- Most of the deadliest heat waves to date have been dry heat waves.

But as our climate warms a possibly even deadlier heat is on the rise.

- It doesn't matter how breezy it is, it doesn't matter how much shade you're under.

It doesn't matter how much water you're drinking.

- These are extreme humid heat waves, often referred to as wet-bulb events.

And these are, in my opinion, one of the scariest byproducts of climate change.

They've been relatively rare historically, but a new study shows that by two degrees Celsius of global warming, many of the most populated places on Earth are likely to experience them.

And the only way to protect ourselves from these dangerous wet-bulb temperatures is to cool our environment.

But what happens if that's not possible?

- The time of year that always has the highest stress on the grid is always just the hottest day of the year because so many people are using AC and so then the AC is liable to go out.

- What happens if there's another extreme weather event?

If you have a tropical cyclone that comes in, wipes out power?

- Stick around as we break down why these temperatures are so deadly and how we can prepare for a world that's so warm, our bodies literally can't handle it.

Let's start by talking about dry versus humid heat.

Dry heat waves are usually driven by large high pressure systems that trap hot air near the surface.

These systems push air downward, and as the air compresses, it heats up in a process called adiabatic heating.

If the high stays stuck in place, the heat builds day after day with no clouds or wind to carry it away.

And that's exactly what happened during the 2003 European heat wave and the 2021 Pacific Northwest heat dome.

Now let's take a look at humid heat.

- As temperatures rise, we can hold a lot more moisture in the atmosphere.

And so this means that if a given location has air temperatures going up with ongoing climate change, it's likely that if they have a moisture source, humidity's also going to increase.

- We just saw how dangerous this can be in 2015 when one of the most extreme humid heat events on record struck Karachi, Pakistan.

Temperatures up to 45 degrees Celsius combined with oppressive humidity, power outages and water shortages made conditions even worse.

At least 1200 people died in just 10 days most from heat stroke and dehydration.

Humans primarily cool down by sweating, but when the air is humid, sweat can't evaporate as easily, so we don't cool off as well.

This is where wet-bulb comes into play.

wet-bulb temperature combines heat and humidity into a single number, which helps us understand when conditions become dangerous or even lethal.

- You can imagine taking a normal thermometer.

If you take that thermometer and you cover the bulb in a little sock that is then placed into a container of water, so it's constantly wetted.

Evaporation's going to take place off of that little sock and it's gonna have a net cooling effect.

- The wet sock at the end of the wet-bulb thermometer functions similarly to human skin.

Water evaporates off the wet sock while sweat evaporates off our wet skin.

In both cases, evaporation carries away heat into the surrounding environment.

- But the more humid the ambient air is, the more difficult and less efficient that evaporation is to take place.

And so that makes any cooling through evaporative cooling, less efficient at high temperatures and high humidity.

- Which is why humid heat feels so muggy and uncomfortable for us and why it can be so dangerous.

There have been a number of attempts to better understand how humid heat affects the human body, in particular where our physical limits are, but it's not easy to identify dangerous wet-bulb thresholds because well, they're dangerous.

- So obviously you can't put someone in a heat chamber and truly cook them.

That would be unethical.

But what you can do is increase temperature and humidity and measure someone's core temperature and see what's the point at which their core temperature starts to increase.

And that's the limit of compensability.

- The human body generally does a good job of regulating its core body temperature through sweat.

But once we cross the limit of livability or compensability, as scientists call it, our core body temperature starts to increase uncontrollably.

And this wet-bulb temperature range is actually a lot lower than we previously believed.

- So this paper is really trying to summarize the most state-of-the-art estimations of these thresholds.

And so what we see is that there is a range.

So uncompensable wet-bulb temperatures range from about 19 to 32 degrees Celsius.

- And you might think the lower end of this range, 19 degrees Celsius sounds really low.

But it's important to understand that a 19 degree wet-bulb does not equal 19 degrees on a regular thermometer.

So 19 degrees Celsius wet-bulb is actually more like 46 degrees Celsius with low humidity or 32 degrees Celsius with higher humidity.

