Change is in the air for Major League Baseball, with numerous new rules going into effect this season aimed at speeding up play and making games more action-packed. But a new study finds that the air itself has also been changing baseball—by making home runs slightly easier to hit.
The study, published Friday in the Bulletin of the American Meteorological Society, analyzed more than 100,000 major league games and attributed 577 home runs hit between 2010 and 2019 to warming caused by climate change. That figure represents around 1 percent of a typical MLB season’s total home runs.
“This effect has been small to date, but the influence of climate change will only become more substantial going into the rest of the century,” says Christopher Callahan, a climate scientist at Dartmouth College and the study’s lead author. “This study is somewhat lighthearted, but it illustrates the pervasive and subtle influence of climate change on our daily lives.”
By raising average global temperatures via prodigious greenhouse gas emissions, humans have also reduced the average density of air, which becomes less dense as it heats up. Warmer, lower-density air means a hypothetical dinger walloped to left field experiences less air resistance or drag, because there are fewer air molecules between the ball and the stands. Any reduction in drag allows the ball to travel farther.
Emeritus University of Illinois physicist and baseball researcher Alan Nathan, who wasn’t involved in the study, says a ball that explodes off a hitter’s bat at 100 miles per hour with a launch angle of about 25 to 30 degrees will travel roughly 400 feet through the air at sea level on a roughly 70 degree Fahrenheit day. The same ball traveling through 100-degree air would carry roughly ten additional feet. If no air resistance slowed the ball down—in a hypothetical vacuum—it would sail an additional 300 feet, Nathan says.
This study is far from the first time that this idea of hotter weather juicing home run stats has been proposed, but an ever-growing wealth of data in modern baseball and an increasingly robust set of statistical methods for teasing out the impacts of climate change are now available.
These developments, combined with an abiding love of America’s national pastime, led Callahan to seek out an answer to just how many more home runs are being hit in MLB because of the roughly two degrees Fahrenheit of warming Earth has already experienced since 1880.
The researchers culled baseball data from more than 100,000 major league games between 1962 and 2019 and 220,000 individual hits from 2015 to 2019. Then the team looked for game day temperatures that were considered anomalously warm for the particular locale and season. The team analyzed this data to look for correlations between home runs and temperature while controlling for a host of other potentially confounding variables, such as stadiums with different dimensions, the rise of steroid use and players altering their swings to hit more balls out of the park. This created an estimate of the contribution of air temperature to the home runs hit in MLB across decades.
To single out the contribution of human-caused climate change to those hot-weather home runs, Justin Mankin, a climate scientist at Dartmouth and the study’s senior author, says they used climate models to simulate a world in which humans hadn’t pumped greenhouse gases into the atmosphere. Comparing this counterfactual Earth to actual temperature data allowed the researchers to estimate just how much warmer a given day was due to climate change, and then to calculate how that extra warming increased the number of home runs hit.
Callahan and Mankin found an average of 58 home runs per year between 2010 and 2019—577 total—that likely wouldn’t have made it out of the park without the assistance of climate change. The study also used different scenarios to estimate the effect of climate-change-induced warming on home runs all the way out to the year 2100.
They found that if greenhouse gas emissions keep climbing and major changes aren’t made to how baseball is played, climate homers could account for nearly 10 percent (437 home runs) of the yearly total in the majors. That pessimistic emissions scenario, known to climate wonks as SSP5-8.5, would entail nearly eight degrees Fahrenheit of warming compared to the pre-industrial era and up to 3.2 feet of sea level rise.
“If we are hitting that emissions scenario, there are probably other concerns that will capture our attention before an increase in home runs,” says Brian McCullough, a researcher studying climate change’s impact on sports at Texas A&M University who wasn’t involved in the paper.
By contrast, strong climate mitigation (dubbed SSP1-2.6) would lead to a temperature increase by 2100 of about 3.7 degrees Fahrenheit and would add roughly 130 home runs to the annual tally.
“Whether you think more home runs are good or bad as a baseball fan, this finding belies the much more serious and sobering risks of global warming,” says Mankin. “Baseball is a game enjoyed outdoors, usually during daytime, and to the extent that the ball will fly farther on a hot day, so too is an elderly person at greater risk of heat stroke or heat stress.”
Night games, the paper suggests, could help mitigate the impacts of increasing daytime high temperatures. Some teams have already taken concrete steps to mitigate the impacts of heat at their ballparks. The Texas Rangers, for instance, opened a new domed ballpark featuring a massive air conditioning system in 2020 to fend off scorching outdoor temperatures that once cracked 100 degrees for 40 straight days in 2011. Air-conditioned domes would also largely nullify the increase in home runs expected under climate change.
As temperatures continue to climb in the coming decades, more and more MLB teams could opt for domed stadiums, rendering America’s pastime an indoor sport at the professional level.
“To me, one of the joys of baseball is being in the open air in the sunlight,” says Callahan. “I think something would be lost if it was played indoors. Climate change is going to continue changing our lives in ways that aren’t always catastrophic but are often sad.”
Mankin says this study underscored just how data-rich MLB is, as well as the privilege that embodies. “The data we take the time to collect is a reflection of our values, and there are so many impacts of warming that are unfolding in ways, especially in the low-income world, that we just don’t have the data to assess,” says Mankin. “So not only are there deep inequities embedded in causes and impacts of climate change, we are also least positioned to quantify the impacts on those who are most vulnerable and least culpable. That’s a tragedy I’d like to rectify.”