- There's a dangerous combination of heat and humidity which is measured by a wet bulb.

It's a kind of tipping point for the human body where we lose the ability to thermoregulate and things start going very wrong very quickly.

We're talking about heat stroke, organ failure, death in just a matter of hours, and it's not anywhere near as hot as you might think.

And as the globe warms, this unique combination of temperature, geography, and meteorology is becoming more common.

- It's becoming more and more concerning 'cause our average global temperatures are only increasing by one degree Celsius, but that increase in temperature allows more water vapor to be held in the air.

And so not only are we going to have longer heat waves, more severe heat waves and more frequent heat waves, but those heat waves are gonna be more humid.

- 14 counties in the US received a 10 out of 10 risk score for wet-bulb temperatures in a recent analysis.

We're gonna look at where they can happen and the conditions that it takes to create a wet-bulb event, including the highest wet-bulb temperature ever measured on Earth.

So stay with us.

(suspenseful music) So what exactly does wet bulb mean?

- So the temperature that we're kind of used to, the ambient temperature that we check on our app, you know, when we wanna know how hot it's gonna be, that's the dry-bulb temperature.

The wet-bulb temperature is a temperature that's read by a thermometer that's covered in a wet cloth over which air is passed.

So it's essentially a measure of the evaporative capacity of the environment and the ability to cool through evaporative mechanism.

- So in a drier environment like here in my living room, water easily evaporates from the wet cloth, taking heat energy with it and causing the thermometer to cool.

This means that the wet-bulb temperature is generally much lower than the dry-bulb temperature as you can see right here.

But as humidity rises, the cloth releases less moisture since the air is already saturated, causing the wet-bulb temperature to rise gradually, approaching the regular air temperature.

- We, humans, our primary avenue of dissipating heat is through the evaporation of sweat.

And so the wet-bulb temperature is kind of a better index of that evaporative capacity of the environment, our ability to dissipate heat into the environment through sweating.

- A recent study shows that for over 6,000 years, most of us have lived in a moderate temperature range called the human climate niche.

Clearly not all humans, but we do tend to like regions with a mean annual temperature between 11 and 15 degrees Celsius.

When you compare this map of the Earth's most habitable regions to where the most extreme wet-bulb temperatures have been recorded, you can see that they tend to fall well outside the human climate niche.

The study also found that now as global temperatures rise, this niche has begun to change in a meaningful way for the first time in human history, creeping gradually towards the poles, leaving somewhere between one and 3 billion people outside the human climate niche by as soon as 2070 with high greenhouse gas emissions.

Now we get a lot of comments about the Earth's climate going through cycles and that's true.

About 50 million years ago, the average global climate was 14 degrees Celsius warmer than today.

But the period of major human expansion on Earth when we went from just a few million to 8 billion happened during a time of very stable climate.

So it's worth taking note when things change rapidly and wet-bulb events are quickly increasing.

Our goal is to keep our core body temperature below 40 degrees.

So Tony and his colleagues at Penn State set out to find the actual wet-bulb temperature where our bodies' systems fail at cooling our core.

Their subjects swallowed a thermometer pill and got in an environmental chamber where researchers created a wet-bulb event.

Now in 2010, a paper theorized that a 35-degree wet bulb was the upper limit of human survivability from heat stress.

Researchers hypothesized that above this threshold, the body would lose its ability to cool itself through sweating, with core temperatures reaching 40 degrees Celsius in just around six hours.

So those rare wet-bulb events that we talked about in the intro, those were all 35 degree readings.

- So over the past couple years, we've been conducting a series of studies aimed toward identifying the upper limits for the maintenance of a stable internal body temperature or core temperature in humans during low intensity physical activity that's meant to approximate the activities of daily living.

And so those studies are done in human subjects.

So they're actual empirical human subject data, not modeled data.

And we showed that the upper limits for a stable core temperature, it was more like 31 degrees Celsius or so in warm, humid environments.

- Below 31 degrees, they found that the human body could keep its core temperature below 40 degrees without a problem, but above 31 degrees Celsius, our ability to thermoregulate quickly breaks down.

