Topline
Forget about analytics-driven swing tweaks, juiced balls or steroids—Dartmouth researchers found another cause for an uptick in home runs in Major League Baseball: Global warming.
Xander Bogaerts of the San Diego Padr admires a home run.
Key Facts
Higher gameday temperatures at MLB stadiums resulted in 577 more home runs than expected between 2010 and 2019, or 58 per season, according to a peer-reviewed study published Friday in the Bulletin of the American Meteorological Society, the first comprehensive, multi-year look at the impact of climate change on America’s pastime.
Using weather data from more than 100,000 MLB games between 1962 and 2019 and batted ball data from 2015 and 2019, the Dartmouth team discovered each additional degree Celsius in game temperature resulted in roughly 2% more home runs per game.
The paper’s senior author Justin Mankin explained that there’s a “very clear physical mechanism” behind this phenomenon: Higher temperatures result in lower air density, creating a less-resistant path for airborne baseballs and allowing balls to travel faster and further.
If global temperatures continue to rise, balls will fly out of the park at an even steeper pace: There will be 192 additional weather-related home runs per season by 2050 and nearly 500 excess big flies by 2100 in a high-end warming scenario in which temperatures spike 5 degrees Fahrenheit by mid-century and 10 degrees by the end of the century, the study estimated.
Key Background
The number of home runs in MLB has risen steadily over the last five decades, soaring from 0.78 per game between 1963 and 1972 to 1.14 per game from 2013 to 2022, a nearly 50% increase, according to Baseball Reference data. There are various non weather-related factors contributing to the multi-year spikes, with the prevalence of performance-enhancing drug use among players helping home run rates climb to then-record levels in the 2000s, an alleged change to the balls over the last decade to encourage more offensive firepower and a greater emphasis on optimizing launch angle for power among hitters. Despite the lack of academic research, the impact of air density on the flight of baseballs has been well-documented, as the thin air at Coors Field, the Colorado Rockies’ home stadium located more than a mile above sea level, puts Coors Field among the most hitter-friendly parks in baseball despite its spacious dimensions.
Big Number
5.7%. That’s the proportion of balls hit in play that were home runs during the 2019 MLB season, the last year of the Dartmouth study, an all-time record. The 2019 home run rate was nearly twice as much as its level 50 years prior.
Further Reading
Is Global Warming Causing More Home Runs in Baseball? (Scientific American)
M.L.B. Hired Scientists to Explain Why Home Runs Have Surged. They Couldn’t. (New York Times)
MLB Study Finds Lower Seam Height on Baseballs, Player Behavior Behind Power Surge (Wall Street Journal)
