The only certainty about Summer Olympics weather is that there's really no certainty at all. Extreme heat is a growing threat for elite athletes, with cases of heat exhaustion and heatstroke becoming more common as fossil fuel pollution pushes temperatures and humidity levels up. Spectators, especially those those who fly in from cooler climates, are vulnerable to extreme heat, as well. Most of the world's cities will be unable to host the Games during summer in the coming decades as they blow past the threshold of safe humid heat, according to a CNN analysis of data from CarbonPlan, a climate science and analytics-focused nonprofit group. Heat stress can be measured with something called wet-bulb globe temperature — a combination of heat, humidity, windspeed, sun angle and cloud cover. CarbonPlan found that by 2050, heat stress in almost all cities in the eastern part of the US would shoot well past the 82.1-degree limit, beyond which experts recommend canceling sporting events. In other words, holding the Summer Games in these cities would be a huge health risk for the athletes. Read more at the link in our bio. Note: Wet-bulb globe temperature (WBGT) is a measurement that combines heat, humidity, windspeed, sun angle and cloud cover. It is a more a nuanced metric for setting exertional heat stress thresholds. The American College of Sports Medicine has set over 82°F WBGT as the threshold to cancel continuous physical activity. * Indonesia's host city will likely be Nusantara — a new capital city currently under construction — which is not included in the source data. The nearest city, Balikpapan, has been used in its place. * For countries south of the equator — Australia and Brazil — the date used was Jan. 20 to Feb. 11. 📸: Illustration; CNN/Adobe Stock