But now, scientists are going further by using the Games to teach a grim climate lesson.

At a high-end scenario for greenhouse gas emissions, a team of researchers write in the influential medical journal the Lancet, fewer and fewer major cities will be able to host a Summer Olympics as the end of the century nears.

The reason? Too much risk of seeing weather conditions get so hot and humid that they would pose a major heat illness danger to athletes.

"You could take a risk, and plan your Olympics, and maybe not get the hot days you expect, but that would be a big risk when there are many billions of dollars at stake," said Kirk Smith, a researcher at the University of California, Berkeley's School of Public Health and the lead author of the study.
Smith completed it with researchers based at institutions in New Zealand, Cyprus and the US.

At the centre of the study is the scary concept of the "wet-bulb globe temperature". What it refers to is a particular combination of temperature, humidity, wind (or the lack thereof), and heat radiation that, at elevated levels, is simply too much for humans to bear for long outdoors, especially when engaging in physical exertion. The core issue is that if there's too much humidity, it limits our ability to use evaporation, through sweating, to cool down our bodies.

The wet bulb globe temperature is determined based on several separate measurements, including one taken while covering the bulb of a thermometer with a wet towel, and another taken after inserting it into a dark globe, and can be either lower or higher than the actual temperature. But humans can't tolerate as high of a wet bulb temperature as an ordinary one. The danger zone is around 25C, for those engaging in major physical activity.

"It's tricky to measure and tricky to predict, but it's come to be understood as the best indicator of heat stress on the body," said Smith of the metric.