The immediate cause of March’s extremes was a weather phenomenon called a “heat dome.” Heat domes form when high pressure in the upper atmosphere traps and compresses warm air below, blocking cloud formation and preventing cooler air from mixing in. That, combined with unusually dry soil—dried further by droughts and low snowpack from a warm winter—rapidly warmed large swaths of the country.
The heat dome was unusual, but it also came during a sustained period of rising temperatures. Last year was the fourth-hottest on record in the contiguous U.S., and the nine warmest years have all occurred since 2012. Almost all scientists attribute this trend to the effects of global warming, as emissions from burning fossil fuels trap heat in the atmosphere.
But it would be going too far to say that global warming caused March’s heat wave, Zeke Hausfather, a climate scientist at the Breakthrough Institute, told TMD. While “there is rapid warming happening underneath all of this, globally and regionally,” he said, warming’s contribution to average temperatures is likely a few degrees. “What’s happening right now, without the last few decades of climate change, still would have been an anomalous heat wave,” Hausfather noted. “It’s just that it becomes a bit more anomalous than it would otherwise be.”
The same atmospheric pattern that superheated the West also displaced cold air elsewhere. In Canada, temperatures in many regions dropped to roughly 20 degrees below normal, and Fairbanks, Alaska, had its coldest March on record, averaging about -10 degrees below normal for the month. “What’s happening right now is more of a redistribution of heat driving this heat wave,” Hausfather said. “We still do need to separate, a little bit, climate from weather.”
But Swain argued that climate change’s contribution to the heat wave was likely larger than the few-degree estimate, noting that the scientific community is actively debating the scale of warming’s effect on extreme weather events. Russ Schumacher, a meteorologist at Colorado State University, told TMD, “Because of the long-term warming trend, the odds are kind of always tilted now towards a warmer-than-average summer being more likely.” He also noted that a potential El Niño—a periodic warming of Pacific Ocean surface temperatures that disrupts typical weather patterns—could cool parts of the Southwestern U.S. by bringing more rain than usual.
Increasingly, extreme heat is a persistent problem across much of the United States. Kai Chen, a researcher at the Yale School of Public Health, recently published a study showing that by 2020, roughly 4,000 deaths a year in the U.S. could be attributed to high temperatures—a 50 percent increase from 20 years earlier, which Chen told TMD was a “conservative” estimate. Heat is now responsible for more deaths than house fires, and roughly as many as a single month’s worth of firearm fatalities.
Rising temperatures have not brought a corresponding decrease in cold-related deaths, which account for roughly 45,000 per year in the U.S.—and as America’s population ages, the toll may even be rising. “What we’ve currently found is that older adults tend to be more vulnerable to both heat and cold,” Chen said. “When you look into the future, the overall [health burden] of both heat and cold combined, unfortunately, is going to increase.”
Jeffrey Howard, an epidemiologist at the University of Texas at San Antonio, told TMD that it’s difficult to pin down the exact number of heat-related deaths, and official heat death numbers, he argued, could easily be an “undercount.” Some well-equipped local health systems have forensic pathologists who use protocols specifically designed to detect heat death when determining the cause of death. But in other places, “it’s probably a justice of the peace with no medical training that’s signing the death certificate,” Howard said.
Many heat deaths result from commonly understood medical conditions, such as heat exhaustion and heat stroke, said Dr. Lauren Siewny, an emergency room physician and assistant professor at the Duke University School of Medicine. But “heat also affects most chronic illnesses, because the body is having to work harder to try to cool itself down,” she said, affecting everything from heart disease to kidney problems to complications arising from pregnancy. Siewny also noted that medications like allergy medicine and beta blockers, which reduce sweating and block heart rate responses, respectively, can also make heat problems worse.
Siewny said those facing extreme heat spikes should make sure they hydrate, wear weather-appropriate clothing, stay indoors when possible, and take frequent breaks if working or recreating outside. “Early heat waves, earlier in the season, are going to lead to higher morbidity,” she cautioned, as the human body takes a few weeks to start sweating more efficiently.
Chen noted that in his team’s survey of heat-related mortality, “a general pattern is that the people in the North tend to have a high risk,” from heat, rather than the much hotter Southeast, likely because historically, cooler regions have had less reason to adapt to hot temperatures. “If you are living in Seattle, you probably don’t need air conditioning,” Chen said, observing that the reverse dynamic applies when typically warm areas, like Texas, are struck by cold snaps.
Even within cities, temperatures vary radically. Average readings—usually taken in fields near airports—can be misleading. “One neighborhood and even one city block can be about 20 degrees hotter than another city block at the same time,” Vivek Shandas, an urban climate researcher at Portland State University who leads heat-mapping research projects, told TMD. Many factors contribute to these variations, Shandas said: Buildings that use “dense and dark” materials (like concrete) absorb the sun’s radiation and retain heat; shade, whether from tall buildings or tree cover, helps cool a neighborhood; and densely packed buildings restrict airflow and trap radiated heat between structures, making closely packed suburban developments or warehouse districts some of the hottest neighborhoods.
The intensely location-based nature of extreme heat, Shandas told TMD, is why community-level outreach is so important, because many neighborhoods, especially in historically cooler regions, are “not quite equipped, or ready, for that kind of heat.”
States have also turned to neighborhood cooling centers, which offer air conditioning and other resources to vulnerable communities. During March’s heat wave, Clark County, Nevada, opened dozens of cooling stations, and Los Angeles County directed residents to hundreds of cooling centers through its 211 hotline. Phoenix—which has a dedicated Office of Heat Response and Mitigation and approved a 23-action heat response plan in February—restricted hiking trails at popular peaks for the first time so early in the year.
But many seasonal cooling programs were not yet fully operational when the heat dome arrived. The heat wave also compounded wildfire concerns. One year after the Palisades and Eaton fires, California Gov. Gavin Newsom has fast-tracked 300 wildfire prevention projects, but many areas remain underprepared. And at the federal level, the response infrastructure remains thin: A September 2025 Government Accountability Office report found that less than 1 percent of FEMA’s flagship hazard mitigation grants have gone to heat as a primary hazard.
Heat waves also drive up electricity demand, as people turn on air conditioning, plug in fans, or sit in front of open fridges. In the spring, this usually isn’t a major problem because states can rely on hydropower as backup, Jordan Kern, an engineer who researches grid design at North Carolina State University, told TMD. But “the absolute worst time to have a heat wave in the western U.S. is actually like September or October, because that’s the driest part of the year,” when water power is less available, he said.
This March was almost certainly an outlier, and future springs will likely be cooler. But as average temperatures continue to rise, the threshold for what counts as an extraordinary heat event rises with them.
