Consider this nightmare scenario. For four days now, temperatures have soared past 110 degrees. Those able to stay home are cranking their air-conditioning while officials scramble to move the unhoused into cooling centers. Even at night, the sweltering is relentless, thanks to the urban heat island effect: The concrete and brick of this metropolis absorbs the sun’s energy during the day and releases it throughout the evening.
There is no relief, and then suddenly there’s disaster: The grid fails, snatching away the AC that’s staving off mass heat illness. If this scenario were to unfold across Phoenix, according to a recent paper, half of the city’s 1.6 million people would need medical attention. More than 13,000 would die.
With climate change, then, won’t living in a place like Phoenix get ever more precarious? After all, the hotter it gets, the more people have to run their AC, adding ever more stress to the grid. Well, there's a reason the United States hasn’t seen such an extreme mass-mortality event: For all its faults, the electric grid is surprisingly resilient to heat emergencies.
And yet, heat is already a hidden disaster. Statistically, it causes more deaths each year than any other weather-related event—an estimated 12,000 per year in the US—but it doesn’t get the headlines that hurricanes, floods, or tornadoes do. And beyond that mortality rate, extreme heat exacerbates underlying health problems, sends people to hospitals, stresses emergency medical transport, and hikes health care spending.
The risk of heat illness and death, like many health problems, falls hardest on those who can do the least to protect themselves. Lower-income neighborhoods are quantifiably hotter than richer areas, because they tend to have fewer trees that cool the landscape. And of course, lower-income households are less likely to have air-conditioning in the first place, meaning their residents are already more susceptible even before the power goes out.
When an urban heat wave rages for several days, and temperatures don’t come down much at night, the body gets no time to recover. “These buildings are very dense: concrete materials push the heat into the residences and expose those folks to levels that it's not going to be easy to comfortably sleep,” says Portland State University climate adaptation scientist Vivek Shandas, who studies the urban heat island effect. “It's not going to be easy for the body to really go into that deep rest state.”
The good news is that the electrical grid is better able to withstand a heat wave than you might think. A hurricane or earthquake causes widespread destruction of the grid infrastructure, downing power lines or damaging power plants. Heat, by contrast, might overload some transformers, or force a power plant to reduce its energy generation to keep from overheating. But the grid as a whole remains intact.
“Those issues are real, and they do occur, but it's not widespread,” says Jeff Dagle, chief electrical engineer at the Pacific Northwest National Laboratory, who studies the grid. “It's not like every time we have a heat wave, we have massive numbers of transformers that fail. It's a relatively small impact on the overall system that’s designed with redundancy and resilience.”
Still, those impacts are occurring more frequently. Earlier research by the multi-university 3HEAT Study that modeled the effect of a heat wave plus grid failure on Phoenix (and also Atlanta and Detroit) has found that the risk of urban blackouts lasting at least an hour and affecting at least 50,000 households increased by 151 percent between 2015 and 2021. “Blackouts that impacted more than a million people, of which there were about one a year, had an average duration of five days,” says Brian Stone Jr., a professor and director of the Urban Climate Lab at the Georgia Institute of Technology, and principal investigator at 3HEAT. “Those are real events that happened, yet every one of those events was considered extremely low probability.”
The challenge of a heat wave is that a grid has to constantly balance its supply of power and the demand for it. When the mercury rises, more people turn on their AC, and a grid operator has to increase generation. This demand typically reaches a peak in the late afternoon, as people return home from work and switch on their appliances. So when you hear about rolling blackouts, that’s a consequence of supply not balancing with demand. But instead of the grid operator blacking out a whole city at once, power is cut to different neighborhoods at different times, reducing demand and stabilizing the system.
In the event of a power outage during a heat wave, officials in Phoenix work to get people into cooling centers, which have backup generators, says Brian Lee, director of emergency management for the city. The unhoused population might also need extra help, since they haven’t been able to enjoy AC in the first place. “We can conduct our own outreach to those affected areas,” says Lee. “And we can offer or extend additional support services to them until that power comes back online.”
But short of Godzilla stomping across the Southwest, it’d be difficult to kick off a lengthy, widespread power outage in Phoenix. “The only thing that's plausible along those lines is if it's some sort of physical attack—sabotage or something like that—where the infrastructure is targeted,” says Dagle. “While cyberattacks against any grid anywhere are possible, it is implausible, in my opinion, for a cyberattack to result in a multi-day outage.”
An electrical utility like Arizona Public Service, which operates in Phoenix, can of course see heat waves coming with weather forecasts. “We plan for 117-degree temperatures,” says Justin Joiner, vice president of resource management at APS. “When you hit 117 degrees, and it goes to 118 or 119, it's all the same at that point, because at 117 every AC unit is already on.”
If APS isn’t able to generate enough energy locally to meet increased demand, it can buy it from neighboring states. So if it’s relatively cool in California, energy can flow into the southwest. Then if California starts roasting, and Arizona is cooler, energy can flow back west. (Texas tends to get in trouble when it’s either too hot—like during the brutal heat wave it’s experiencing right now—or too cold, because it runs its own isolated grid, so it can’t import large amounts of power from other states.)
