The effects of high temperatures on the human body — and on livestock and crops too

I was lucky enough to spend time in Sante Fe. One day a colleague walked into the office, took one look at me and said, “Drink Kate!”  I was wilting before her eyes.

I didn’t feel hot, or unwell. The reason why is that it’s dry heat in the high desert of New Mexico, so the sweat body was making to keep me at the right temperature, evaporated fast and cooled me down.

It took me a couple of days to remember to drink a lot, and often.

I felt less fortunate working in Kuala Lumpur. Like everyone else, I can’t handle heat and humidity. It’s 90/90 there; i.e. 90F ( or 32C) and 90% humidity.

Sweat doesn’t evaporate with that humidity. Inevitably I felt physically stressed as soon as I stepped outside. And physiologically, I was indeed stressed.

Our body temperature is about 36C, so anything approaching that temperature causes us problems, especially when our cooling system (i.e. sweating) doesn’t work:

It’s only in the last couple of decades that air-conditioning has been widespread in cities like Kuala Lumpur. This fossil-fuelled phenomenon has allowed rapid economic development in places that previously had people doing not-much in the heat of the day. (A full 70% of all energy consumption in Saudi Arabia is spent on air-con.)

As climate breakdown wreaks its damage across the world, people who work outside to get food on to our plates, aren’t going to be able to do so. It’s  not just those who plant, tend and harvest crops, either. Sometimes machines can do the work. Others crops are labour-intensive; rice, for example, needs constant tendering. Add in too, the people who process, package and transport produce.

The optimal temperature for humans is 15C-20C; it’s the temperature in which we work best.

Living in some cities will become increasingly difficult, too. Already, the average summer temperature in 350 of the world’s megacities is 35C; i.e. dangerous to human life.

David Wallace-Wells estimates there will be 950 megacities cities, with a total population of 1.6bn people, with 35C summer temperatures by 2050.


All animals, not just humans, have the same problems as the ambient temperature gets close to blood temperature. Like us, they can suffer heat stress in the twenties, with potentially lethal consequences as the temperature edges upwards.

Literally millions of chickens died last July. From heat stress.

Plants don’t thrive in very hot conditions, either. Our staple crops start to suffer at 30C, affecting yields. As the temperature rises, the yields are non-linear, with losses 4% for each degree of heat above 32C.


Previously sparsely occupied lands where food was scarce a few decades ago are now teeming with people. All of whom need food. Every day.

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