In the mid-1970s when professional baseball player Yogi Berra coined the phrase “The future ain’t what it used to be,” he could not have been more correct. Around the same time, geochemist Wally Broecker first used the term “global warming” in a scientific paper predicting global temperatures would continually rise this century due to the buildup of CO2 in our atmosphere from fossil fuel burning.
We now know that Wally was similarly prophetic. Global warming has raised mean annual temperatures by about 0.6°C (about 1°F) since the 1980s, with another 0.5 to 1°C (about 1 to 2°F) increase predicted to occur by 2100. At the same time, fans of Major League Baseball (MLB) have witnessed a correlative increase in the number of home runs per game since the 1980s.
A home run occurs when a batter hits the baseball hard enough to send it skyrocketing outside the field of play, scoring their team a run in one fell swoop. These “long balls,” or “dingers,” are some of the most pivotal events in baseball, as they can alter the course of an entire game, series, or even championship. As such, researchers want to understand why players keep hitting more home runs.
The coincidence of rising home runs with increasing temperatures has led some baseball analysts to speculate climate change could be contributing. Warmer air is less dense, so the same baseball hit at the same angle with the same speed should travel further in warm weather than in cold weather. But no scientists have rigorously analyzed the link between global warming and home runs in baseball. Fans want to know – can we expect more home runs as future temperatures continue to rise?
To address this question, Christopher Callahan and colleagues from Dartmouth College recently analyzed more than 240,000 baseballs hit in 100,000 different MLB games played between 1962 and 2019. They wanted to test whether global warming contributed to a prominent surge in home runs that occurred between 2015 and 2019.
Researchers before have shown that players tend to hit the ball farther and to hit more home runs on hotter game days. But other researchers have suspected evolving sports trends like changes in ball construction, use of performance-enhancing drugs, and advances in training technology and analytics could cause the same trends. Different major league ballparks also have different sizes and elevations, which can change the number of home runs players hit.
To isolate the effects of global warming on home runs in baseball, Callahan and his team had to separate the effects of temperature from these other potentially confounding factors. To do so, they used a statistical method called Poisson regression. Poisson regression analyzes the relationship between a set of independent variables, like temperature or elevation, and a countable outcome, like a home run.
The researchers used this method to analyze home run occurrences against data for game-day temperatures, game dates, stadiums, and teams playing, to see how much each variable contributed to the overall number of home runs. They only used data from games played in open-air stadiums or in stadiums with open roofs to avoid artificially-controlled temperatures.
The scientists confirmed temperature affected home runs more than any other variable they tested. They calculated the number of home runs increased by nearly 2% for every 1°C increase in maximum game-day temperatures. The effect was larger for daytime games, at 2.4%, and smaller for nighttime games, at 1.7%. They found the same temperature control even when they accounted for local weather effects, like rainfall, humidity, and wind speeds.
Higher temperatures could also affect home runs indirectly through physiological controls on the players. To confirm air temperature and density were the main factors boosting home runs, Callahan’s team compared the launch angle and launch speed of each batted ball, using data from the MLB Statcast system. They found the same temperature trends when comparing balls hit at an identical speed and angle, effectively canceling out any differences in batter or pitcher performance.
Finally, the researchers compared temperatures in climate models run with and without fossil fuel burning to calculate how many extra home runs can be attributed to global warming. They estimated global warming led to an additional 58 home runs per year between 2010 and 2019, and could result in nearly 200 more home runs per year by 2050.
The researchers concluded MLB should develop strategies to mitigate the influence of global warming on player performance. They suggested playing only evening games, adding domes to stadiums, or changing the makeup of bats could help.
