John Coates, chairman of the Coordination Commission for the Games, said an Olympic expert medical and scientific committee has recommended that the marathon begin at 5:30 or 6 a.m., although a final decision still needs to be confirmed with the International Association of Athletics Federations.
Morning rugby matches will now start at 9 a.m., 90 minutes earlier than first planned, while mountain biking events have been shifted an hour later to start at 3 p.m. to reflect concerns about the heat, Yoshiro Mori, president of Tokyo 2020, said at a news conference.
It was the second shift in the scheduling to combat the heat, only five months after Tokyo announced in July that the marathon start time would be brought forward to 7 a.m. and triathlons to 8 a.m., with race walking events to start between 6 and 7 a.m.
Tokyo weather in July and August has shot to the top of the agenda since a summer heat wave killed at least 138 people in Japan, sent more than 70,000 people to hospitals and was declared a national emergency. Temperatures peaked above 40 degrees Celsius (104 Fahrenheit) in parts of Tokyo, while humidity exceeded 80 percent.
Coates said the International Olympic Committee took the issue very seriously.
“It will continue to be front of mind for us, and for the organizers, and front of mind for the teams that are coming here,” Coates said. “We will do everything to ensure that they are not competing at risk or watching at risk.”
There were similar concerns when Atlanta hosted the games in 1996, and temperatures soared above 100 degrees Fahrenheit during the opening weekend. Temperatures in Athens reached similar levels.
Organizers of the 2022 World Cup in Qatar also have made sweeping concessions to the scorching heat in the Persian Gulf. The tournament was shifted from summer to begin in late November, and stadiums will still be outfitted with air conditioning.
In its bid for the 2020 Games, Tokyo dramatically downplayed the heat issue, but it has been forced to act.
“With many days of mild and sunny weather, this period provides an ideal climate for athletes to perform at their best,” it said at the time of its bid.
The IOC’s Evaluation Commission even appeared to buy the claims without a murmur, noting in its 2013 report that Tokyo’s dates were selected “for climatic reasons.”
Mori said this summer was “abnormal” and that adjustments had to be made to reflect public opinion and medical advice.
But these are intensely hot and humid months at the best of times, with maximum temperatures averaging 32 degrees Celsius (90 Fahrenheit) over the past two decades, according to Japan Meteorological Agency data.
When Tokyo last hosted the Olympics in 1964, the games began in mid-October. But these days the IOC prefers July and August because it better suits broadcasters, which pay billions of dollars for television rights.
The summer dates avoid clashes with other major sporting events, including the National Football League season and Major League Baseball playoffs in the United States and the soccer season in Europe.
Organizers had also discussed shifting the 2020 marathon to the evening, but broadcasters said televising the race after sunset would be “problematic,” said Toshiro Muto, chief executive of Tokyo 2020.
Tokyo plans to coat 60 miles of roads, including the marathon course, with a resin-based surface that reflects infrared rays and lowers the surface temperature by as much as 8 degrees Celsius (14 Fahrenheit). Organizers have also promised to keep spectators cool with tents, fans and sprays of cooling mist.
But more ambitious measures to combat the heat have either proved too expensive or too disruptive. An initial plan to build air conditioning into a new national stadium in Tokyo was abandoned because of rising costs. Instead, a new design for the stadium is supposed to allow breezes to enter from outside.
Mori, the committee president, even visited Prime Minister Shinzo Abe in July to suggest the entire nation shift to daylight saving time over the summer to combat the heat and allow earlier start times.
The government, though, was not keen on the idea, arguing it would be disruptive to sleep and work. Daylight saving time was unpopular when introduced in 1948 during the Allied occupation of Japan, and it was scrapped in 1952.
Makoto Yokohari, a professor of urban engineering at the University of Tokyo, said he has been warning about the problems for the past two years, but Japan’s Olympic Committee did not take him seriously at first.
“The Tokyo marathon normally gets [a half-million] to 1 million spectators. Even if 0.1 percent of them get ill from heat, that would amount to 500 to 1,000 people,” he said. “Tokyo currently does not have the capability to rescue that many people, though starting at 6 may mean fewer spectators.”
There really is no way to race outside for two to three hours in Tokyo at the height of summer, he added. “The ideal conclusion is not to do it in Tokyo at that time of the year.”
Takaaki Matsumoto, a professor at the School of Health and Sport Sciences at Chukyo University in Nagoya, led research over the past two summers about the combined effect of heat and humidity at the racetrack.
He said that after 8:30 a.m., conditions move into a “high alert” status, when marathon running should be banned, according to Japan Sports Association guidelines.
“We knew it would be a problem the moment Tokyo was chosen,” Matsumoto said.” I am also a marathon runner. Normally, marathon races are not held beyond June. I got a sense of urgency, and that’s how I began scientific research.”
Correction: An earlier version of this article misspelled the name of Makoto Yokohari.