July 2, 2021 - 7:00am

We don’t usually associate the Great White North with blistering heat, but on Tuesday a new record was set for Canada — 49.6°C in Lytton, British Columbia. 

Of course, heatwaves happen; and Canada isn’t immune. Nevertheless what’s happened this week is extraordinary. The country’s temperature record wasn’t just broken, it was smashed — by nearly five degrees centigrade. 

We can’t be absolutely sure that this is directly related to climate change — or that anything as bad (or worse) will happen again soon. However, we can be certain that the planet is warming up and that all that extra energy won’t be distributing itself evenly. 

On a geological timescale, what we’re doing to the climate is happening very fast. But what about the human timescale? We’ve still got years — indeed, decades — to adapt, haven’t we?

Well, yes, as a species, we have. But as individuals, there’s literally a point at which we can’t cope with extreme heat. 

You may have heard a few mentions this week — e.g. here and here — of something called ‘wet-bulb temperature’. This takes a bit of explaining, but bear with me because your life might just depend it.

It’s got nothing to do with planting tulips, but rather refers to the bulb of a thermometer. If you cover the bulb in a wet cloth then, because of the cooling effect of evaporation, a lower temperature is recorded than if the bulb is left dry. 

The purpose of the exercise is to simulate what human bodies do to stay cool, i.e. perspire. When the moisture on our skin evaporates, the cooling effect allows us to tolerate higher ambient  temperatures than if we didn’t sweat like pigs.

However, there are limits. At a wet-bulb temperature of around 35°C, human bodies gain more heat than they lose, no matter how much they sweat. It’s at this point that people start dying.

Though many parts of the world regularly experience temperatures above 35°C, these are ambient or ‘dry-bulb’ temperatures that don’t take into account the cooling effect of evaporation. Currently, wet-bulb temperatures very rarely reach this danger level (except in very local environments like a stifling apartment or a locked car). However, if we continue to push global temperatures upwards at the current rate, whole parts of world will slip into the danger zone. 

Outside of artificially-cooled spaces, these areas would become uninhabitable — which is obviously a very bad thing, even if, at first, it only happens occasionally.

Environmental campaigners are missing a trick by not doing more to highlight this issue. While global warming has all sorts of nasty consequences well before we hit the wet-bulb danger zone, it strikes me that a hard limit on human survival is a great way of concentrating minds. 

Peter Franklin is Associate Editor of UnHerd. He was previously a policy advisor and speechwriter on environmental and social issues.