The energy-sapping race walks are scheduled to open the championships on Saturday morning, with the women’s marathon on Sunday and the men’s on Monday.
Temperatures have been as high as 33C this week and, with the heatwave set to continue into next week, organisers have been forced to move the road race starts to 7.30am.
“Due to expected elevated heat conditions that could pose a health and safety risk to competing athletes, all road events on the first three days … will start 30 minutes earlier than scheduled,” said a joint statement from the organisers and World Athletics.
“The start time for the road events had originally been set at 8am in consideration of climate conditions, operational aspects, and maximising spectator attendance.”
World Athletics chief Sebastian Coe admitted in Tokyo on Tuesday that the high temperatures would be an issue for athletes.
Marathon and race walk events at the pandemic-delayed 2021 Tokyo Olympics, which were held from July 23 to Aug 8, were moved to the cooler northern city of Sapporo because of heat concerns.
However, this time round they have remained in Tokyo where the temperatures in mid-September “have remained at those of mid-summer”, the statement noted.
“(The decision) has been communicated to the athletes as early as possible to allow them to prepare and adjust to the new start time.”
Fears over the heat in Doha at the 2019 world championships resulted in the marathons beginning at midnight with the walks half an hour earlier.
Japan’s average temperature between June and August was 2.36C above “the standard value”, making it the hottest summer since records began in 1898, the Japan Meteorological Agency (JMA) said.
It was the third consecutive summer of record high temperatures, the agency noted.
Coe said after Tuesday’s meeting of the World Athletics Council the future risks of global warming had been discussed.
“These are not transient, they’re here to stay,” he said.
“Governments have not stepped up to the plate and sport is going to have to take some unilateral judgments and decisions here.
“And we have reflected in the past, if we are committed to athlete welfare, then we should probably be openly committed to that,” he added.
In-stadium competition start times on each of the first three days remain unchanged.