In March, a month traditionally known for heavy mountain snows and dreary lower-elevation weather, a heat wave settled across the West, shattering temperature records from Tucson, Arizona, to Casper, Wyoming.
The heat wave’s intensity and early arrival shocked many climate scientists. “It is exceptionally difficult for the Earth system to produce temperatures this warm so early in the season,” wrote Daniel Swain, a climatologist with the University of California Agriculture and Natural Resources who runs the Weather West blog.
Yet not only did Western locations set new March highs; many exceeded temperature records for May. And those high temperatures kept hanging on, said Zachary Labe, a climate scientist at the nonprofit science center Climate Central, for nearly two weeks.
While heat waves are a natural phenomenon, this was the earliest and most widespread one ever recorded in the Southwest. And it was caused by climate change, which is making intense heat waves much more likely. Researchers say this means understanding their fallout is even more important.
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Scientists are just now beginning to understand the ramifications of a devastating 2021 heat wave, when a massive heat dome brought 120-degree temperatures to the Pacific Northwest, causing widespread ecological damage. Tens of thousands of trees died. Baby birds that could not yet fly plummeted to the ground as they tried to escape the heat. Salmon and trout suffocated in small streams. Millions — perhaps even billions — of mussels and barnacles cooked.
This year’s heat wave may not have had the same immediate ecological impacts, but it comes on the heels of an already record-breaking hot, dry winter. Researchers say 2021 holds lessons about what lies ahead for both vulnerable and resilient species. Ecosystems, they warn, are likely to permanently change as some species simply can’t handle the heat.
FULLY UNDERSTANDING the impact that events like heat waves have on long-lived tree species takes time. Research is now trickling out from places like Washington, Oregon and British Columbia, and it’s not good.
The 2021 heat wave either killed or otherwise harmed more than three-quarters of species surveyed, including by limiting their reproductive success, according to Julia Baum, a professor at University of Victoria who co-wrote a recent paper on the long-term impacts. The hardest hit, perhaps unsurprisingly, were those unable to move to seek shade or cooler temperatures. Marine species like acorn barnacles and green rope seaweed fared the worst, as did kelp, surfgrass and rockweed.
“The rocky shorelines they live on heated up to (122 Fahrenheit). Think of being glued to hot concrete on the most scorching summer day: They essentially baked and died,” said Baum. “On land, wildflowers wilted and died, preventing entire populations from reproducing that year, and there was widespread leaf scorch and death in forests.”
Some species that could move modified their behavior: Ferruginous hawks reduced their flight time by about 81%, while wolves moved around more, perhaps seeking hunkered-down prey like mule deer and moose.
Meanwhile, species already adapted to hotter or more variable temperature ranges adjusted better than others.
The heat wave’s timing also mattered, said Adam Sibley, a remote sensing scientist and co-author of a 2025 paper that examined the impact on trees and forests. Plants tend to acclimate to heat throughout a season, so the triple-digit temperatures that struck in June hit harder than they would have in August.
So many tree needles died, in fact, that when Sibley drove to the Oregon coast with friends a few days after the heat wave ended, the tree canopy looked as though it had been dusted with orange snow.
New buds and needles are fragile for a number of reasons, said Christopher Still, a forest ecology professor at Oregon State University. Many contain fatty membranes that, when super-heated, will melt and cause the leaf to fall apart. Young leaves and needles also lack “heat hardening” mechanisms like specialized proteins that stabilize mature leaves and needles when it’s hot.
Many larger, more well-established trees, such as Douglas fir, lost a growing season: Their needles fell off, but grew back the following year. Other trees died, especially younger ones and species like Sitka spruce and western red cedar that require cooler, wetter temperatures.
The 2021 heat wave also rapidly dried grasses, flowers and other fine fuels, leading to record-breaking wildfires in the Pacific Northwest, according to a 2024 paper in the journal Nature.
WHILE THE TIMING of this year’s heat wave surprised many climatologists, the fact that it arrived in March may have ultimately saved some Southwestern plants, said Osvaldo Sala, a professor and director of Arizona State University’s Global Drylands Center.
During the hottest period, he explained, many plants were still dormant. Desert plants tie their growing cycles to rain and moisture instead of heat or sun duration. That means that, unlike in places like Wyoming, where cherry trees started blooming in March instead of May, desert plants were still waiting for rains to come.
Unfortunately, that early blooming has left the cherry trees and other flowering plants particularly susceptible to spring frosts, Still said.
The effects of this year’s heat dome have only exacerbated the winter’s record-setting heat and drought, Still added. Snowpack across much of the West was abysmal; in many places, it was the worst in recorded history.
“The heat dome put an exclamation point on the worst winter in a century,” said Still. “It was the worst possible way to end the winter that was already worse than normal.”
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