The opening ceremony of the Summer Games treated an estimated 3 billion viewers around the globe to a two-minute primer on climate change. The video traced the growth of carbon pollution and the rise temperature, and envisioned the flooding of vulnerable regions like the Netherlands, South Florida and, most pointedly, Rio de Janeiro.

For Brazil, climate change is personal, a fact highlighted in a new report from Brazilian environmental group Observatório do Clima. Today, the world’s finest athletes compete in Rio, but authors warn that as the mercury climbs, Brazil will become less and less hospitable to outdoor sports. Extreme heat will hamper performance, putting records out of reach and placing athletes in serious danger.

Imagine a world-class runner in the first miles of a marathon. Every muscle fiber is working at maximum efficiency, and yet three-quarters of the energy she burns is released as heat. As she warms up, she begins to sweat. On a hot and humid day, her sweat won’t evaporate and she won’t cool off. Her muscle fibers will become less efficient, generating more heat. She will sweat more, grow dehydrated, and this will wear her concentration and slow her reaction time. If she loses enough water, she could become ill and lose consciousness.

Doctors warn that sweltering temperatures could cause the immune system to react as though it’s being attacked by a virus or bacteria. For our runner, the temperature outside could mean the difference between a career-defining performance and a trip to the hospital on a stretcher.

Projected warming in Brazil in degrees centigrade by the end of this century, according to the Miroc-5 climate model (top) and the HadGEM2 climate model (bottom) in low and high emissions scenarios (left and right, respectively). Source: Observatório do Clima

For a game played in Manaus during the 2012 FIFA World Cup, it was so hot that officials mandated players break every 30 minutes to rehydrate. The report from Observatório do Clima projects that by the end of the century, it could become too dangerous to play outdoor sports in Manaus at any time of year.

Even in the more temperate corners of Brazil, athletes will need to adapt to warmer conditions. That could mean training in a sauna-like chamber to coax the body into dealing with heat more efficiently, using advanced metrics to gauge how much water and electrolytes an athlete needs, or wearing smart fabrics or ice-filled helmets to stay cool.

These adaptations have one thing in common — they are are expensive. High-tech apparel and sports equipment will come at a significant cost, which could widen the gap between the haves and have-nots. As temperatures rise, Brazil risks sidelining potentially stellar athletes who lack the means to compete. Authors argue that an “active search of new talent needs to be implemented as a measure of adaptation of Brazilian sport.”

Climate change could mean a “loss of excellence in sports practice.” Fans may see more sluggish performances from top athletes and a steady decline of record-breaking performances. During the games in Rio, outdoor competitions are only taking place in the mornings and late afternoons to avoid the warmest part of the day. But in the future, competitors will need to become more adaptable.

For Brazilian athletes, the heat is on.

Jeremy Deaton writes for Nexus Media, a syndicated newswire covering climate, energy, politics, art and culture. You can follow him at @deaton_jeremy.