Immediate Action Needed to Avoid Thousands of Heat-Related Deaths in U.S., New Study Says

Days of Extreme Heat Will Become Weeks as Climate Warms, U.S. Study Warns

Even regions of the U.S. where extreme heat and humidity have been rare should expect significant increases in the number of hot days by mid-century.

Kristoffer Tigue


JUL 16, 2019

Outdoor laborers, athletes and the elderly are among those most at risk as global temperatures rise. Sarah Reingewirtz/Pasadena Star News via Getty Images
Outdoor laborers and the elderly are among those most at risk as global temperatures rise. Sarah Reingewirtz/Pasadena Star News via Getty Images

Nearly every part of the United States will face a significant increase in extremely hot days by mid-century, even if some action is taken to reduce greenhouse emissions, a new study says. If nothing is done to rein in climate change, it warns, the impact will be worse.

Large parts of the Central and Eastern U.S. will get a taste of what that feels like over the coming days as a muggy heat wave settles in.

The study, published in a peer-reviewed journal and as a longer report by the Union of Concerned Scientists, used 18 climate models to predict changes in the heat index—the mix of heat and humidity that reflects how hot it feels—across the contiguous U.S. as global temperatures rise over the coming decades.

Sign up for InsideClimate News Weekly

Our stories. Your inbox. Every weekend.I agree to InsideClimate News’ Terms of Service and Privacy Policy

It found that the number of days when the average temperature will feel like 100 degrees in the Lower 48 states will more than double, from about two weeks at the end of the last century to 30 days by mid-century, even with some efforts to reduce the greenhouse gas emissions that drive global warming.

And the number of days with a heat index of 105°F or more will more than triple to nearly three weeks, the study found.

Maps: U.S. Forecast: More Extremely Hot Days

The study is part of a growing body of research linking the frequency of extreme heat days and heat waves to climate change.

When the researchers broke down the data regionally, they found that even areas of the U.S. where extreme heat has been historically rare will see significant increases in the number of hot days.

For example, states like Connecticut, Massachusetts and Rhode Island have averaged only about a week out of every year with a heat index of 90°F or higher, the study says. But by midcentury, even with some action taken to reduce emissions, that number jumps to more than a month. For historically hot states, like Florida or Texas, the number of days hitting 100°F or higher on the heat index jumps to more than 100.

“That’s the equivalent of all of June, July and August,” said Kristina Dahl, the study’s lead author. “There are almost no places in the U.S. that won’t be affected by extreme heat in the coming decades.”

More Need for AC Hurts Low-Income People

For people living in states unaccustomed to the heat, where homes often don’t have air conditioning, the rising temperatures mean new expenses.

And for those who can’t afford the cost of air conditioning, the increase in extreme heat days can become a health threat, Dahl said. A heat wave in Chicago in 1995 killed more than 700 people. “Most of them were elderly residents who didn’t have air conditioning. So, we know that air conditioning saves lives,” she said.

As the need for cooling grows, so do utility bills—meaning low-income families who can’t afford to pay the extra cost could be most at risk from the heat.

“The concern is that people who can’t afford to use air conditioning won’t be using it,” said Andrew Grundstein, a University of Georgia professor of geography and climate science. “And they will feel the full effects of heat increase over time.”

Chart: The Heat Index at a Glance

The growing demand for air conditioning also contributes to global warming. Air conditioning units often run on electricity from power plants fueled by coal, oil and gas; and they can leak hydrofluorocarbon (HFC) coolants, short-lived climate pollutants many times more potent than carbon dioxide.

The number of window air conditioning units is expected to more than triple globally by 2050, and those systems could increase the planet’s temperature by almost 1°F by the end of the century, according to a reportlast year by the Rocky Mountain Institute.

Young Athletes and Laborers at Risk

The rising heat has also been a growing problem for outdoor laborers and high school sports.

Seven years ago, a string of high-profile deaths involving high school athletes who suffered heat stroke pushed the Georgia High School Association to adopt new policies to prevent heat-illness among student athletes. As of last year, 28 states had adopted similar policies to those in Georgia—known as preseason heat acclimatization guidelines—according to the National Athletic Trainers’ Association

“Things have come a long way since 2012,” Grundstein said. “Georgia has a really good heat policy now, and a lot of other states are following suit.” 

