate.

HERSHER: That makes it impossible for scientists to study a specific fire and say this was X amount worse because of climate change. Other weather disasters are somewhere in between, like hurricanes. They're more complicated than heat waves, but less tricky than wildfires. So scientists have made some progress by focusing on individual parts of the storm, like how much rain fell or how intense the wind was. There's a lot of pressure for this research to move quickly, says Wehner.

WEHNER: There is a clear demand for this from the public.

HERSHER: In the future, concrete information about the effects of climate change could just be part of the normal weather forecast. In fact, the Weather Service for the European Union is already trying it out for heat waves and floods. Rebecca Hersher, NPR News.

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