When Will Extreme Heat Become Unlivable? Wet-Bulb Temperature Explained

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Transcript

00:00Most 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.

00:08It 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.

00:14These 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.

00:24They'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.

00:35And the only way to protect ourselves from these dangerous wet bulb temperatures is to cool our environment.

00:42But what happens if that's not possible?

00:45The 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.

00:52And so then the AC is liable to go out.

00:55What happens if there's another extreme weather event? If you have a tropical cyclone that comes in, wipes out power.

01:00Stick 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.

01:15Let's start by talking about dry versus humid heat.

01:19Dry heat waves are usually driven by large high pressure systems that trap hot air near the surface.

01:26These systems push air downward and as the air compresses, it heats up in a process called adiabatic heating.

01:33If the high stays stuck in place, the heat builds day after day with no clouds or wind to carry it away.

01:39And that's exactly what happened during the 2003 European heat wave and the 2021 Pacific Northwest heat dome.

01:46Now let's take a look at humid heat.

01:49As temperatures rise, we can hold a lot more moisture in the atmosphere.

01:53And so this means that if a given location has air temperatures going up with ongoing climate change,

01:58it's likely that if they have a moisture source, humidity is also going to increase.

02:02We just saw how dangerous this can be in 2015 when one of the most extreme humid heat events on record struck Karachi, Pakistan.

02:11Temperatures up to 45 degrees Celsius combined with oppressive humidity.

02:16Power outages and water shortages make conditions even worse.

02:19At least 1,200 people died in just 10 days, most from heat stroke and dehydration.

02:26Humans primarily cool down by sweating.

02:29But when the air is humid, sweat can't evaporate as easily, so we don't cool off as well.

02:35This is where wet bulb comes into play. Wet bulb temperature combines heat and humidity

02:40into a single number which helps us understand when conditions become dangerous or even lethal.

02:46You can imagine taking a normal thermometer. If you take that thermometer and you cover the bulb

02:52in a little sock that is then placed into a container of water so it's constantly wetted,

02:58evaporation is going to take place off of that little sock and it's going to have a net cooling effect.

03:03The wet sock at the end of the wet bulb thermometer functions similarly to human skin.

03:08Water evaporates off the wet sock while sweat evaporates off our wet skin.

03:13In both cases, evaporation carries away heat into the surrounding environment.

03:18But the more humid the ambient air is, the more difficult and less efficient that evaporation is

03:25to take place. And so that makes any cooling through evaporative cooling less efficient at high

03:30temperatures and high humidity. Which is why humid heat feels so muggy and uncomfortable for us and

03:36why it can be so dangerous. There have been a number of attempts to better understand how humid heat

03:42affects the human body. In particular, where our physical limits are. But it's not easy to identify

03:49dangerous wet bulb thresholds because, well, they're dangerous.

03:53So obviously you can't put someone in a heat chamber and truly cook them. That would be unethical.

03:59But what you can do is increase temperature and humidity and measure someone's core temperature

04:05and see what's the point at which their core temperature starts to increase. And that's the

04:10limit of compensability. The human body generally does a good job of regulating its core body temperature

04:17through sweat. But once we cross the limit of livability or compensability, as scientists call it,

04:23our core body temperature starts to increase uncontrollably. And this wet bulb temperature

04:28range is actually a lot lower than we previously believed. So this paper is really trying to summarize

04:36the most state-of-the-art estimations of these thresholds. And so what we see is that there is a range.

04:41So uncompensable wet bulb temperatures range from about 19 to 32 degrees Celsius.

04:46And you might think the lower end of this range, 19 degrees Celsius, sounds really low. But it's

04:52important to understand that a 19 degree wet bulb does not equal 19 degrees on a regular thermometer.

04:59So 19 degrees Celsius wet bulb is actually more like 46 degrees Celsius with low humidity or 32 degrees

05:07Celsius with higher humidity. And the reason there's a range is because this threshold depends on a number

05:14of variables including activity level, exact air temperature and humidity levels, and age.

05:21Older adults' thermoregulatory responses are worse. They sweat less efficiently. They also have higher

05:27risks of comorbidities, so things like cardiovascular and respiratory diseases.

05:31So according to their study, where on earth are we likely to cross these thresholds? Well, at 2 degrees

05:38Celsius 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.

05:45But 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.

05:54Most of India, most of the populated area of China. This is most of the eastern portion of the U.S.