Heat Exhaustion or Heat Stroke? A Guide

As the effects of global warming become more prominent, Grundstein said, he believes more athletic organizations will start talking about heat-illness in tandem with climate change. 

Already, some worldwide sports organizations have begun adjusting for the warming climate, Grundstein said. FIFA plans to hold the 2022 World Cup in winter for the first time because of concern over high temperatures in the host country, Qatar.

“You can kind of see these patterns in terms of these international competitions,” Grundstein said. “You’re just seeing more and more heat-related problems popping up.”

Jordan Davidson Jun. 17, 2019, Taking action to stop the mercury from rising is a matter of life and death in the U.S., according to a new study published in the journal Science Advances.

Current greenhouse gas mitigation ambition is consistent with ~3°C global mean warming above preindustrial levels. There is a clear need to strengthen mitigation ambition to stabilize the climate at the Paris Agreement goal of warming of less than 2°C. We specify the differences in city-level heat-related mortality between the 3°C trajectory and warming of 2° and 1.5°C. Focusing on 15 U.S. cities where reliable climate and health data are available, we show that ratcheting up mitigation ambition to achieve the 2°C threshold could avoid between 70 and 1980 annual heat-related deaths per city PER EXTREME EVENT (30-year return period). Achieving the 1.5°C threshold could avoid between 110 and 2720 annual heat-related deaths. Population changes and adaptation investments would alter these numbers. Our results provide compelling evidence for the heat-related health benefits of limiting global warming to 1.5°C in the United States.

Extreme heat due to the climate crisis will cause thousands of deaths around the country unless immediate action is taken to stop global temperatures from rising over 1.5 degrees Celsius, or 2.7 degrees Fahrenheit. Already this month, San Francisco, typically cool in June, has seen triple digit temperatures, as the San Francisco Chronicle reported. And meanwhile, parts of India have seen scores of heat-related deaths from temperatures sizzling around 122 degrees Fahrenheit, according to CNN.

“The more warming you have, the more heat waves you have,” said Michael Wehner, a scientist at Lawrence Berkeley National Laboratory, who was not involved in this study, as reported by the New York Times. “The more heat waves you have, the more people die.”

By examining the effects extreme heat will cause in 15 U.S. cities, the study shows the pressing need for immediate action for nations around the world to ratchet up their efforts to contain global warming to 1.5 degrees Celsius above pre-industrial levels.

“Our results demonstrate that strengthened mitigation ambition would result in substantial benefits to public health in the United States,” the study’s authors concluded.

In the absence of those efforts, the outlook is bleak. If the global average temperature rises 3 degrees Celsius, or 5.4 degrees Fahrenheit, above pre-industrial levels, a heat wave could claim 6,000 lives in New York City, 2,500 in Los Angeles and 2,300 in Miami, the study says, as NBC News reports. The greatest risks are in northern cities like New York, Boston and Philadelphia.

That also means a densely-populated and vulnerable city like New York has the most to gain from meeting the 2 degree Celsius rise laid out in the Paris agreement, and even more to gain if temperatures rise only 1.5 degrees.

“New York City … could see 1,980 1-in-30-year heat-related deaths avoided in the 2C warmer world relative to the 3C warmer world under the assumption of constant population. If the 1.5C world is realized, 2,716 of 1-in-30-year heat-related deaths could be avoided, relative to 3C,” according to the paper, as Carbon Brief reported.

In the gravest scenario where temperatures rise 3 degree Celsius, Seattle, known for its cool summer temperatures, could see over 700 heat-related deaths while daily temperatures hover around 97.5 degrees Fahrenheit, the Seattle Times reports. That’s just heat-related and doesn’t factor in the decline in air quality that Seattle has seen thanks to nearby wildfires. Not to mention, as stark as the numbers are, the study might have underestimated the toll taken by heat waves by failing to consider non-fatal heat-related injuries, which send patients to American emergency rooms about 65,000 times each summer, said Aaron Bernstein, co-director of the Center for Health and the Global Environment at Harvard’s T.H. Chan School of Public Health, to NBC News.

