Human beings function in a fairly constrained temperature band. The healthy body temperature is 37° Celsius, plus or minus about one degree. The human body cools itself by evaporation. If water can’t evaporate from your skin, you can no longer cool yourself. Rather, you take on heat from the environment. Body temperature above 40° Celsius is a life threatening emergency, and above 42.3° denaturing of proteins, especially in the brain, may occur rapidly. This is not survivable.
But the operation of the human body generates heat continually. The beating of the heart generates heat. The movement of the chest cavity in breathing generates heat. Even brain activity – thinking – generates heat.
So in order to survive we need to be able to dump a small amount of heat into our environment continually. If the air is humid enough that sweat can’t evaporate, we need a small heat gradient to make that possible. So we need it to be actually cooler than 37°, and the survivable number generally quoted is a “wet bulb temperature” of 35°.
What does ‘wet bulb temperature’ mean? It’s the temperature at which water (and sweat) evaporates, and that’s a function of the absolute temperature of the air, and of the humidity: the saturation of water vapour in the air. As the air becomes increasingly humid, so the wet bulb temperature falls. In very dry atmospheric conditions, you can easily survive air temperatures well above 35° Celsius, provided you can drink enough fluids to enable you to sweat.
So: there is a temperature and humidity at which everyone exposed to the air just dies, and just dies pretty quickly. How close are we to hitting those temperature and humidity conditions?
The answer, of course, is that there are places on Earth which regularly exceeded those limits even before the days of significant anthropogenic warming, but that people don’t live in those places. Examples include Death Valley in California, parts of the Arabian Peninsula and of the Sahara, and Pakistan’s northern Sindh province, in which a Victorian British Brigadeer had the bright idea of building a city. What’s interesting is that these are not generally humid places; on the contrary, they’re exceptionally dry.
But the consequence of anthropogenic heating is that we’re seeing both higher average temperatures and higher variations in temperature. Which means we’ve been seeing a lot more exceptional heat events than we’ve been used to.
Over the past decade, ten places on Earth have recorded wet bulb temperatures at or above the theoretical ‘everybody dies’ limit. So far, we haven’t had an event in which very large proportions of the population have died suddenly of heat stroke, despite the fact that three of the places which have seen the highest wet-bulb temperatures, Ras Al Khaimah, Jacobabad and Mecca have significant populations.
But we cannot be very far away from an ‘everybody dies’ event, and the first probably won’t be somewhere that’s accustomed to very high temperatures.
Lytton, in British Columbia, Canada, wasn’t, until this year, accustomed to exceptionally high temperatures. Yes, it had frequently been the hottest place in Canada, but Canada is not on the whole a very hot place. The extreme heat event that hit Lytton in June 2021 – 49.6° Celsius – was fully 5° Celsius warmer than had ever been recorded there before. This is, as I’ve said before, because having more heat in the atmosphere makes it more turbulent and thus more heterogenous.
Lytton is 50°13’52" North. For comparison, that’s about eleven miles south of Plymouth in Devon, England, or about six miles north of Prague, in the Czech Republic.
Lytton didn’t quite exceed the ‘everybody dies’ limit, although it came very close. But it did exceed the ‘everything burns’ limit, which is what it isn’t there any more. And what Lytton proves is that, in an era of increasingly unstable weather, extreme heat events do not only happen in the tropics.
- We’re currently on 1.19° Celsius of anthropogenic warming over pre-industrial levels;
- At 1.19°, we’re seeing local temperature records being exceeded by 5° not only in the tropics but even in temperate zones;
- At 1.19°, our margin of safety for ‘everyone dies’ events in previously habitable zones appears to be completely exhausted;
- If capitalism continues (SSP4), we’re heading for 3.2° Celsius of warming; even on the much more optimistic SSP2 track, we’re now heading for 2.7°.
So: if at 1.19° average warming, we’re seeing local records broken by 5°, by what amount will we see them broken at 3.2° average? This isn’t going to be a simple linear curve; it seems to me that it must be at least to some degree exponential, because there are clearly accelerating effects and feedback loops in there.
But let’s assume that it’s simply linear. Then we would be looking at current local records being exceeded by fifteen degrees, more or less anywhere in tropical or temperate zones. Indeed, given what we saw in Siberia in 2020, we could easily see ‘everyone dies’ events happening as far north as the Arctic Circle. Obviously, they will be more common in the tropics. But no currently populous place on Earth will be immune.
So, just sit for a moment, and imagine. It’s 2035. It’s June. You’ve seen ‘everyone dies’ events, covering thousands of square kilometres, happening in other parts of the world already. Now the weather forecast tells you that there’s a 30% probability of an ‘everyone dies’ event in the area where you live in the next ten days.
What do you do?
That weather forecast is going to trigger everyone who can move, to move. It will cause gridlock on every means of transport. It will cause mass civil unrest. A fortnight later we’ll see news reports of ‘rescue’ workers – probably troops – going into the places people have gathered in their last desperate attempts to survive, have died in heaps. Those places may be subway stations, or underground vaults; they may be public buildings where there had been some hope the air conditioning system would not fail.
And each such event will leave a dead zone behind it to which – although we’ll know that these events will happen more or less randomly, like lightning strikes – few people will want to return.
Let’s be clear about this. 3.2° of warming isn’t ‘survivable’. 2.7° of warming isn’t survivable. We’re on the very edge of seeing ‘everyone dies’ events, covering areas the size of small European countries, now, at 1.19°. We cannot afford any more warming.
Yet already we’re hitting cascade effects. As the Arctic warms, so methane frozen in permafrost or beneath the sea thaws and is released into the atmosphere, ratcheting up warming. As ice melts, new, darker sea and land surfaces are exposed, ratcheting up warming. As ocean circulation systems are disrupted, their moderating effects break down, ratcheting up warming. Even if we all stop burning fossil fuels today, the temperature will still rise to about 1.6° Celsius above pre-industrial levels.
We are already in the disaster zone. Every barrel of oil we pump makes it worse. We have to just stop.