In Warming World, Global Heat Deaths Are Grossly Undercounted

Yale Environment 360: This summer is shaping up to be one of the hottest on record in the U.S. You have studied the question of excess deaths during heat waves. Do we know how many people are dying?

Kristie Ebi: The number of excess deaths is much higher than what is reported because those numbers come from death certificates, and those typically are people who die from heat stroke. But that’s just a tiny fraction of the total number of people who die during and immediately after a heat wave. Excess death analyses after a heat wave indicate about 50 percent of all deaths are from cardiovascular causes.

The CDC’s estimate of heat deaths, about 1,200 Americans a year, is probably at least a tenfold undercount. An estimate from several years ago was that, in the U.S., there are actually in the range of 10,000 to 12,000 heat-related deaths, and that is likely higher today.

e360: Why is there such a gap between the actual number of excess deaths and the numbers that are attributed to heat?

Ebi: Think of two people going into an emergency department with a heart attack. One was pushed over the edge into the heart attack because of the heat. The other was not. There’s not a way to tell at the individual level, in the middle of a crisis, which heart attack could have been exacerbated by the heat and which was not. In both cases, the cause of death will be recorded as a heart attack.

But we can tell at the population level. As with COVID, you look at the number of deaths during a particular time period, compare it with earlier time periods when there wasn’t an epidemic, when there wasn’t a heat wave, and that gives you a relatively accurate count of excess deaths. But again, at the individual level, making that attribution is pretty challenging.

e360: So it’s more accurate to look at the excess deaths for a period than to look at, say, what a doctor writes on the death certificate?

Ebi: That’s correct. Because the focus of the emergency department has to be on trying to save that life, and you don’t really want to change that dynamic. But there’s also people who die from kidney causes, from respiratory causes, from a range of other issues, and parsing out how heat contributed to those illnesses is challenging at the individual level. But at the population level it’s relatively straightforward.

e360: Over the past several months the Federation of American Scientists, which works to minimize the risks of significant global threats, including climate change, has made several statements calling for the adoption of a standard methodology to measure heat deaths. Would that help?

Ebi: The CDC is actively considering doing this because they are acutely aware that current estimates of heat-related mortality significantly underestimate the burden. The goals of understanding the numbers of deaths from heat include raising awareness and ensuring that the level of investment reflects the magnitude of the problem.

While it would be desirable to have precision, there are tradeoffs in achieving it. Investigating all deaths during a heat wave to determine which were associated with heat requires resources and time, which would take away from other priority health issues.

e360: Do we have any idea how sharply the excess deaths are increasing due to heat waves in the past, say, decade or two?

Ebi: We do have studies looking at the numbers of excess deaths, and they are increasing with climate change in some places but not in others because people are adapting to those changes. And of course, communities are adapting to those changes by implementing early warning systems, putting in cooling shelters, and by making other changes to try and protect the population.

e360: Do we know what percentage of heat deaths is actually due to climate change?

Ebi: There is abundant evidence that heat waves are increasing in frequency, intensity, and duration. There was a big modeling study that came out a couple of years ago, it looked at 43 countries, and the answer was, on average, 37 percent of heat-related summer mortality over the last several decades was due to climate change across those countries.

e360: You’ve said elsewhere that people don’t think of heat as something that they should be worried about. But what you just said suggests this may be changing, that people are becoming more aware of the dangers of heat than they were a few years ago.

Ebi: And thank you to the media for covering this story and helping people to understand that indeed they are at risk. Heat is a silent killer. Some early warning signs of heat stress are nausea, fatigue, and dizziness. But it can be challenging, because often the first sign that people are in trouble is they get confused and can’t always judge for themselves what is happening. It means that all of us need to pay attention to family, friends, and colleagues to make sure that they’re doing okay when the temperatures are high.

e360: People don’t typically start dying or having really severe reactions on the first day of a heat wave. Why is that?

Ebi: It goes back to your core body temperature and how that temperature affects your cells and your organs. It’s not instantaneous. It does take a while for the cells and the organs to start reacting to the high temperature. And that’s why it’s 24 hours after the start of a heat wave when you start seeing the impacts.

e360: We’ve been talking mostly about the U.S., but in large parts of the world, heat is getting to levels that are threatening to any human life outside. Are there whole countries that are going to be entering the unlivable zone pretty soon?

Ebi: I think it’s important to step back and recognize that those deaths are preventable, that people don’t need to die in the heat. This emphasis on whether it’s going to be livable doesn’t take into account the creativity of human beings. We live in all kinds of environments that are not livable. People live in Antarctica for several months a year. Humans have found ways to modify infrastructure, to modify clothing, behavioral patterns, to be able to survive in very extreme conditions. Our emissions of greenhouse gases are making it more complicated, of course. And so putting the emphasis on how we can prevent those deaths is, from my perspective, a priority.

e360: What are some of the things that we should be doing now to design the world to better withstand these heat events?

Ebi: We need to develop heat action plans with two major components. One is heat-wave early warning and response systems, with the emphasis on the response. It’s not just telling people it’s hot. It’s telling them what they can do about it and ensuring they have access to services, making sure that there are cooling shelters, making sure that there’s outreach to the unhoused, making sure that your outdoor workers are protected, that people living in elderly care institutions have access to cooling.

The other part of a heat action plan is the long-term planning. What are our cities going to look like in 30 to 50 years? Will everybody be required to have a white roof? Will buildings over a certain size be required to have a green roof? Are we planting the trees that will cool our neighborhoods and that are going to survive in a hotter future?

e360: So you’re hopeful?

Ebi: I’m basically a worried optimist.

e360: The worry I can understand. Where does the optimism come from?

Ebi: The optimism comes from the fact that our society is always changing and that there are constant advances. And it also comes from my students. They really care about climate change issues, and they’re committed to helping us find even more solutions in the future.

This interview was edited for length and clarity.