A deadly wave of heart attacks and strokes is headed for the US, borne by extreme heat waves spawned by climate change—and those deaths are most likely to occur in people who are older or Black.

By mid-century, according to research published Monday, cardiovascular deaths could triple to almost 5,500 extra deaths per year, if nothing is done to curb the greenhouse gas emissions that are driving climate change and extreme heat events. And even if the US achieves some emissions control by staying on its current planned path of reductions, cardiovascular deaths are likely to more than double to 4,300 extra deaths per year. Thanks to the combined influences of age, genetic vulnerability, geography, and the heat-trapping aspects of urban development, the investigators predict that older adults will be at higher risk, and Black adults will be at higher risk than any other group.

“The public health impact of climate change is falling on individuals who live on the margins of our society,” says Sameed Khatana, a cardiologist and assistant professor at the University of Pennsylvania’s Perelman School of Medicine. “Any policy action or mitigation efforts really need to be tailored towards individuals who are most vulnerable.”

The prediction originates with Khatana’s group at the University of Pennsylvania, who previously modeled the relationship between current deaths from heart attack and stroke and the rising number of “extreme heat days” (possessing a heat index—a measure of apparent temperature that is a product of ambient temperature and relative humidity—at or above 90 degrees Fahrenheit). Using data for the 3,108 counties in the contiguous US between 2008 and 2017, they found rising rates of cardiovascular deaths along with a trend of increasing numbers of extreme heat days. By 2019, they said, there were 54 extreme heat days per year, and each year, 1,651 people died as a result.

That is a small proportion of all cardiovascular deaths in the US currently. But given the expectation of heat events rising with climate change, they thought it worth inquiring how increases in temperature would affect death rates. The results were dramatic.

To perform the new analysis, they combined the earlier work with predictions of rising global temperatures, migration to warmer parts of the US, and aging of the US population, along with demographic shifts that will tilt the majority of the population away from whites who are not Hispanic. The team then plotted the likely effects of those combined factors within two scenarios. In one, the US manages to hold down greenhouse gas emissions to a moderate increase, a scenario known as RCP 4.5 that represents existing policies likely to be implemented. In the other, known as RCP 8.5, emissions rise essentially unchecked.

Under the RCP 4.5 scenario, they predict, deaths from cardiovascular events would rise 162 percent between the years 2036 and 2065. Under RCP 8.5, extreme heat would occur on 80 days—one-fifth of the entire year—and deaths would rise 233 percent. And those rises will not occur evenly across the population. Compared with the young and middle-aged, elderly people of any background would suffer a 3.5 times greater risk of death. For Black adults, the risk would rise to 4.6 times that of whites.

“Even in our somewhat more optimistic scenario, there is still a significant increase in extreme heat predicted,” Khatana says, “and we projected a more than doubling of extreme heat-associated cardiovascular mortality in that scenario.”

The projections confirm what other researchers have been finding: a reliable relationship between climate change and harm to human health, not just in the US but worldwide. “It helps us understand what the trajectory of disease is on the increasingly urbanized globe, and how we can quantify the health impact of globalization and environmental shifts,” says Sadeer Al-Kindi, a physician and associate director of preventive cardiology at Houston Methodist DeBakey Heart and Vascular Center, who studies the effects of global extremes of temperature on cardiovascular disease. “There has been a significant increase in cardiovascular disease throughout the world from non-optimal temperatures.”

But though that accumulated evidence makes for grim predictions, it also provides a possible menu for action. Extreme heat is already an unacknowledged disaster; it causes more deaths each year in the US than any other weather-related event, exacerbating multiple health conditions beyond cardiovascular diseases, and US cities are behind the curve in protecting themselves against its impact. But multiple analyses show that heat’s influence is not uniform; it has more impact in certain areas of countries and cities, where socioeconomic status is lower and minority and recent immigrant occupancy may be higher.

“One way we can explain those differences is by looking at the impact of historical drivers, for example, redlining,” says Edith de Guzman, a heat researcher and cooperative extension specialist at UCLA, referencing a 20th-century policy in which banks refused to sign mortgages in minority or poor neighborhoods. “Even years after the end of redlining, there are legacy impacts that are very obviously detectable in how hot neighborhoods get, even in the same city.”

Multiple studies, for instance, show that less affluent neighborhoods tend to have substandard housing, with less air-conditioning to cool them and less insulation to protect against heat incursion; less tree cover to shield houses; and more road surfaces and parking lots that absorb heat during the day and release it at night, keeping the air unhealthily warm. Less affluent neighborhoods may also be sited near highways or industrial zones, making them doubly vulnerable to the synergistic risks of heat plus particulate air pollution and ozone.

Census tracts in California that are more likely to experience days of high heat and high pollution exposure also tend to score low on socioeconomic status, says Shahir Masri, an air pollution scientist at UC Irvine. It’s the same pattern, he points out, that occurred in the Covid pandemic. “We saw massive disparities between the survival rates of Caucasians and those of Hispanic and African American communities,” he says. And while those groups might suffer genetic vulnerabilities to certain diseases, “largely what we find is those populations have much less access to health care and suffer from comorbidities, such as chronic obstructive pulmonary disorder, that make these populations much more vulnerable.”

Work by de Guzman and others has shown that cities can be reconfigured to protect residents against extreme heat, by identifying zip codes that are most at risk and then installing reflective surfaces and planting trees. “We’re literally talking about changing land cover in cities,” she says. “We found we could reduce the number of deaths, depending on the heat wave and depending on the community, in the neighborhood of 25 to 50 percent.”

Allied to those efforts, she says, is a need to make communication about heat risks more organized and public, along the lines of cities that have created “heat officer” positions and begun scoring heat waves on scales of how much risk they pose. Knowing whether they are at risk, and knowing what resources, such as cooling centers, exist to protect them, could save city residents from the death waves of heat days. “With this global climate change phenomenon that we’re living through, the prospect of actually effecting positive change can feel really dismal,” she says. “We’ve asked: ‘Can we move the needle locally, regardless of what happens at the international level?’ And the answer we found is: ‘Yes.’”