Major League Baseball is trying to make the sport more exciting, but global warming might be doing the league’s job for it.
A study from Ivy-League Dartmouth University found that global warming has led to an increase in home runs hit across the league. More than 500 home runs in the past decade were due to climate change, it found.
“Home runs in baseball … have risen since 1980, driving strategic shifts in gameplay,” the study’s abstract states. “Myriad factors likely account for these trends, with some speculating that global warming has contributed via a reduction in ballpark air density … We isolate human caused warming with climate models, finding that >500 home runs since 2010 are attributable to historical warming.”
The study begins by establishing its fundamentals. In physics, the ideal gas law states that air density is inversely proportional to temperature: at high temperatures, air is less dense, and vice versa. Furthermore, the laws of ballistics state that air density affects the trajectory of a batted ball; in principle, a batted ball will fly farther in less dense, warmer air. With these in mind, the study analyzed more than 100,000 games between 1962-2019 and more than 200,000 individual batted balls tracked by Statcast cameras between 2015-2019, and estimated the number of home runs as a function of temperature.
The study notes other factors that may have influenced the increase in home runs over the years, including different ball construction, the use of performance-enhancing drugs, technological advances, the rise of analytics and data on pitcher tendencies, and changes in training, all of which have coincided with the global warming trend. But the study isolates these factors out of its data, as well as the well-noted variable field dimensions and geography of individual ballparks, and the fact that baseball season happens mostly during spring and summer, when temperatures rise on average anyway. It also excluded domed stadiums, which were not affected by ambient temperatures.
The study found that a temperature increase of 1 degree Celsius during a game increased the number of home runs by 1.96%. The effect was higher for early afternoon games, a 2.4% increase, and lower during night games, an increase of just 1.7%. The effect was the same for both home and away teams. The results were unaffected by controls for wind speed, though it may have been because there is no effective measurement for wind speed and direction in the parks; they were similarly unaffected by humidity, which has only a minor effect on air density; or by precipitation, since games are called off in inclement weather. The study also controlled for other impacts of temperature, like heat stress on pitchers; those control models found that temperature’s effect on home runs is mostly from the effect on air density.
In summation, the study concluded that climate change resulted in a decrease in home runs between 1962 and 1995, then an increase from 1995-2019. Between 2010-2019, global warming led to 58 additional home runs per season, and 577 over the decade. The change was driven by policies meant to curb pollution, leaving greenhouse gas emissions as the primary driver of climate change, accelerating home runs.
However, the impact was small. “Other factors such as changes in the height of the stitches on the baseball appear to have been more important in driving recent home run trends,” the study says.
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Despite this, the study projected that home runs will increase with future global warming: for every degree increase in global temperature, it projected an average of 95 more home runs per season. By 2050, that number would increase to 192 more home runs per year, and 467 by 2100. “Strong climate mitigation” keeps the increase to just 130 per year by 2100.
Absent climate mitigation, the study recommends playing all MLB games at night, or adding domes to stadiums, to reduce the effects of global warming. It also recommends altering pitching strategies, game management, and bat composition.