Baseball’s home run boom is due, in part, to climate change

Baseball is the best sport in the world for numberphiles. There are so many stats collected that the analysis of them even has its own name: sabermetrics. Like in Moneyball, team managers, coaches and players use these statistics in game strategy, but the mountain of available data can also be put to other uses.

Researchers have now mined baseball’s number hoard to show that climate change caused more than 500 home runs since 2010, with higher air temperatures contributing to the sport’s ongoing home run heyday. The results appear April 7 in the Bulletin of the American Meteorological Society.
Many factors have led to players hitting it out of the park more often in the last 40 years, from steroid use to the height of the stitches on the ball. Blog posts and news stories have also speculated about whether climate change could be increasing the number of home runs, says Christopher Callahan of Dartmouth College (SN: 3/10/22). “But nobody had quantitatively investigated it.”

A climate change researcher and baseball fan, Callahan decided to dig into the sport’s mound of data in his free time to answer the question. After he gave a brief presentation at Dartmouth on the topic, two researchers from different fields joined the project.

That collaboration produced an analysis that is methodologically sound and “does what it says,” says Madeleine Orr, a researcher of the impacts of climate change on sports at Loughborough University London, who was not involved with the study.

The theorized relationship between global warming and home runs stems from fundamental physics — the ideal gas law says as temperature goes up, air density goes down, reducing air resistance. To see if home runs were happening due to warming, Callahan and colleagues took several approaches.

First, the team looked for an effect at the game level. Across more than 100,000 MLB games, the researchers found that a 1-degree Celsius increase in the daily high temperature increased the number of home runs in a game by nearly 2 percent. For example, a game like the one on June 10, 2019, where the Arizona Diamondbacks and Philadelphia Phillies set the record for most home runs in a game, would be expected to have 14 home runs instead of 13 if it were 4 degrees C warmer.

The researchers then ran game-day temperatures through a climate model that controls for greenhouse gas emissions and found that human-caused warming led to an average of 58 more home runs each season from 2010 to 2019. The analysis also showed that the overall trend of more home runs in higher temperatures goes back to the 1960s.

The team followed that analysis with a look at more than 220,000 individual batted balls, made possible by the Statcast system — where high-speed cameras have tracked the trajectory and speed of every ball hit during a game since 2015. The researchers compared balls hit in almost exactly the same way on days with different temperatures, while controlling for other factors like wind speed and humidity. That analysis showed a similar increase in home runs per degree Celsius as the game-level analysis, with only lower air density due to higher temperatures left to explain higher numbers of home runs.
While climate change has “not been the dominant effect” causing more home runs, “if we continue to emit greenhouse gases strongly, we could see much more rapid increases in home runs” moving forward, Callahan says.

Some fans feel that the prevalence of home runs has made baseball duller, and it’s at least part of the reason that the MLB unveiled several new rule changes for the 2023 season, Callahan says.

Teams can adapt to rising temperatures by shifting day games to night games and adding domes to stadiums — the researchers found no effect of temperature on home runs for games played under a dome. But according to Orr, climate change may soon cause even more dramatic changes to America’s pastime, even with those adaptations.

Because the sport is susceptible to snow, storms, wildfires, flooding and heat at various points during the season, Orr says, “I don’t think, without substantial change, baseball exists in the current model” within 30 years.

Callahan agrees. “This sport, and all sports, are going to see major changes in ways that we cannot anticipate.”