And the reason there's a range is because this threshold depends on a number of variables including activity level, exact air temperature, and humidity levels and age.

- Older adults thermo regulatory responses are worse, they sweat less efficiently, they also have higher risks of comorbidities, so things like cardiovascular and respiratory diseases.

- So according to their study, where on earth are we likely to cross these thresholds?

Well, at two degrees Celsius of global warming, which could come in as little as two decades for older adults.

It's about a third of Earth's land area, but that only tells part of the story.

- It's 35% of the land area, but it's also many of our most populous places on earth.

Most of India, most of the populated area of China.

This is most of the eastern portion of the U.S. - And if we let global warming get really out of hand or if climate sensitivity is just on the higher end of our projections at four degrees Celsius above pre-industrial levels, over 60% of Earth's landmass could see these dangerous wet-bulb temperatures.

Heat is the deadliest weather disaster in the U.S. in many parts of the world, and it's projected to get a lot deadlier.

During the 2003 European heatwave, up to 3,400 people died in Paris alone.

At two degrees Celsius, which is again only a couple decades away, scientists found that Paris will see deadly heat waves like this once every few years rather than once a century.

And though this heat wave in 2003 was among the deadliest in recent history, it didn't get close to the dangerous wet-bulb thresholds we've been discussing in this episode.

But the 2015 heat wave in Karachi did it passed the uncompensable wet-bulb threshold for older adults for something like eight to 10% of the time.

But at two degrees Celsius of warming, we can see that the uncompensable threshold is crossed for younger adults as well.

And at four degrees, models show that the same event would cross the uncompensable threshold for older adults for about 90% of the heat waves duration.

And the thing to remember about these critical wet-bulb threshold is that once you cross one, the only way to bring your internal temperature down is by cooling yourself or your environment.

- It doesn't matter how breezy it is, it doesn't matter how much shade you're under, it doesn't matter how much water you're drinking.

The only way you can protect yourself from these conditions is by cooling your environment.

And so that is why air conditioning is really the major adaptation measure and protection measure that we have.

- And although AC is one of the most important tools for surviving extreme heat, it's far from perfect.

First of all, it's expensive.

AC is energy intensive, and as our climate gets warmer, many people won't be able to afford to stay cool.

And due to its financial inaccessibility, many people in places most vulnerable to deadly heat don't have access to AC.

Second, air conditioning doesn't eliminate heat, it just moves it.

Cooling the indoors means transferring the heat outside, which warms the surrounding air.

And in cities, this added heat can worsen the urban heat island effect.

Now, let's imagine that everyone could afford air conditioning.

It still wouldn't automatically solve the problem.

Our power grids in the U.S. and globally weren't designed for everyone to crank up their units at once.

During heat waves, widespread outages become more likely leaving the most vulnerable at even greater risk.

- If AC units were our only way of protecting people as soon as the power system fails, whether that's from the grid being overloaded, or if we have another weather driven sort of infrastructure damage that's going to put people immediately at risk.

- And these compounding hazards like a heat wave after a hurricane are a growing concern as we head towards unprecedented levels of warming for humans.

And as weather across the board just gets more extreme, but there's one more catch.

The more we rely on ac, the more we could be fueling the base problem of global warming.

- If you tried to just build out air conditioning to deal with heat risk, how much would that contribute to carbon emissions across the world?

And how much would that positively feed back on global temperature making the temperature even higher?

- Right now, air conditioning accounts for about 3% of global greenhouse gas emissions.

And according to the International Energy Agency, the number of AC units worldwide is expected to nearly triple by 2050 leading to a massive rise in energy demand.

So what do we do about this paradox?

Well, I think it's a powerful argument for transitioning as quickly as possible to clean energy.

The only way to meet the growing need for AC without accelerating climate change is to power our grids with renewables.

And of course, there's a lot we can and should do beyond just air conditioning.

We can rethink the design of our cities by installing white or green roofs and planting trees.

We can also build more cooling centers, upgrade our electrical grids to be more resilient and prioritize protecting the most vulnerable during heat waves.

The truth is that we are headed into uncharted territory, but it's also true that we still have time to choose how bad it gets.