"Every 10 degrees slows the race by two minutes," Primack said. "So between 2011 when it was relatively cool at 57 degrees and 2012 when it was around 89 degrees, there was a 9-minute difference in winning times."
And that's just the elite runners. It's likely that the rest of the field feels the impact more than the winners.
"It would affect average runners much more, and weaker runners even more dramatically," said Primack, who logged a 3:56 time at Boston on cold, drizzly day in 1970. "If you're exposed for a longer period of time to high temperatures, you're losing more water."
Running coach Jason Karp, author of Running a Marathon for Dummies, agrees.
"The elite runners are only out there for a little over two hours, and others are out there for four or five hours," Karp said. "In a lot of ways it's more stressful to be out there longer. And, the more you're in shape, the better you are at cooling yourself."
Even though the average temperature in Boston has already warmed by about 5 degrees over the past 80 years, Boston weather is so variable that it will take much more of a warming trend to affect the average of one specific day -- in this case, Patriots' Day. The researchers calculate that if temperatures warm at the same speed, there will be a 64 percent chance that winning times will slow. But if the temperatures warm faster, as high as 9.4 degrees as some models predict, that chance rises to 95 percent.