The study authors’ hope that putting the numbers in context on a city-by-city basis will trigger leaders to reassess and ramp up their commitment to reducing greenhouse gases.

“You might think, what’s the difference 1.5 degrees will make to human lives?” said Eunice Lo, a climate scientist at the University of Bristol and the paper’s co-author, as reported by NBC News. “Actually, thousands of lives in one year can be saved in a city. Meeting the 1.5-degree target is essential and would be substantially beneficial to public health in the United States.”

Sheldon Whitehouse✔@SenWhitehouse People dying as India hits 123 degrees temperature. Nothing to do with fossil fuel, of course. …1591:01 PM – Jun 13, 2019 36 Die in India Heat Wave, Delhi Records Its Highest All-Time TemperatureIn one shocking incident, around 15 monkeys died in Joshi Baba forest range, possibly of heat stroke after another group of monkeys prevented them from accessing the closest water

Limiting warming to 1.5C could prevent ‘thousands’ of heat deaths in US cities

ROBERT MCSWEENEY, Carbon Brief, 05.06.2019 | HEATWAVES Limiting warming to 1.5C could prevent ‘thousands’ of heat deaths in US cities 

Holding global temperature rise to 1.5C above pre-industrial levels, rather than 2C or 3C, could help prevent thousands of deaths in US cities during heatwaves, a new study says.

The research projects heat-related deaths for 15 cities across the US under the different levels of future warming. The results suggest that major cities, such as New York City and Los Angeles, could see hundreds or thousands more deaths in extreme heat without greater ambition in global emissions cuts.

The findings provide “compelling evidence for the heat-related health benefits of limiting global warming to 1.5C” in the US, the study concludes.

The work “breaks new ground”, a scientist not involved in the work tells Carbon Brief, and shows that “the strongest climate policies, both for mitigation and adaptation, will save lives and help us to avoid never-before-seen human suffering from extreme heat”.

Heat deaths

As global temperatures increase, the likelihood and severity of summer heatwaves is expected to rise – and so too is the number of heat-related summer deaths. For example, a recent report by the Lancet Countdown on Health and Climate Changeproject – an annual review of the scientific evidence of climate change’s effect on human health – found that the number of vulnerable people exposed to heatwaves is already increasing by millions.

Attribution studies are increasingly able to link rising global average temperatures with the frequency and severity of heatwaves, and even heatwave deaths. A study from 2016, for example, found that hundreds of deaths in London and Paris during the 2003 European summer heatwave could be attributed to the impact of climate change.

The new study, published in Science Advances, estimates the future heat deaths in 15 US cities under different levels of warming. The researchers compare the two warming limits enshrined in the Paris Agreement – 1.5C and 2C above pre-industrial levels – with the estimated 3C of warming that existing worldwide mitigation commitments would amount to.

Using data on daily mortality and temperature data for 1987-2000, the researchers identified the “exposure-response” relationship between temperature and heat-related deaths for each city. This essentially describes how the risk of death relates to temperature.

Typically, the risk of death is highest in very cold and very hot conditions. Somewhere in between will be a “minimum mortality temperature” (MMT) where the number of heat-related deaths is at its lowest. The lowest MMT of the cities studied in St Louis in Missouri at 15C, while the highest is 34.5C in Phoenix, Arizona.

Applying these observed relationships between temperature and heat-related deaths in climate model simulations, the researchers projected the number of deaths under each level of warming. They focus on “1-in-30-year heat-related deaths” – the annual number of people dying from heat where that year is the warmest in 30 years.

The researchers kept all other factors constant in their experiment, explains lead author Dr Eunice Lo, a research associate at the University of Bristol. She tells Carbon Brief:

“We’ve assumed no adaptation, we’ve kept global population to be the same, we’ve kept the relationship between mortality and temperature to be the same – so that the only difference between these worlds would be temperature.

“We know that there are many factors that would affect future level of mortality and temperature-related mortality, but we’re asking the question of how global mean temperature rise – the level of global mean temperature rise, or difference – would affect mortality, and only this factor.”

In the video clip below, recorded at the American Geophysical Union Fall Meeting last year, Lo explains her study to Carbon Brief.

