TOKYO — For more than three years, Randy Wilber has been on a mission. Wilber, the U.S. Olympic and Paralympic Committee’s senior sports physiologist, specializes in environmental physiology, and his duties cover a broad range, from fighting jet lag to preventing exercise-induced asthma. Long before anyone had heard of coronavirus protocols, the Tokyo Olympics offered a different but equally clear challenge.

“We were given the task by one of our vice presidents of performance,” Wilber said. “ ‘Every one of those athletes, Olympic and Paralympic, when they go to train and especially when they go to compete in Tokyo, you make damn sure that every single one of them is optimally prepared.’ We’ve been working on it since that day we got called on the carpet. Make sure every kid is ready physically, physiologically, emotionally, confidence-wise that heat and humidity is not going to be a negative factor or a limiting factor to our athletes.”

Before the pandemic postponed the Olympics for a year and created both massive risks and massive challenges, Tokyo’s pummeling heat and humidity had been the gravest Olympic worries for organizers and athletes. In early 2020, the International Olympic Committee and Tokyo 2020 agreed to move the marathon to Sapporo, a mountainous region more than 500 miles north, for safety concerns. While the virus now overshadows the weather, the promise of high temperatures and the possible threat of typhoons — or even earthquakes — will have no less of an impact on the competition.

At a news conference Sunday, Tokyo 2020 organizers displayed a slide that read, “The rainy season is over in Tokyo, and the hot summer has come!” On Sunday, temperatures in Tokyo crept into the low 90s, the air so thick it felt as if you had to chew it before you could breathe it. As competitions begin next week, intermittent afternoon rain and thundershowers are forecast. Organizers passed out sodium tablets to volunteers and journalists. Stadiums and fields will be adorned with shade tents, portable air conditioners, ice baths, coolers packed with bottled water, mist fans and, for the equine athletes, a horse cooling station.

At the track and field stadium, USOPC officials have installed what they call the NIV Center. The NIV stands for Nike Ice Vest. Athletes stuff ice packs inside zippered pockets on their chests and backs and wear the vests for the minutes just before heading to the start line.

“The goal is to keep your core body temperature as cold and cool as possible to the start of the race,” said American middle-distance runner Rachel Schneider, who will run the 5,000 meters. “You’re going to get hot quick, so the trick is to not be too hot on that start line and delay the onset of how fast that heat will impact your internal temperature.”

The USOPC’s high-performance staff will provide its athletes with cooling devices at the Games, but its plan has been prepared for years and at work for months. In its labs at the Olympic Training Center in Colorado Springs, USOPC staff tested athletes for sweat rate, electrolyte loss, body core temperature and body response to sport drinks in hot, sticky conditions.

The most important process is what Wilber called “pre-acclimatization.” Many sports, including track and field, will hold training camps in Japan. But athletes can acclimate to Japan’s climate weeks before arriving. By wearing layered clothing, Wilber said, athletes can train their bodies to better respond to hot conditions during exertion in a matter of weeks. Some of the strategies came from studying how the U.S. military trains soldiers to serve in desert climates.

“It’s pretty much a money-back guarantee,” Wilber said. “In human beings, the most robust physiological response to a threatening situation is the heat and humidity response.”

The schedules are highly individualized. Wilber and his staff rely on a scientific database, but they realize that a study in, say, the Journal of Applied Physiology needs to be tweaked for someone such as swimmer Caeleb Dressel.

For each athlete who asked, Wilber constructed an detailed, color-coded training schedule. For a woman who trains in Boulder, Colo., for the 10,000, a light morning workout July 12 would be done wearing a long-sleeve shirt (yellow), but a week later, the same workout would be performed in long sleeves, a sweatshirt and a cold-weather beanie (green). Even once the runner arrives in Tokyo, she will take light runs in long sleeves.

