Killer Heat in the United States

• Kristy talks about UCS' recently released report on extreme heat
• Colleen asks how disruptive extreme heat will be
• Colleen and Kristy discuss what we can do to have a fighting chance against the warming planet

• Opener (0:00-0:55)
• Intro (0:55-2:30)
• Interview part 1 (2:30-10:40)
• Break (10:40-11:38)
• Interview part 2 (11:38-23:24)
• Science FTW Throw (23:24-23:31)
• Science FTW (23:31-26:57)
• Outro (26:57-28:23)

• Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days (2019)
• Top Ten Heat-Related Terms You Need to Pay Attention to While Reading UCS’s Killer Heat Report
• 5 Great Public Health Resources for Dealing With Extreme Heat

Colleen: Kristy, thanks for joining me on the Got Science podcast.

Kristy: Thanks for having me, Colleen.

Colleen: So, you've just finished an analysis looking at extreme heat and climate change. Why did you and your colleagues decide to do this analysis?

Kristy: Well, for many years we've been working on sea level rise and while sea level rise will affect millions of people over the course of the coming decades, it's also a very coastal problem. It's limited to a really narrow band along the coast. And it's just one of many, many potential impacts of climate change. So, as we saw our sea level rise work build, we thought we would like to be engaging people who live away from the coasts as well in places that may not experience something like sea level rise but will certainly be feeling the effects of climate change. So we turned to extreme heat.

Colleen: So, what data did you use and what was the methodology?

Kristy: Sure. So, we used climate models to try to understand how extreme heat will change in the future. And when scientists use climate models, the best practice is not just use one model, but to use many. There are dozens of climate models that have been developed all over the world. And what we've done is taken a suite of those, we used 18 of them. And each of those models the same experiment and then we average the results from those climate models. So, we specifically take the temperature data and the relative humidity data. Those are the two components of what's called the heat index or the feels like temperature. And we have daily data from each of those climate models that allowed us to see what the heat index was on any given day between now and the year 2100. And we use that daily data to ask the question, how often would the heat index be above a certain temperature, above 100 degrees Fahrenheit? So for each of those models we looked at the number of days above those different temperature thresholds we were interested in and then we average that number of days over all those climate models to get sort of a coherent result.

Colleen: What is a climate model?

Kristy: You can think of a climate model as a giant computer program that you have fed information into and that information includes things like how the oceans circulate, how water exchanges between the oceans and the atmosphere, how ice forms in the Arctic in the winter and recedes in the summer. So essentially, it's all of the physics that we know about our planet. And then you can force those climate models with different questions or scenarios. You can say, "Okay, climate model. What happens if we increase the amount of carbon dioxide in our atmosphere from now through the end of the century?" And the climate model will give you an answer, right? It will show you at every point on the earth what the temperature change is, what the humidity change is, how it changes the winds. And so that's how scientists do these experiments to say, " What happens if we start decreasing emissions now versus letting them go up and up and up through the end of the century?"

Colleen: tell me about the timeframe that you looked at and what different scenarios you fed into the model.

Kristy: So we looked at the period from 1970 through 2000, And we used that as our historical baseline. And then we looked at mid-century and used 30 years centered around the year 2050. And we looked at the end of the 21st century, the last 30 years of this century. The reason we use 30 years is because there's a lot of variability in our climate from year to year. You can think of things like El Niño which happens every three to five years and creates very different conditions on Earth's surface depending on where you are in that cycle, right? So, we use a 30-year period to make sure we're getting what we call a climatological average. And then we ran the models with different scenarios. We had one scenario that we could call business as usual where greenhouse gas emissions continue to rise between now and the end of the century largely unabated, without any attempts to decrease them. We ran a scenario that has greenhouse gas emissions starting to decrease around mid-century. So they continue to rise until about 2045 and then they start to decrease. And then we also looked at a scenario that we call the Paris scenario. So this is a scenario in which future warming is limited to two degrees Celsius or about three and a half degrees Fahrenheit. And that's the goal of the Paris Climate Agreement. And so we were able to compare these different scenarios and see how extreme heat in the U.S. would differ under those different scenarios.

Colleen: So, let's talk about the heat index for a minute. You said the feels like temperature, and that's often you hear in a weather report. It's going to feel like X Tell me what exactly the heat index is.