- Above these critical wet-bulb temperatures, your core temperature continuously rises.

And so again, we're able to calculate the rate of change in core temperature over time and predict how long it would take your core temperature to reach 40.2 degrees Celsius.

And in our studies, we found that it would only take about three to five hours.

- And this means that if you're caught in a wet-bulb event without air conditioning, you don't have much time to act, making this tipping point very deadly.

- At these critical wet-bulb temperatures, it's important that we have interventions to mitigate the rise of core temperature, such as, you know, moving into air conditioning, replacing lost fluids, cooling down with a cold towel around your neck, an ice bath, if possible.

- But we know that the human body gets worse at thermoregulation as we age, making the elderly far more vulnerable to heat and humidity.

But extreme wet-bulb events are rare and that's because it takes really specific conditions to create them.

According to climate scientist Colin Raymond, you need heat, moisture, and stability.

- So in a place like Florida, for example, it's pretty hot and humid much of the time, but the reason they don't have really extreme wet-bulb temperatures is because there's not enough stability in the atmosphere.

Basically, what that means is if it gets too hot and humid, thunderstorms are going to very quickly developed.

So sort of high temperatures to start with, a whole bunch of moisture and a mechanism that stops that moisture from escaping the near surface that we walk around in.

And then we experience high wet bulb when all those three conditions are met.

There's a very sort of geographically constrained set of places that these sort of, let's say, greater than 30 degrees Celsius wet-bulb temperatures are really possible with any meaningful frequency.

So in terms of life-threatening or potentially life-threatening wet-bulb temperatures, there's two regions that really stand out from my work and others in the world.

One of those is the Persian Gulf, the immediate coastline of it.

And the other is South Asia.

So basically Pakistan, Northwest India, especially.

- [Maiya] Earlier, we looked at a map of the most extreme wet-bulb events where temperatures reached 35 degrees, the prior theorized limit of human adaptability.

But if we expand the map out to include events hitting the threshold of 31 degrees, we see many more regions affected.

You can see that most of these extreme events have happened near coastlines in the subtropics where there's intense heat from land and humidity from warm water, both of which are increasing in a warmer climate.

- It's like a rising tide lifts all boats, right?

So if we're interested in a certain level of heat stress, which of course we are because our physiologies are not going to change and those of, for example, wildlife or livestock aren't going to change either, then with a rising tide of heat stress, we are just going to expose more and more people and systems that we care about to levels of heat stress that we're not designed for.

- So what parts of the US are most likely to see these events?

The 14 counties most at risk of extreme wet-bulb temperatures are mostly concentrated around the Gulf Coast.

And if we zoom out to include counties that received a six out of 10 risk score or higher, the area of impact expands up to Chicago and along the south and southeast coastlines encompassing millions of people.

- [Colin] Now there's a much smaller area but that is the irrigated desert areas of southeastern California and southwest Arizona.

They're very hot just like in the Middle East and they're also affected by the North American monsoon.

So that there are situations where there can be extremely high heat stress in that area.

- But what about the highest wet-bulb temperature ever registered on Earth?

Well, that happened at 3:00 p.m. on July 8th in 2003 in Dhahran, Saudi Arabia when wet-bulb temperature touched 36.5 degrees Celsius, the heat index or felt temperature measured 81 degrees Celsius.

That's 178 degrees Fahrenheit, an almost unimaginable extreme.

- So as heat stress becomes more extreme and our predictions of it become more precise, I think there will be growing understanding and appreciation of heat stress as something that's not a situation that you can toughen yourself through.

But a situation that really reflects a force of nature just like a severe storm, like a tornado or a hurricane, for example, that everyone knows one has to take shelter from it.

As we start to experience things that are hotter and more humid than we've experienced at least in the past hundred thousand years, we're going to need technology or other adaptations like just seasonally moving out of places that don't support our natural functioning there anymore.

- So what do you think, as our planet continues to get hotter and more humid, what can we do to ensure the safety of our communities, especially the most vulnerable?