“There’s no one in the world who knows how to heat-train and put together a protocol better than Randy Wilber,” three-time Olympian Jenny Simpson said. “He’s mentored a lot of athletes, a lot of coaches along the way. He puts the U.S. team in a position to excel in really tough weather conditions. That’s been proven over and over.”

In a presentation to Team USA, Wilber highlighted the United States’ success in the Athens 2004 marathon as an example of how acclimatization provides an edge. Meb Keflezighi won silver, and Deena Kastor claimed bronze, giving the United States its best marathon results ever on days when the temperature neared 100 degrees.

“Athletes who prepare effectively for heat and humidity can beat a lot of athletes who have more talent than they do but who have not prepared effectively for heat and humidity,” Wilber said. “That 2004 experience really cemented that for us. They really beat a lot of people who on a mild day, a moderate day probably would have beat them.”

While seeking an advantage, the USOPC also must protect athletes from heat stroke, dehydration and other conditions caused by overexertion. Elite athletes are at acute risk of suffering from those conditions. They separate themselves by their capacity to train and compete through discomfort that would cause others to quit. In extreme heat, such doggedness can transform from admirable to life-threatening.

“It’s what makes them great, but it’s also what can get them in trouble, particularly if they’re not careful in a hot and humid environment,” Wilber said.

Heat is not the only issue Tokyo’s climate poses. Typhoon season comes in August and September, and heavy rains could be particularly problematic late in the Olympics and during the Paralympics. Wilber also has run USOPC staff through earthquake drills and worked to normalize for athletes that they compete in places filled with earthquake risk reminders.

“It’s an additional stressor,” Wilber said. “How many Games have we gone to where we’ve had to put that on our list?”

The U.S. track and field team received a surprising preview of extreme weather at the Olympic trials in Eugene, Ore. A historic heat wave pushed the weather, typically moderate, into dangerous territory. Temperatures climbed into the triple-digits, forcing some events to be rescheduled and postponing the final day for five hours after a heptathlete fainted on the track. Multiple athletes were hospitalized.

Wilber viewed the issues as unfortunate but also a timely warning. After the trials, he received several requests from track and field athletes seeking a weeks-long program to acclimate to the Tokyo heat.

“I think everybody knew it,” Wilber said. “But until you go through those kind of environment conditions in conjunction with a major race, you really don’t get the full wake-up call.”

The trials also could foreshadow an event in Tokyo being delayed because of weather. The heat already has impacted the Games’ schedule. Organizers originally envisioned the marathon as snaking through Tokyo streets in the morning, screaming fans lined up along the route, but sports scientists convinced them to move it to Sapporo out of concern for athletes’ safety.

“Enough people spoke up and said you can’t have it in Tokyo in middle of afternoon or even morning,” Wilber said. “I commend them on doing that. I’ve seen many situations where they just give into television, to politics, to other factors that discount the health of the athletes.”

IOC Executive Director Christophe Dubi insisted Sunday that any postponement decision would be decided with athletes’ safety as the priority. In every sport, the IOC and international federations have guidelines for what conditions would trigger a postponement. The IOC’s Competition Schedule Change Committee makes the decision on when to delay and how to reschedule.

“If there is a risk those thresholds could be crossed, there is a very well-documented process,” IOC Olympic Games Operations Director Pierre Ducrey said.

Climate change ensures Wilber’s task will not change. Japan’s climate provides stark challenges but perhaps not in a way that will be unique for long. For previous Olympics, he studied the past 50 years of a region’s weather patterns to predict would what happen. For Tokyo, he focused on just the past five years because climate change had made anything before that less relevant. In a presentation given to athletes in early May, Wilber noted that the average heat index in Tokyo was 86 degrees Fahrenheit in 2016. In 2018, it was 106.

“Probably every Olympics, we are going to have this conversation about, what are you doing for heat and humidity?” Wilber said. “I’m telling my crew here, take lots of pictures, don’t pack too much stuff away, because you’re going to be bringing it out in three years when we go to Paris.”

Rick Maese contributed to this story from Washington.