Is global warming slowing Boston Marathon winning times?

Winning times in endurance events such as marathons have improved over the last century, a trend that may slow or reverse should temperatures warm as climate models predict. Determining the impact of global warming on endurance racers is complicated by a number of variables, such as different start times, weather conditions on race day, and general improvements in race performances that have occurred over the past century. While previous studies have shown that warm temperatures diminish the performance of endurance athletes by affecting their ability to regulate internal body temperature, it is not known whether warming trends have already affected athletes' performance during endurance competitions. However, the models used in the BU study indicate that if race starting times had not changed and average race day temperatures continue to warm at the current high-end estimated rate (0.058°C/yr), researchers would have had a 95 percent chance of detecting a consistent slowing of winning marathon times by 2100.

"The potential for warming temperatures to affect marathon athletes depends on the magnitude of warming and inter-annual variation in daily temperatures.," says study co-author Richard Primack, professor of earth and environment at Boston University. "If temperatures do not increase relative to temperature variability on race days, the effects of warming on marathon times may not be detectable. However, at some point, temperature increases may be large enough to affect marathon times."

The researchers used the Boston Marathon to see whether warming has affected the winning of endurance runners because it provides an ideal case for measuring an effect of warming on marathon times: It is the oldest continuous annual marathon in the world; it has been run on the same course on approximately the same day each year since 1924; and its consistent route and the length of the recorded time series are exceptional.

"We examined a time series (1933-2004) from the Boston Marathon to test for an effect of warming on winning times by men and women," says Primack. "We found that warmer temperatures and headwinds on the day of the race slow winning times. However, the warming in annual temperatures in Boston between 1933 and 2004 did not consistently slow winning times because of high variability in temperatures on race day."

Starting times for the race, another variable, changed to earlier in the day beginning in 2006, making it difficult to anticipate effects of future warming on winning times. If temperatures warm as much as is predicted by the high end of IPCC (Intergovernmental Panel on Climate Change) estimates, which is reasonable to expect in Boston, warming would likely have a detectable effect on marathon times by the year 2100, assuming starting times remained the same. Even if temperatures warm at the mid-range rate of 0.028°C/yr, there would be a 64% chance of detecting an effect of warming on Boston Marathon times by the year 2100. By changing starting times for the race to earlier in the day when temperatures are cooler, race organizers have effectively counteracted any effects that long-term warming would have had on winning times. If this change had not been made, it is likely that warming would have resulted in fewer record-breaking times in the Boston Marathon.

In summary, despite the well-known effect of temperature on marathon performance, the BU researchers found that warming trends in Boston have not caused winning times to slow over time because of high variability in temperatures on race day. However, their models indicate that if race starting times had not changed and average race day temperatures continue to warm by 0.058°C/yr, a high-end estimate, they would expect a 95 percent chance of detecting a consistent slowing of winning marathon times by 2100.