Global warming now pushing heat into territory humans can’t tolerate

The explosive growth and success of human society over the past 10,000 years has been underpinned by a distinct range of climate conditions. But the range of weather humans can encounter on Earth – the “climate envelope” – is shifting as the planet warms, and conditions entirely new to civilisation could emerge in the coming decades. Even with modern technology, this should not be taken lightly.

Being able to regulate our temperature has played a key role in enabling humans to dominate the planet. Walking on two legs, without fur, and with a sweat-based cooling system, we’re well designed to beat the heat. But hot weather already limits our ability to work and stay healthy. In fact, our physiology places bounds on the level of heat and humidity we can cope with.

The normal temperature you see reported on weather forecasts is called the “drybulb” temperature. Once that rises above about 35°C, the body must rely on evaporating water (mainly through sweating) to dissipate heat. The “wetbulb” temperature is a measure that includes the chilling effect from evaporation on a thermometer, so it is normally much lower than the drybulb temperature. It indicates how efficiently our sweat-based cooling system can work.