06:01And if we let global warming get really out of hand, or if climate sensitivity is just on the higher

06:08end of our projections, at 4 degrees Celsius above pre-industrial levels, over 60% of Earth's landmass could

06:16see these dangerous wet bulb temperatures. Heat is the deadliest weather disaster in the U.S.

06:22in many parts of the world, and it's projected to get a lot deadlier. During the 2003 European heat wave,

06:29up to 3,400 people died in Paris alone. At 2 degrees Celsius, which is, again, only a couple decades

06:36away, scientists found that Paris will see deadly heat waves like this once every few years, rather

06:43than once a century. And though this heat wave in 2003 was among the deadliest in recent history,

06:49it didn't get close to the dangerous wet bulb thresholds we've been discussing in this episode.

06:54But the 2015 heat wave in Karachi did. It passed the uncompensable wet bulb threshold for older adults

07:01for something like 8 to 10% of the time. But at 2 degrees Celsius of warming, we can see that the

07:08uncompensable threshold is crossed for younger adults as well. And at 4 degrees, models show that the same

07:15event would cross the uncompensable threshold for older adults for about 90% of the heat wave's duration.

07:22And the thing to remember about these critical wet bulb thresholds is that once you cross one,

07:27the only way to bring your internal temperature down is by cooling yourself or your environment.

07:33It doesn't matter how breezy it is. It doesn't matter how much shade you're under. It doesn't

07:37matter how much water you're drinking. The only way you can protect yourself from these conditions

07:42is by cooling your environment. And so that is why air conditioning is really the major adaptation

07:49measure and protection measure that we have. And although AC is one of the most important tools

07:54for surviving extreme heat, it's far from perfect. First of all, it's expensive. AC is energy intensive,

08:02and as our climate gets warmer, many people won't be able to afford to stay cool. And due to its

08:07financial inaccessibility, many people in places most vulnerable to deadly heat don't have access to AC.

08:14Second, air conditioning doesn't eliminate heat. It just moves it. Cooling the indoors means transferring

08:23the heat outside, which warms the surrounding air. And in cities, this added heat can worsen the urban heat

08:30island effect. Now, let's imagine that everyone could afford air conditioning. It still wouldn't

08:38automatically solve the problem. Our power grids in the US and globally weren't designed for everyone

08:44to crank up their units at once. During heat waves, widespread outages become more likely,

08:50leaving the most vulnerable at even greater risk. If AC units were our only way of protecting people,

08:56as soon as the power system fails, whether that's from the grid being overloaded or if we have another

09:02weather driven sort of infrastructure damage, that's going to put people immediately at risk.

09:07And these compounding hazards like a heat wave after a hurricane are a growing concern as we head

09:13towards unprecedented levels of warming for humans and as weather across the board just gets more

09:19extreme. But there's one more catch. The more we rely on AC, the more we could be fueling the base

09:26problem of global warming. If you tried to just build out air conditioning to deal with heat risk,

09:34how much would that contribute to carbon emissions across the world? And how much would that positively

09:40feed back on global temperature, making the temperature even higher?

09:44Right now, air conditioning accounts for about 3% of global greenhouse gas emissions. And according to the

09:50International Energy Agency, the number of AC units worldwide is expected to nearly triple by 2050,

09:58leading to a massive rise in energy demand. So what do we do about this paradox? Well, I think it's a

10:05powerful argument for transitioning as quickly as possible to clean energy. The only way to meet the

10:11growing need for AC without accelerating climate change is to power our grids with renewables. And of

10:18course, there's a lot we can and should do beyond just air conditioning. We can rethink the design of our

10:25cities by installing white or green roofs and planting trees. We can also build more cooling

10:32centers, upgrade our electrical grids to be more resilient, and prioritize protecting the most vulnerable

10:39during heat waves. The truth is that we are headed into uncharted territory. But it's also true that we

10:45still have time to choose how bad it gets. So do you live in an area that's already experiencing dangerously hot

10:53in humid conditions? What strategies have worked for you to handle the heat? Let us know in the comments

11:00and we'll see you next time on Weathered. Oh, and before you go, we also wanted to let you know about

11:05something special. PBS and the BBC have teamed up to bring you Walking with Dinosaurs Unearthed, an amazing

11:12show about the prehistoric beasts that ruled the earth before us and experienced some pretty extreme weather

11:18of their own. The premiere episode just published earlier this week on PBS Terra and we think it's

11:24something our viewers will really enjoy. Click the link on screen or below to check it out and tell them

11:30Weathered sent you.

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