The complexity of drawing power from elsewhere becomes more challenging when the reason for the grid failure isn’t mere demand, but damage—the kind that major weather can wreak. Parts of Florida were without power for a week last year following Hurricane Ian. A straight-line derecho in June 2012 took out power for 4 million people in 11 states and Washington, DC. And after Hurricane Katrina in 2005, areas in Louisiana were without power for a month, according to assessments by the Department of Homeland Security in 2017. To underline the cascading risks: Almost half of the prolonged outages studied by Stone and his colleagues occurred between May and September, when temperatures rise.
When blackouts take away air-conditioning—or trap heat in buildings that were never fitted with climate control, a factor in deadly heat waves in the UK and Europe—the occurrence of heat exhaustion and heat stroke spikes. And health care, especially emergency departments, detects them first. In the Pacific Northwest's record-breaking heat wave in 2021, ER visits increased 69 times over, and heat-related deaths among people over age 65 rose by half. Outages also pose a risk to people who rely on powered equipment such as oxygen concentrators and electric wheelchairs—and excess heat places a much greater burden on those with chronic illnesses such as cardiovascular disease, asthma, diabetes, and pulmonary insufficiency.
After New Orleans’ Charity Hospital catastrophically lost power during Hurricane Katrina—because its backup generators were stashed in the basement, which flooded—the federal Center for Medicare and Medicaid Services created emergency preparedness rules that mandate uninterrupted power in critical parts of buildings: operating rooms, ICUs, and call centers, for instance. (Though not necessarily exam rooms. Or plumbing plants.)
“So on the one hand, hospitals might be an island of power and air-conditioning, able to cool water and make ice,” says Jeremy Ackerman, an emergency physician and associate professor at Emory University School of Medicine. “But on the other hand, we’re suddenly faced with an increased volume of patients, and that may mean we need to be using more IV fluids and other cooling measures.”
Because hospitals are now more likely to stay cool, some jurisdictions have folded them into extreme-heat emergency plans. New York City, for instance, asks hospitals to allow unhoused people to rest in their waiting rooms during heat waves, even if they have no other medical need.
Ensuring that hospitals stay cool isn’t the sole solution for the health stresses of extreme heat, though, because many other facilities, such as nursing homes, don’t have the same back-up power rules. Extracting people from buildings that can’t be cooled—whether that’s a long-term care facility or a high-rise apartment with a nonfunctioning elevator—becomes a challenge for emergency medical transport. A further challenge: The things you need to cool people down—misters, cold blankets, chilled IV fluids—are impractical to transport in an ambulance.
So municipalities that have drafted extreme-heat plans, such as Phoenix and New York City, are looking at ways to get people out of hot places and into cold ones without taking them all the way into an ER’s ambulance bay. “One of the lessons of the pandemic was that we need more flexibility in hospital design,” says Craig Zimring, an emeritus professor of architecture at Georgia Tech and an expert in evidence-based design. “We’d want, as we did in the pandemic, to evaluate people before they get into the [emergency department]. That could be in tents in the parking lots, or closing off streets around the hospital, developing a portable infrastructure.”
In extreme heat, though, even those cooling centers need advance consideration. Stone’s team, for instance, found in 2022 that the number of cooling centers planned for Phoenix, Detroit, and Atlanta could care for no more than 2 percent of those cities’ populations. And none of the municipalities required them to have back-up generators. (The city of Phoenix says its facilities are equipped with redundant or back-up power, and that its emergency operations plan can be scaled up to accommodate all affected community members if need be.)
People who’ve been dealing for a while with the public health impact of heat stress say that it’s crucial to plan for this in advance—which means not just a week or so ahead when forecasters announce a heat wave is coming, but months or years ahead, as part of municipal and health care disaster planning. At the time of the Pacific Northwest heat wave, Seattle had no heat action plan that could be activated—for instance, to protect residents in neighborhoods likeliest to heat up, or to redirect ambulances to hospitals that were not already maxed out. A year later, it created an “extreme heat mitigation strategy” that coordinates the actions of government departments, EMS, hospitals, and nonprofit and volunteer organizations.
“There are specific things you can do in advance of a heat wave to prepare, and then there are things you do that are all-hazards preparedness, which make you more resilient,” says Jeremy Hess, a professor of emergency medicine at the University of Washington who worked clinically during that heat wave and who researches the impact of climate change on health care. “Because when things fail, they often don’t fail in quite the ways you expect.”
Phoenix’s APS, for its part, is confident it can provide enough energy to power all that AC during a heat wave, and it can import electricity if needed. “It's like an airplane—we have multiple redundancies for everything,” Joiner says. Bits and pieces of it might fail at times, Joiner continues, but the whole thing is unlikely to crash. “So we can have a plant come offline, we can have a transmission line come off. But to take all of them off at once, we’d have to feel something from New Mexico all the way to California, and even north of us, for that to be something that was even in the realm of possibility.”
