Too hot to handle (Image: Jeff Hutchens/Getty)

THERE may come a point, if the world warms enough, when parts of the tropics will become so hot and humid that humans will not be able to survive. Models predict that this could start to happen in places in as little as 100 years in the worst case scenario. Now, observations show the process is already under way.

As humidity rises, sweating cools us less, so we suffer heat stress at lower air temperatures. For now, no place on Earth exceeds the human threshold for heat tolerance, with the exception of a few caves like the Naica cave in Mexico. That is thanks to a fortunate natural thermostat: when humid air gets hot, it rises and causes storms that cool things down.

But there is a catch. The point at which air begins to rise – the stability threshold – depends on how warm and moist surrounding air is. Models predict that as the entire tropics warm, the stability threshold will rise.


Nathaniel Johnson and Shang-Ping Xie at the University of Hawaii studied satellite and rain-gauge data from the last 30 years and found that sea surface temperatures in the tropics now need to be about 0.3 °C higher than they did in 1980 before the air above rises and produces rain (Nature Geoscience, DOI: 10.1038/ngeo1008).

This means the stability threshold has already started to rise, says Johnson. He adds that the same should be true over land masses in the tropics.

Earlier this year, Steven Sherwood at the University of New South Wales in Australia pointed out the serious implications for humans. Heatwaves already kill tens of thousands of people, and even more will die as the thermostat rises.

Eventually, some tropical areas could get so hot and humid on occasion that even someone standing naked in the shade in front of a fan would die (New Scientist, 23 October, p 36).

Some tropical areas could get too hot and humid on occasion for humans to survive