How Hot is too Hot for you to survive?

Just how high does the temperature need to get before it becomes too hot for human life to be maintained?

It’s actually quite a complicated question to answer and yet there is an answer – 35C – go beyond that and your life is at risk.

Wait, that’s crazy, it goes over that temperature in many places all around planet, yet people happily thrive.

Why is 35C deadly for human life?

I did say that it would get a bit complicated. OK, permit me to explain.

What I’m referring to is not the normal air temperature of 35C, but rather a wet-bulb temperature of 35C. The number might be the same but a wet-bulb temperature of 35C is something quite different. It is a measurement that also considers humidity. There is a cooling effect from evaporation.

A key to understanding this is to remember that a high temperature, of say 46C, is something humans can easily survive if the air is dry, but bump up the humanity, by let’s say 50%, and it becomes a very different story. That would yield a wet-bulb temperature of about 35C. Prolonged exposure to that will result in most strong normal healthy humans dying from hyperthermia. Basically you overheat.

What is a Wet-Bulb Temperature and how do you measure it?

If you wrap a thermometer with a water soaked cloth and then let the atmospheric air pass over it, you are getting a cooling effect via the evaporation of the water. The hot air might be 46C, but the evaporation from the wet cloth will cool the temperature and so the actual reading will not be 46C, but rather a lot less.

How much less?

That will depend upon the humidity of the air. The more humid it is, the less evaporation can cool things. If for example the air at 35C was at 100% humidity, then evaporation can’t take place because the air can’t absorb any more moisture. Net effect, the wet-bulb temperature would in that case be the same as the atmospheric temperature … 35C.

In drier hot air you don’t just feel cooler, you actually are cooler. The evaporation enabled by the drier air enables you to stay a lot cooler. If right now you were in Death Valley where the current forecast is expected to be 110°F to 120°F+ (43°C to 49°C+) then you would be OK. This is because it is also very very dry, so your sweat keeps you cool and brings the temperature down.

Why is a 35C wet-bulb temperature fatal?

Your core body temperature is 37 C and so your skin maintains a temperature of 35 C (or below) so that the core temperature can be maintained. If the wet-bulb temperature rises above 35C then no matter how much you sweat, it will not cool you below that critical 35C. Unless you gain access to air conditioning then you will overheat. The word used to describe that is Hyperthermia – the opposite of hypothermia (too cold). It is perhaps more common to call it heat stroke.

Early symptoms include confusion, delirium, dizziness, weakness, agitation, combativeness, slurred speech, nausea, and vomiting. Lack of access to cooling leads to vital organ damage, unconsciousness, organ failure, and death. Even if you do recover, it can also lead to long-term disability.

Today the wet-bulb temperature generally never exceeds 30 or 31C anywhere, but in a warming world we are fast approaching a time when we will hit and then pass that 35C limit.

Not even the fittest of humans can survive this limit. Even in well-ventilated shaded conditions, when the wet-bulb temperature stays above 35C you will overheat.

There was a paper a few years ago that presented a stark warming of what is coming.

Future temperature in southwest Asia projected to exceed a threshold for human adaptability

Published in Nature back in 2015, the abstract for the above paper explains it as follows …

A human body may be able to adapt to extremes of dry-bulb temperature (commonly referred to as simply temperature) through perspiration and associated evaporative cooling provided that the wet-bulb temperature (a combined measure of temperature and humidity or degree of ‘mugginess’) remains below a threshold of 35 °C. (ref. 1). This threshold defines a limit of survivability for a fit human under well-ventilated outdoor conditions and is lower for most people.

We project using an ensemble of high-resolution regional climate model simulations that extremes of wet-bulb temperature in the region around the Arabian Gulf are likely to approach and exceed this critical threshold under the business-as-usual scenario of future greenhouse gas concentrations. Our results expose a specific regional hotspot where climate change, in the absence of significant mitigation, is likely to severely impact human habitability in the future.

They also had a map with some rather glaring colours that identify where this will happen.

You might think that while it will be there, it will not impact us here. I’d hit pause on that thought.

There is a Wet Bulb Temperature Map

Via another study published last year (2020) in Science Advances we have a far more detailed map. Using data gathered, they created an interactive map that plots the highest wet-bulb temperatures recorded at stations across the world.

Side Note: This study is titled “The emergence of heat and humidity too severe for human tolerance” and was published in May 2020.

The illustration below shows locations that experienced extreme heat and humidity levels briefly (the hottest 0.1% of daily maximum wet bulb temperatures) from 1979-2017. Darker colors show more severe combinations of heat and humidity.

What this reveals is that some areas have already experienced conditions at or near humans’ survivability limit of 35°C.

Breaching the 35C wet bulb limit is not some theoretical issue several generations away, it is here now.

There is also one more recent paper that I would like to flag up.

Projections of tropical heat stress constrained by atmospheric dynamics

This last paper (link above) was published in Nature last March (2021) just a few months ago.

The reason to pull this one up is to point out something rather important. By running climate models the researchers were able to do some very revealing projections. We do still have a choice …

limiting global warming to 1.5 °C will prevent most of the tropics from reaching a TW [Web Bulb Temperature] of 35 °C, the limit of human adaptation.

In other words, while doing nothing about climate change will have dire consequences, taking decisive meaningful action to limit global warming will keep many people safe.

We are not doomed, we do still have choices.