‘Mitigate more’

The chart below shows the projected increase in heat deaths for warming of 1.5C (dark blue), 2C (blue) and 3C (red) compared to present day (2006-15) for a 1-in-30-year heat event.

Additional annual heat deaths in 15 US cities for a 1-in-30-year heat event, compared to 2006-15, for a 1.5C (dark blue), 2C (blue) and 3C (red) warmer world. Whiskers show 2.5-97.5% uncertainty range. Data provided by Eunice Lo. Highchart by Carbon Brief.

The results show an increasing number of heat-related deaths as the climate warms. At 1.5C, the study estimates between 35 and 779 additional annual deaths, depending on the city. For 2C, this increases to 70-1,515, and then at 3C to 139-3,495.

The results highlight the benefits of meeting the limits set out in the Paris Agreement, says Lo:

“Our main finding is, basically, for most of our studied cities, if we mitigate more – so if we increase our climate action to meet the 2C target – then fewer people will die from heat. But this level of mortality – heat-related mortality – would be substantially even lower if we mitigate to 1.5C of warming. So meeting the Paris Agreement’s targets – especially the 1.5C target – would be substantially beneficial to the population in the US.”

New York City stands out with the most to gain from limiting warming, the paper says:

“New York City…could see 1,980 1-in-30-year heat-related deaths avoided in the 2C warmer world relative to the 3C warmer world under the assumption of constant population. If the 1.5C world is realised, 2,716 of 1-in-30-year heat-related deaths could be avoided, relative to 3C.”

This is partly because New York City has the largest population of all the cities that the team studied, says Lo, which means it is likely to have more people who are vulnerable during heatwaves.

Similarly, the second and third most populous cities in the US – Los Angeles and Chicago – see substantially lower numbers of projected heat deaths under stricter warming limits.

The chart below shows the results on a relative scale, where the number of deaths are estimated per 100,000 of population. With the data presented in this way, the cities of Miami and Detroit see the largest reductions in heat deaths with lower warming levels.

Additional annual heat deaths per 100,000 of population in 15 US cities for a 1-in-30-year heat event, compared to 2006-15, for a 1.5C (dark blue), 2C (blue) and 3C (red) warmer world. Whiskers show 2.5-97.5% uncertainty range. Data provided by Eunice Lo. Highchart by Carbon Brief.

There are some cases where the results don’t show clear differences at each warming levels the study notes. For example, in Atlanta, San Francisco and St Louis, the increases in heat deaths at 2C and 3C are statistically similar.

The paper also finds that some of the future 1-in-30 year maximum temperatures in these cities “may be hotter than what’s been observed in the last 30 years”, says Dr Vijay Limaye, a climate change and health science fellow at the US Natural Resources Defense Council science centre. Limaye, who wasn’t involved in the research, tells Carbon Brief:

“We may be literally off the charts in terms of unprecedented, widespread human exposure to insufferable heat if we don’t meet the challenge of limiting carbon pollution.”


The projected number of avoided deaths from limiting global temperature “may actually be underestimated”, says Limaye, because the study does not take into account future population growth.

The research also assumes that the existing relationship between temperature and heat-related deaths remains constant. This could change in the future as it has done over past decades. For example, adaptation measures could help reduce the future health burden of extreme heat. However, this is “not a given” says Limaye:

“For example, we see present-day access and affordability issues with air conditioning. We’re in new territory as public health scientists, and it’s not clear that this sobering picture even represents the full magnitude of the threat posed by deadly heat.”

The results draw attention to the raised health risks of global warming beyond the Paris limits, says Lo. This is particularly relevant as existing commitments by governments to cut emissions, known as “Nationally Determined Contributions” (NDCs), need to be ramped up in order to hold warming to 1.5C or 2C, she concludes:

“One of the motivations of this work is that the next round of NDC submissions for the Paris Agreement will happen in the year 2020, and we hope that our results could motivate increasing climate action internationally in these NDCs.”

Lo, Y. T. E., et al. 2019. Increasing mitigation ambition to meet the Paris Agreement’s temperature goal avoids substantial heat-related mortality in U.S. cities, Science Advances, doi:eaau4373