STEVE INSKEEP, HOST:
One forecast for Washington, D.C., calls for a high of 100 degrees tomorrow and a high of 100 again on Sunday. That's going to be within the normal range for much of the United States this weekend. It was already hot, and it's getting hotter. We sent a producer out to a flag football camp in Washington, D.C., to see how people are doing, and here is Coach Jake Janikowski.
JAKE JANIKOWSKI: Drink lots of water tonight. Drink lots of water tonight. If you come in tomorrow and you have a picture of yourself drinking water - five points for your team. Five points for your team.
(SOUNDBITE OF CROSSTALK)
INSKEEP: Listen to that man. Here to talk with us about the science of heat waves like this one is NPR science correspondent Rebecca Hersher. Hi there.
REBECCA HERSHER, BYLINE: Hi.
INSKEEP: The question that naturally comes to mind for many people is - can we blame climate change?
HERSHER: Yes, sort of. Average temperatures are rising, right? The hottest days are getting hotter, heat waves are getting longer. So that means weather like this is more likely.
INSKEEP: Mmm hmm.
HERSHER: But, you know, the part of global warming that really helps you understand something like this is not the overall warming, it's extreme weather. So as the Earth gets warmer - and it already is getting warmer. So it's already about two degrees Fahrenheit hotter on our planet than it was in the late 1800s.
INSKEEP: Mmm hmm.
HERSHER: Weather gets more extreme. So that means hotter hots, it means colder colds, it means wetter wets.
INSKEEP: This is useful to know. I mean, it's hard to know when we're in it. I mean, I've seen 100 degree temperatures before, and if it's a 100 degree day, do I think that's worse or not? You're telling me the average is up and that the extremes are worse. What kinds of extremes are we talking about?
HERSHER: Well, let's just look at this year. So there was record-breaking flooding all over the central U.S. So there was extreme rain that was falling, back-to-back-to-back storms, five, six, seven inches of rain falling in a short period of time. That's because hotter air can hold more moisture, so that falls as rain. Or go back to the winter. Remember the polar vortex?
INSKEEP: Oh, yeah.
HERSHER: Really cold, very terrible for a big part of the northern U.S. That was in part because the changing climate allowed cold air to seep down from the Arctic. Or look outside the U.S. There's this really stark example happening right now in India. The city of Chennai, it flooded terribly a couple years ago. This spring, there was so little rain, the city is running out of water. So that's just what it looks like when extremes get more extreme and more variable.
INSKEEP: That's quite a list. And I think you could probably go on, but I want to note one other thing about this particular heat wave that we're experiencing here in the United States. A hundred degree temperatures during the day - that happens. I mean, people have experienced that before. But you might have expected it to get down into the 60s at night, be a little cooler. That's not happening.
HERSHER: Yeah. And part of the reason is that moist air holds more heat. And this is probably actually kind of intuitive to people. So in humid places, tropical places, they don't get as cool at night - right? - as the desert, for example, where it gets quite cold...
HERSHER: ... Even though it's really hot during the day. But so remember that's because hotter air, it sucks up more moisture, and so we're getting these really high overnight temperatures. And that happens especially in cities. So people who live in really dense areas, they're actually at more risk. And that's because concrete or asphalt, it soaks up heat and creates this island of extra hot air. And then at night, all those surfaces - they're like little furnaces. They just radiate that heat back out into the surrounding area. It's particularly dangerous for people who are already sick.
INSKEEP: Are some parts of the country more affected than others by all these trends?
HERSHER: Yes. The farther north you live, the bigger the temperature increase. And that's not great news. It means that places that are getting hottest fastest are least equipped to deal with that. It's just a reminder that we all need to change where we build and how.
INSKEEP: Oh, my goodness. So it's like when there's a snowstorm in the South, people don't know what to do. And now we have heat waves in the north.
HERSHER: Exactly, yeah.
INSKEEP: Rebecca, thanks.
HERSHER: Yeah, thanks so much.
INSKEEP: That's Rebecca Hersher, who reports on all things science for NPR News. Transcript provided by NPR, Copyright NPR.