WASHINGTON – The growing risk of overlapping heat waves and power outages poses a serious threat that major American cities are unprepared for.

Power outages have increased more than 60 percent since 2015, despite climate change making heat waves worse, according to the new study published in the journal Environmental Science & Technology. Using computer models to study three major US cities, the authors estimated that a combined blackout and heat wave would expose at least two-thirds of those cities’ residents to heat exhaustion or heat stroke.

And while each of the cities studied has public cooling centers for people who need to be freed from the heat, according to the authors, those centers could not accommodate more than 2 percent of the population of a given city, so an overwhelming majority of residents are at risk.

“A widespread power outage during an intense heatwave can be the deadliest climate-related event we can imagine,” said Brian Stone Jr., professor in the Georgia Institute of Technology’s School of City and Regional Planning and lead author on the study. However, such a scenario is “increasingly likely,” he said.

The changing climate also seems to make power outages more frequent. From 2015 to 2020, the number of power outages in the US doubled annually, said Dr. Stone. And these blackouts were more likely to be in the summer, suggesting they were caused in part by high temperatures that increase demand for the power grid when people turn up their air conditioners.

With both heat waves and power outages becoming more common, “the probability of a simultaneous heat wave and power outage event increasing,” said Dr. Stone.

Therefore, Dr. Stone worked with a team of eight other researchers – from Georgia Tech, Arizona State, the University of Michigan, and the University of Guelph in Ontario, Canada – to measure the impact on human health when power outages coincide with heat waves.

To do this, they selected three major cities – Atlanta, Detroit, and Phoenix – and looked at the recorded temperatures during some of their heaviest heat waves.

They next used computers to model the temperatures in different parts of the city if those heat waves occurred at the same time that a city-wide power outage disabled air conditioners.

Crucially, the researchers wanted to know how hot the interiors of houses would get under these conditions – something Dr. Stone said it had never been tried before. They collected data showing the building characteristics for every single residential structure in every city – for example the age of the building, the construction material, the degree of insulation and the number of floors.

The results were alarming. In Atlanta, more than 350,000 people, or approximately 70 percent of the population, would be exposed to indoor temperatures of at least 32 degrees Celsius (89.6 degrees Fahrenheit). Heat stroke is possible.

In Detroit, more than 450,000, or about 68 percent, would be exposed to this indoor temperature. In Phoenix, where the vast majority of residents depend on air conditioning, the entire population would be at risk – nearly 1.7 million people.

Even in the absence of a power outage, some residents of every city do not have access to air conditioning, exposing those residents to dangerous indoor temperatures during a heatwave. Those numbers range from 1,000 people in Phoenix to 50,000 in Detroit based on the characteristics of their homes, the authors found.

This exposure is most pronounced in the lowest-income households, who are 20 percent less likely to have central air conditioning than the highest-income households.

The authors reported that each city had designated public cooling centers for extreme heat. However, they found that each of these centers could only accommodate 1 to 2 percent of the total population.

In none of the three cities do these cooling centers need emergency generators to operate air conditioning systems in the event of a power failure.

“Based on our findings, a simultaneous heatwave and power failure would require a far more extensive network of emergency cooling centers than is currently set up in any city with mandatory backup power generation,” the authors write.

The New York Times asked officials in Atlanta, Detroit and Phoenix to comment on the paper’s findings and describe their plans to respond to a combined blackout and heat wave.

A spokeswoman for the city of Phoenix, Tamra Ingersoll, said that in a crisis situation such as a heat wave that coincides with a prolonged power outage, many residents would leave the city alone. Emergency response for the remaining people would focus on “vulnerable populations such as the elderly, the frail or the low-income,” she said.

Christopher Kopicko, a spokesman for the Detroit Office of Homeland Security & Emergency Management, said only one of the city’s 11 cooling centers had an emergency generator. But he said Detroit recently purchased mobile generators that could be sent to cooling centers that needed them, and residents could go to any of the city’s 12 police stations that have replacement generators. He also said some of the city’s largest venues have agreed to act as mass shelters.

The Atlanta Mayor’s Office Keisha Lance Bottoms did not comment.

The Federal Emergency Management Agency referred to questions about whether it had plans to help a major city cope with a combined power outage and heatwave, to a 2017 plan to cope with the effects of a long-term power outage.

However, this document did not address how the agency would react if a heat wave occurred during such a power outage, nor did it suggest that “a lack of electricity requires the provision of uniform heating or air conditioning and adequate hygiene poses challenges in emergency shelters or other mass care facilities. ”

Other cities in the United States are at risk of similar health threats from a combined heat wave and power outage, which affects the proportion of their population that is very likely to be at risk, the authors found.

“We find that millions are at risk,” said Dr. Stone. “Not years in the future, but this summer.”