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Warmer temperatures may be linked to a rise in baseball home runs, study finds

MARY LOUISE KELLY, HOST:

Baseball season just started. And a new study says that home runs may be on the rise due to warming temperatures. Ari Daniel covered the bases for us.

ARI DANIEL, BYLINE: Eleven years ago, baseball commentator Tim McCarver said this about home runs.

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TIM MCCARVER: I think, ultimately, it will be proven that the air is thinner now. There have been climatic changes over the last 50 years in the world, and I think that's one of the reasons that balls are carrying much better now.

CHRISTOPHER CALLAHAN: He was widely mocked for making this statement on air.

DANIEL: Christopher Callahan is a Cubs fan and a climate science grad student at Dartmouth College. He says McCarver's comment also generated articles that did some rough calculations.

CALLAHAN: I had read some of those and said, hey; we could try and find this in the actual data.

DANIEL: Callahan considered the number of home runs for more than a hundred thousand Major League Baseball games going back 60 years combined with temperature data.

CALLAHAN: Instead of saying, is it warm or cold, we say, is it unseasonably warm for that location at that time?

DANIEL: A question that's got nothing to do with other things, like what the bat's made of or whether players were doping.

CALLAHAN: And then we say, well, are there unusually more home runs than there are normally?

DANIEL: The answer, says Callahan, is yes. Dartmouth climate scientist Justin Mankin, who worked with Callahan, says it's due to warmer air being less dense than cooler air.

JUSTIN MANKIN: So more space between the air molecules. And so a ball is just going to encounter less air resistance, and it's going to fly farther.

DANIEL: I spoke with Michael Mann, who directs the Penn Center for Science, Sustainability and the Media and wasn't involved in the research. He says there may be another factor at play - heat stress on pitchers who have to hurl the ball over and over.

MICHAEL MANN: But a hitter just has to get up there once and hit the ball, and then they're done for a while. And so hitters have more of an advantage over pitchers.

DANIEL: So on warmer-than-usual days, the thinking goes, they'd hit more home runs. But that's where a second data set steps up to the plate.

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UNIDENTIFIED SPORTSCASTER #1: That is a missile, and it's two-nothing Yankees.

UNIDENTIFIED SPORTSCASTER #2: I know we're going to get the stats on this. That's one of the fastest home runs I've seen here at Yankee Stadium.

DANIEL: That's from a Yankees home game against the Cubs last June. Within seconds, the exiting velocity of Giancarlo Stanton's homer appears on screen - almost 120 miles per hour, according to a tracking system called Statcast. Here's Christopher Callahan.

CALLAHAN: This system of high-speed cameras. So we have the launch speed and launch angle of individual baseballs coming off the bat.

DANIEL: And at that point, all the other factors, including pitcher fatigue, don't matter. You're just looking at the speed and angle of a ball the moment it's hit. Callahan compared over 200,000 Statcast measurements from a five-year span. And sure enough...

CALLAHAN: We can say that the same ball leaving the same bat ends up being a home run more often in warm conditions.

DANIEL: Probably due to lower air density, he says. And if things continue to heat up, by the year 2100, we're likely to see several hundred more home runs per baseball season. This work is in the Bulletin of the American Meteorological Society. Marshall Shepherd is an atmospheric scientist at the University of Georgia who wasn't involved in the research. He says if the finding holds up, it's got broader implications.

MARSHALL SHEPHERD: It's more than just the novelty of more home runs. I think it does raise a caution flag about the health and safety of both players and fans at these games.

DANIEL: Finding this connection between home runs and temperature was only possible because of the vast amounts of data the MLB collects, says Justin Mankin. And these kinds of impacts are lurking everywhere, if only we could measure them.

MANKIN: Climate change - it is fundamentally going to restructure our lives and livelihoods and recreation and well-being. Nothing escapes its touch.

DANIEL: Nothing, not even a baseball sailing through a less resistant sky. For NPR News, I'm Ari Daniel.

(SOUNDBITE OF LOST IN KIEV'S "THUMOS") Transcript provided by NPR, Copyright NPR.

Ari Daniel is a reporter for NPR's Science desk where he covers global health and development.