Kristy: Sure. So the heat index is a combination of temperature and relative humidity. And you can think of it this way. If you were in your car sitting in Arizona and the dashboard temperature on your car was 90 degrees, when you stepped out of the car, you would feel one thing in Arizona. But if you drove your car to say, South Carolina, and the dashboard temperature said 90 degrees, when you stepped out, it would likely feel very much hotter, right? Because South Carolina tends to be a much more humid place. So, our bodies don't just experience temperature, they experience this combination of temperature and humidity. When the humidity is high, the temperature feels much hotter.

Colleen: So, what's happening to the human body Why does that happen?

Kristy: A lot of things happen to the human body as the heat index goes up. As temperature increases, our hearts have to pump blood faster and we start to pull water away from the core of our body and toward the skin and that's why your skin becomes flushed when it's hotter. But as that happens, there's a risk of dehydration, there's a risk of your organs like your kidneys not having enough fluid to function properly. So, all of those changes are happening physiologically in your body as the heat index rises. And the important part about humidity and how it factors into our experience of heat is that when it's very humid outside, there's a lot of water vapor in the air. It's actually harder for your body to cool itself by sweating because it's the evaporation of that sweat that cools you off. When it's more humid, that evaporation is harder.

Colleen: So, ideally, in a humid area where you're sweating and cooling your body, you're replenishing by drinking water you're replenishing yourself.

Kristy: Absolutely.

Colleen: That in this extreme heat your body can't release its own water so you're just heating up inside essentially.

Kristy: You're just heating up. That's right. And rehydrating is incredibly important and it's one of the reasons why children are more vulnerable when there's extreme heat because they often don't recognize that need to rehydrate, and so they tend to become dehydrated more quickly.

Colleen: So, if people that are living in areas in the south, say, where it's very humid are going to be greatly affected, so what about the areas like Arizona where it's dry? Do those areas do they not have to worry or... I'm wondering if I'm imagining a mental map of the United States that there are particular areas that will be really hard hit.

Kristy: There definitely are going to be areas that are more hard hit than others. And we certainly see that in the western half of the U.S. where humidity is generally lower than it is in the east that the increases in the number of extremely hot days that we predict tend to be somewhat smaller. However, that doesn't mean that they're any less dangerous, right? So, in places like Southern Arizona where there's already a really high number of extremely hot days. That means that even if there's a one and a half times increase in the next 30 years or so, that's still that many more days that you're at risk of dehydration, heat cramps, heat stress, heat stroke.

Colleen: Talk to me about the timeframe that you covered in the analysis.

Kristy: So we looked at a few different time periods. We looked at a historical time period that covers the 1970s through 2000, so 30 years' history. Then we looked at 30 years around mid-century, so the years 2036 to 2065. The middle of that is at 2050. And then we looked at the last 30 years of this century.

Colleen: And what did you find?

Kristy: So what we found is that overall for the country, just by mid-century, there's a doubling in the number of days with a heat index above 100.

Kristy: And we also found that there's a tripling in the number of days with a heat index above 105. We also looked at these off the charts kinds of conditions. And this is a really fascinating concept that's not something to our knowledge that anyone's looked at before. So, the heat index was designed to be calculated and to be valid for conditions that we experience on earth today. The thing is that as climate warms our overall temperatures, we're going to be getting outside of the range that the heat index was designed to calculate. So right now in our country, the only place where we have these conditions that we call off the charts where we can't reliably calculate a heat index are in the Sonoran Desert, that's Southern Arizona, very southern edge of California. And even there, there are only two or three of these sorts of days each year historically. But what we found was that by mid-century, about 25% of the country by area would experience these conditions at least once a year.

[Break]

Colleen: What is a climate model?

Kristy: You can think of a climate model as a giant computer program that you have fed information into and that information includes things like how the oceans circulate, how water exchanges between the oceans and the atmosphere, how ice forms in the Arctic in the winter and recedes in the summer. So essentially, it's all of the physics that we know about our planet. And then you can force those climate models with different questions or scenarios. You can say, "Okay, climate model. What happens if we increase the amount of carbon dioxide in our atmosphere from now through the end of the century?" And the climate model will give you an answer, right? It will show you at every point on the earth what the temperature change is, what the humidity change is, how it changes the winds. And so that's how scientists do these experiments to say, " What happens if we start decreasing emissions now versus letting them go up and up and up through the end of the century?"

Colleen: tell me about the timeframe that you looked at and what different scenarios you fed into the model.

