David Murphy knows that after 11 marathons, and at the age of 42, the window may be closing on his chances to PR. With that in mind, he’s throwing everything he’s got at his fall marathon. Front and center in that plan is picking a race that will likely deliver cooler temperatures. For this reason, he is running the Indianapolis Monumental Marathon November 9.

Indianapolis Monumental Marathon
Cool times at the Indianapolis Monumental Marathon / photo: courtesy Beyond Monumental

The healthcare consultant, who’s based out of Evanston, Ill., has run his hometown favorite Chicago Marathon several times in the past. While he loves just about everything about it, he doesn’t like the fact that it’s mid-October date has led to some hot temperatures of late. “The reality is that increasingly, you can’t count on the weather that time of year,” says Murphy. “I love that race but if I want to go fast, it’s not where I should go.”

Clear Evidence: Heating Up and Slowing Down

Murphy’s hunch on the increasing Chicago temperatures isn’t unfounded. According to a new study by RunRepeat, the mercury in the windy city is on the rise. The average temperature at the marathon has risen by five degrees in the last 18 years. When you overlay that with marathon finish times during that same period, it’s easy to see a trend.

chicago marathon temperatures
graph: RunRepeat

Marathon times are getting slower, says RunRepeat data cruncher Paul Ronto, and the correlation to climate change is clear. “We focus on data-based content and we had hypothesized that warming temperatures played a role in slowing marathon results,” he says. “We didn’t expect the evidence to be so overwhelming, however.”

The study’s analysis revealed that the average runner adds one minute and 25 seconds to his/her marathon finish time for every additional degree Fahrenheit. Given the general trend in increasing temperatures, that has meant an average increase of six seconds a year in marathon times worldwide. These increases are similar to those reported by other studies correlating heat and running pace. “This applied across the board, regardless of age,” says Ronto. “Even a 25- to 30-year old is slowing down as the temperature rises.”

Chicago Marathon water stop
photo: 101 Degrees West

Climate is a big factor in results, responsible for over 30 percent of the variance in finish times, according to RunRepeat. The report also points out that as more races are added around the world, more of them take place at high temperatures. Nevertheless, the trend for most marathons, tracked individually over time, is an upward-sloping line.

Choosing Late and Early

Given the data, runners looking to go big at a coming marathon are likely best advised to follow Murphy’s example. “I love running in the 40s and 50s, both for race day, and for my long runs,” he says. “I think later fall or early spring races are where I’ll remain while I care about my time.”

Elite runner and coach Esther Atkins of Greenville, SC, has seen her athletes disappointed due to warm marathon days. “Yes, some people run better in the heat than others,” she says, “but nobody actually runs better in temperatures over 75 than in temperatures closer to 60.”

Chicago Marathon
photo: 101 Degrees West

Knowledge in hand, Atkins steers her athletes toward later fall marathons that offer a safer bet on temperature. “Coaching in the Southeast and with many of my athletes on the East Coast, I steer them toward marathons in November and December, like the Richmond or Monumental Marathons,” she explains. “Even though Chicago may be a flatter course, the risk of it being windy or hot almost negates that positive.”

Ryan Warrenburg, a coach with North Carolina-based ZAP Fitness, has also narrowed down a window of time that he recommends to his PR-aiming clients. “I strongly recommend marathons in the mid-November to February time frame,” he says. “Any dates before that in the fall seem more and more likely to get warm weather.”

Changing the Calendar?

There’s another factor that goes hand-in-hand with this, and that’s the training through increasingly brutal summers for early fall marathons. “For many people, these runs can become too taxing to make positive progress and not get run down,” says Warrenburg.

Late spring can also be risky: “The kicker in spring is that with each passing day, you are more likely to see the hottest day of the year on your race day,” Warrenburg says. “Unlike the fall, you haven’t trained through the summer to acclimate to that, which makes those warm marathons even tougher to manage.”

London Marathon Temperatures
graph: RunRepeat

Ronto suggest there is a case for race directors to consider date changes as the climate warms. “Other than elites who train for heat, the average runner isn’t going to get a fast race day in early fall or late spring these days,” he says.

This is the conclusion Murphy has reached as he looks to PR a few final times: “There just aren’t any guarantees these days, so I’m doing what I can to improve my odds.”