[“TAKE ME OUT TO THE BALL GAME” PLAYING] IRA FLATOW: Yes, the baseball season is in full swing, and you know I love baseball. And the new rules imposed this year are aimed at making the game even more enjoyable. But Mother Nature may be imparting a bit of her own rulemaking.

Research published this week in the Bulletin of the American Meteorological Society says that climate change– specifically, global warming– may make the weather more favorable for home runs. And it may favor some teams over others.

Joining me is Christopher Callahan. He’s a PhD candidate in geography at Dartmouth College, in Hanover, New Hampshire, and lead author on that report. Welcome to Science Friday.

CHRISTOPHER CALLAHAN: Hi. Thank you very much for having me.

IRA FLATOW: You’re welcome. OK. So how does warming make for more home runs?

CHRISTOPHER CALLAHAN: Well, when temperatures go up, the air is less dense. This is a basic physical mechanism that we know about. And we know that when the air is less dense, there’s less air resistance. So it’s easier for a ball to fly through it. And so a batted ball will simply carry farther and is more likely to be a home run.

IRA FLATOW: And how big of an effect are we talking about here?

CHRISTOPHER CALLAHAN: At present, the effect is relatively small. We can link climate change to about 500 home runs over the last 10 years, which is only about 1% of the number of home runs in total that were hit in Major League Baseball.

That being said, if we move into the future and keep emitting greenhouse gases substantially, this effect could get much larger. And we could be talking about hundreds more home runs per year later in this century.

IRA FLATOW: Wow. Would we need an asterisk, you know, in the pre-climate change days?

CHRISTOPHER CALLAHAN: Yeah, maybe. A new type of asterisk.

IRA FLATOW: Can you actually compare a hit in two different ballparks?

CHRISTOPHER CALLAHAN: Well, there’s so many things that are different between ballparks. We know that they have different dimensions. They’re at different elevations. The teams are different, if it’s two different teams playing. And so it is difficult for us to say that any two individual hits are comparable.

But we have enough data over the course of the last 60 or 70 years on nearly every baseball game, including the home runs and the temperature in that game. And so we can make these sort of general claims about average changes in home runs due to climate change independent of any particular hit in any particular ballpark.

IRA FLATOW: And can you actually put a number on it? I mean, how many hits per every degree of temperature.

CHRISTOPHER CALLAHAN: Yeah. So we found that, for every 1 degree Celsius increase in temperature, there’s about a 1.9% increase in the number of home runs in that game.

IRA FLATOW: Wow.

CHRISTOPHER CALLAHAN: Now, there’s only two or three home runs in any given game, on average. And so that effect is not all that large from the perspective of any one game. But once you start thinking about climate change over the last several decades across many ballparks and many years and then moving into the future, the numbers can start accumulating.

IRA FLATOW: This is Science Friday, from WNYC Studios.

A lot of research papers are statistical. They look at the statistics of stuff and they extrapolate. Are you looking at actual hits by actual batters in your work?

CHRISTOPHER CALLAHAN: We are. I mean, we are using the statistics of those actual hits. So we have data on the number of home runs in each game, the temperature of that game. And then we also use data on individual batted balls from the more recent era, when we have this data from the Statcast system of high resolution, high speed cameras in each ballpark. And so we can use data on actual hits by actual batters and say, when the temperature is warmer, those hits go farther. And we can observe that using real data.

IRA FLATOW: Can you factor in any other changes– perhaps differences in the ball? You hear people talking about some years the ball is juiced or they think that there’s something wrong with it. Can you factor that in?

CHRISTOPHER CALLAHAN: Absolutely. So we do our best to control for those other factors. We know that there are changes in the construction of the ball over time. We also know that different ballparks have different dimensions and different climates and different elevations. And so we can factor out those other things and say, what is the influence of temperature independent of those other things?

Now, to be clear, because we find that temperature has not been the single thing driving increases in home runs recently, it is still very valid to say that much of the home run surge might be due to, for example, a juiced ball. And so our results should not be taken to disprove that theory. It’s just an additional thing on top of that.

IRA FLATOW: So are there teams or ballparks where this would be most significant?

CHRISTOPHER CALLAHAN: So we find, moving into the future, there’s going to be an increase in home runs due to climate change. But as you said, it’s going to be different in different places. And the things that shape how different that effect is, aside from simply how much global warming there is in each individual place, is, do these ballparks have domes on them that insulate them from ambient conditions, and are they playing games in the middle of the day or in the evening?

So, for example, Wrigley Field, which is walking distance from where I grew up in Chicago, is going to see the highest increase in home runs in the future. The reason for that is because it’s open air. So it doesn’t have a dome that insulates it from the ambient weather conditions. And most of its games are played during the day, when it’s hottest, rather than in the milder evening conditions

IRA FLATOW: Wow. Are you ready to go and make book on that?

CHRISTOPHER CALLAHAN: I am not.

IRA FLATOW: [LAUGHS] So luckily, Red Sox versus Yankees should have similar effects?

CHRISTOPHER CALLAHAN: We’ll see. Potentially.

IRA FLATOW: This is great. It looks to me like you’re obviously a Chicago Cubs fan, correct?

CHRISTOPHER CALLAHAN: Absolutely.

IRA FLATOW: Did you get into this line of reasoning and line of research because you’re a baseball fan?

CHRISTOPHER CALLAHAN: Yeah, I did. I definitely would not have been motivated to think about it if I was not already thinking about baseball in my spare time. I knew that people had hypothesized about this sort of link between temperature and air density and home runs.

And I’m interested enough in baseball that I decided, well, I’d like to go see if that effect is actually there in the large-scale data that we can use. And so that’s what motivated me to go do it.

IRA FLATOW: Well, in a season where they’re looking for more base hits than home runs, we’ll take your advice and keep an eye out on it. Thank you for taking time to be with us today.

CHRISTOPHER CALLAHAN: Thank you very much for having me.

IRA FLATOW: Christopher Callahan, PhD candidate in geography at Dartmouth College, in Hanover, New Hampshire.

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