Over the past decade, baseball has witnessed a power surge, as home runs have flown out of ballparks at record rates. In 2019, for example, pitchers gave up 1.4 home runs per nine innings, the highest home run rate for any reason on record, and a 55.6% increase over the 2011 rate. (Through the first five days of this season, batters continued to slug homers at a record pace.) Many fans and pundits believe that this rise in home runs, and an accompanying surge in strikeouts—batters are swinging for the fences—has plagued the game, robbing baseball of “small-ball” action (think singles and stolen bases and great fielding plays) that once made the national pastime exciting. A host of reasons have been offered to explain this trend: livelier baseballs, improvements in player training, batters swinging the bats at angles more likely to lift the ball out of the park.
And according to a new study published in the Bulletin of the American Meteorological Society, another factor is definitively at play: the warming earth.
An empirical analysis in the journal article, “Global warming, home runs and the future of America’s pastime,” found that after separating out factors like the physical characteristics of the baseball and advanced data analysis that clue batters in to pitcher tendencies, a 1℃ increase in the daily high temperature on the day of a game played in an outdoor stadium increases the number of home runs in that game by 1.95%. The study analyzed more than 100,000 games played between 1962 and 2019. A more granular analysis of recent data provided by Statcast, the system of high-speed cameras installed in MLB stadiums that track data on every fly ball, allowed the researchers to effectively control for the skill of the batter and hitter. Comparing balls leaving the bat at the same angle and speed on a warm day versus a cold day in outdoor ballparks, the Statcast analysis found that between 2015 and 2019, a 1℃ increase in temperature yielded a 1.7% increase in home runs per game.
In all, the researchers conclude global warming led to an additional 58 home runs per year, and 577 home runs in all, between 2010 and 2019. That number accounts for 1.1% of all MLB home runs hit during that period. Though climate change played a relatively small role in the home run surge of the last decade, researchers project future warming will continue to give pitchers headaches. If greenhouse gas emissions and climate change continue unabated, modeling predicts that warming could account for 10% or more of all home runs hit by 2100.
Basic physics explains this phenomenon. According to the ideal gas law, air density is inversely proportional to temperature. Warmer air is less dense, allowing a ball to carry farther. “Just from ballistics, we know there’s going to be less drag on the baseball on a warm day,” says climate scientist and Dartmouth geography professor Justin Mankin, senior author on the study. “There’s a really good physical relationship at work here.”
While this research may appear frivolous outside the cozy confines of baseball, to Mankin and his co-authors, it’s anything but. “The big thing that this hammers home is that data poverty is a massive issue,” Mankin says. “Major League Baseball is a privileged activity for which we have a trove of data. High speed cameras in every ballpark, capturing the launch angle and launch velocity of every batted ball, that’s absurd. I’m concerned about the things for which we don’t have great measures.”
For example, what is the impact of heat exposure on fans in the ballparks? Or on a more broader scale, how does climate change influence public health in the Global South? ”The human costs of climate change are far greater than altering the number of home runs in Major League Baseball,” says Mankin. “But we’re able to conceive of adaptation and intervention measures, like building domes or switching to night games. We’re able to attribute the effect of global warming on this in the first place because we have the data.”
Sports stakeholders, however, are sure to gloss over such issues of global consequence. Agents may be on the horn with these researchers, asking for their assistance in arbitration hearings and free agent negotiations. Maybe it wasn’t their client’s crappy curveball that’s responsible for an inflated era.
It had to be global warming, right?
“That is not something that I thought about, but now it’s something I’m very worried about, so thanks for that,” says study co-author Christopher Callahan, a doctoral candidate in geography at Dartmouth who conceived of the study. “We’re not pinpointing the influence on any individual home run. We’re just saying that, on average over time, the probability of home runs is going to go up. Now, that’s not going to stop people from calling me. But it’s worth saying.”
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