Kristy: So we looked at the period from 1970 through 2000, And we used that as our historical baseline. And then we looked at mid-century and used 30 years centered around the year 2050. And we looked at the end of the 21st century, the last 30 years of this century. The reason we use 30 years is because there's a lot of variability in our climate from year to year. You can think of things like El Niño which happens every three to five years and creates very different conditions on Earth's surface depending on where you are in that cycle, right? So, we use a 30-year period to make sure we're getting what we call a climatological average. And then we ran the models with different scenarios. We had one scenario that we could call business as usual where greenhouse gas emissions continue to rise between now and the end of the century largely unabated, without any attempts to decrease them. We ran a scenario that has greenhouse gas emissions starting to decrease around mid-century. So they continue to rise until about 2045 and then they start to decrease. And then we also looked at a scenario that we call the Paris scenario. So this is a scenario in which future warming is limited to two degrees Celsius or about three and a half degrees Fahrenheit. And that's the goal of the Paris Climate Agreement. And so we were able to compare these different scenarios and see how extreme heat in the U.S. would differ under those different scenarios.

Colleen: How disruptive will this extreme heat be for people that are living in the hardest hit regions?

Kristy: So, there are a couple of different ways to think about which regions will be hardest hit by changes in extreme heat. You could argue that it would be places like the south of Florida. So right now, Orlando, Florida experiences about 20 days with a heat index above 100. So that's about three weeks. If we look to mid-century depending on what scenario we look at, in Orlando, they could be experiencing between 87 and 107 days with a heat index above 100. And if we look at late century in Orlando the end of the century, they could see over 140 days per year with a heat index above 100. And some of those days would be even hotter. So there'd be about 110 days where the heat index would be above 105 and about 15 days that would have off the charts conditions where we can't even really calculate a heat index.

Colleen: So, this is... Many people in Florida have air conditioning, but many people don't. And this type of weather sounds dangerous.

Kristy: It is. It is dangerous. And the danger of this type of heat in Florida was really highlighted in the wake of Hurricane Irma in 2017. So, Irma came up the Florida peninsula from south to north and caused widespread power outages for thousands of people. Without the air conditioning, there were about 12 residents of a nursing home who died because of extreme heat. So, it wasn't the storm itself that caused it but the lack of air conditioning in the heat wave that followed. And that happened with a heat index that hovered right around 100 degrees for a couple of days. So some of the heat we're talking about is even more extreme than that.

Colleen: But also for people who work outside who just don't have access to air conditioning but... I'm imagining landscapers...

Kristy: Construction workers, farm workers. Yes.

Colleen: Exactly.

Kristy: We all rely on a huge group of people in this country who do work outside in conditions where they can't have air conditioning.

Colleen: So, they either lose work or they potentially lose their health.

Kristy: And that's one of the big problems with trying to understand the impact of extreme heat on outdoor workers. So, if you think of a migrant farm worker, for example, that person may be getting paid by the pound or by the bushel. And if they don't have the opportunity to pick or harvest, then they're not getting paid. And so there is a disincentive to reporting that you're feeling uncomfortable due to heat, there's disincentive for taking the breaks that you need in order to rehydrate to sit in the shade. So we know that incidences of heat-related illness in outdoor workers particularly in the agriculture sector are underreported.

Colleen: So what areas of the United States will be hardest hit?

Kristy: In terms of the absolute number of extremely high heat index days, it's places that you wouldn't find surprising, right? Southern parts of Texas, Florida, the deep Gulf Coast states, Louisiana, Mississippi. But then there are places like New England where the absolute number of days that they'll be experiencing extreme heat in the future is much lower than a place like Texas, but the people who live there are much less accustomed to it and the infrastructure isn't designed for it. So, in Texas, most buildings, most homes are air-conditioned. That's not necessarily the case in New England or in the San Francisco Bay area where I live, nobody has air conditioning, no schools are air-conditioned. And so when an extreme heat event hits in these kinds of places, it has an outsized impact.

Colleen: Well, also, if you then have to air condition a lot more places, then you're not exactly reducing emissions.

Kristy: Right. The solution here can't be simply add air conditioning because that's extremely energy intensive. And we know that it's at these times when our energy demand is the greatest that the energy we're getting is the dirtiest because we have to turn on additional power plants that are typically burning coal. So we know that any response that we design for adapting to a future of extreme heat has to be coupled with one that is addressing clean energy infrastructure.

Colleen: So, these extreme heat days that are above the current heat index chart, we don't even have a name for that. Will a new chart be developed or will the heat index be expanded?

I think the question of how we handle the fact that we can't calculate a heat index is an interesting one and I think one that we can surmount, right? We can figure out other ways to describe the feeling of this heat. And some of them may be mathematical, there may be other measures that combine heat and humidity that don't have this upper limit like the heat index does. But I think it also raises this interesting question of how we communicate that risk to the public. So right now the National Weather Service issues heat advisories and excessive heat warnings that alert people to the fact that there are unsafe conditions outside. Typically, when the heat index is expected to be above 100, they issue a heat advisory, and that'll say things like, "Make sure you're drinking water. Don't leave pets and children in parked cars. Children and elderly are particularly at risk. Check in on your neighbors." Things like that. And at 105 heat index, they'll issue and an excessive heat warning which communicates that this is a risk not just for those sensitive groups but for everyone. But how do we start to communicate when conditions surpass that? And how will the National Weather Service take that on and say, "Hey, this is heat that's a different category."

And interestingly, we've heard from many local National Weather Service offices that they're concerned about issuing too many heat warnings because people get desensitized to it and they don't take it as seriously. So, given that that's the case, we need to make sure that we're designing systems that really communicate the risk to people of these off the charts, extremely hazardous days.

Colleen: is there anything we can do?

Kristy: There's a lot that we can do here. And one of the most exciting things about our analysis is that it points to the incredible impact that emissions reductions can have not just in the long term at the late century timeframe, but even in the mid-century timeframe. We looked at two different emissions scenarios, a higher emissions scenario where it's our heat-trapping gases continue to rise in kind of a business as usual, no change through the end of the century. We also looked at a lower emissions scenario where we aim to cap future global warming to less than two degrees above the pre-industrial levels. So, it's important to note we already have undergone about a degree of that warming. We don't have a whole lot left until we hit that two-degree mark. And it would require really substantial, fast, dramatic reductions that are greenhouse gas emissions. But when we look at the difference between those two scenarios, it's incredible. So, by late century, if we manage to cap warming to less than two degrees, we see about half the number of these extremely hot days than we would see with that business as usual scenario. So, the way we look at it is that by rapidly reducing our emissions, we can hold the line, right? We'll still see increases in the number of extremely hot days. There will still be really widespread extreme heat. But we could prevent it from getting a lot worse.

Colleen: Do you think it's realistic that we can reduce emissions in that way that rapidly?

Kristy: That's a great question. And I wish that I had an optimistic answer for you. I think different people have different takes on this. We know that we have a lot of the technology we need and we know that the problem at this point is largely one of will and politics, particularly in the U.S. So, some people look at that problem and say, "Well, it's just a problem of politics. We can fix that. It's not like a problem that we can't fix, like, the sun is getting hotter. There's not much we could do about that. We can do something about this." But It is a problem that has been intractable for decades in the U.S. And so while we see really encouraging signs like the fact that Congress is now discussing climate change at all, like, the fact that in states like California and New Mexico we've passed really inspiring laws that put us on a path to 100% clean energy by the mid-century timeframe. They are encouraging signs, but we need so much more.

Many of the impacts of climate change would persist for a long time even if we stopped emitting greenhouse gases today. Something like sea level rise has a certain amount baked into the system, but that's much less the case with extreme heat. And the actions that we take today will truly be felt by the children who will become adults in mid-century. So what we do today will have a relatively quick impact on the future of extreme heat. So what that means is that we need to pull together the energy that we already have in this country and we need to be insisting that our leaders take this on at the federal level to make sure that it's a priority.

Colleen: So if the clean energy momentum that we're seeing already continues, we really have a fighting chance of curbing this.

Kristy: We do. And if you think about the progress that we made during the Obama administration toward strengthening fuel economy standards towards incentivizing clean energy. If we can keep things like that going with the next administration, it will give us the best chance we have.

Colleen: Well, Kristy, thanks so much for joining me.

Kristy: Thanks, Colleen. It was great to be with you.

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Science FTW: Cynthia ReRocco
Editing and music: Brian Middleton
Research and writing: Jiayu Liang and Pamela Worth
Executive producer: Rich Hayes
Host: Colleen MacDonald