In March, traditionally a time for heavy mountain snows and dreary weather in lower elevations, a heat wave swept across the West, breaking temperature records from Tucson, Arizona, to Casper, Wyoming.
The intensity and early arrival of the heat wave took many climate scientists by surprise. “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.
Not only did Western locations set new highs for March, but many also surpassed temperature records for May. These high temperatures persisted, according to Zachary Labe, a climate scientist at the nonprofit science center Climate Central, for nearly two weeks.
While heat waves are naturally occurring events, this was the earliest and most extensive one ever documented in the Southwest. It was attributed to climate change, which is increasing the likelihood of intense heat waves. Researchers emphasize the importance of understanding their consequences.
Scientists are just beginning to comprehend the effects of a devastating 2021 heat wave when a massive heat dome brought 120-degree temperatures to the Pacific Northwest, resulting in widespread ecological damage. Tens of thousands of trees died, baby birds unable to fly fell to the ground to escape the heat, salmon and trout suffocated in small streams, and millions — potentially billions — of mussels and barnacles were cooked.
Number of daily record highs broken in March 2026
This year’s heat wave may not have caused the same immediate ecological impacts, but it followed an already record-breaking hot, dry winter. Researchers believe that 2021 provides insights into what the future holds for vulnerable and resilient species. Ecosystems, they caution, are likely to undergo permanent changes as some species cannot withstand the heat.
Understanding the impact of events like heat waves on long-lived tree species takes time. Emerging research from Washington, Oregon, and British Columbia is not promising.
The 2021 heat wave either killed or harmed more than three-quarters of species surveyed, including by limiting their reproductive success, according to Julia Baum, a professor at the University of Victoria who co-wrote a recent paper on the long-term impacts. The most affected were those unable to move to seek shade or cooler temperatures. Marine species like acorn barnacles and green rope seaweed suffered the most, along with 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 adjusted their behavior: Ferruginous hawks reduced their flight time by about 81 percent, while wolves moved around more, possibly seeking prey like mule deer and moose.
Meanwhile, species already adapted to hotter or more variable temperatures adjusted better than others.
The timing of the heat wave also played a role, said Adam Sibley, a remote sensing scientist and co-author of a 2025 paper that examined the impact on trees and forests. Plants generally acclimate to heat over the course of a season, so the triple-digit temperatures that hit in June were more severe than they would have been in August.
So many tree needles died that when Sibley drove to the Oregon coast with friends a few days after the heat wave ended, the tree canopy appeared as if it had been dusted with orange snow.
New buds and needles are fragile for several reasons, explained Christopher Still, a forest ecology professor at Oregon State University. Many contain fatty membranes that melt when super-heated, causing the leaf to disintegrate. Young leaves and needles also lack “heat hardening” mechanisms like specialized proteins that stabilize mature leaves and needles in hot conditions.
Many larger, well-established trees, such as Douglas fir, lost a growing season: Their needles fell off but regrew the following year. Other trees died, particularly younger ones and species like Sitka spruce and western red cedar that require cooler, wetter climates.
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 caught many climatologists off guard, its arrival in March may have ultimately protected some Southwestern plants, said Osvaldo Sala, a professor and director of Arizona State University’s Global Drylands Center.
During the hottest period, many plants were still dormant, he explained. Desert plants base their growing cycles on rain and moisture rather than heat or sunlight duration. This means that, unlike in places like Wyoming, where cherry trees began blooming in March instead of May, desert plants were still awaiting rains.
Unfortunately, this early blooming has made cherry trees and other flowering plants especially vulnerable to spring frosts, Still noted.
The repercussions of this year’s heat dome have only worsened the winter’s record-setting heat and drought, Still added. Snowpack across much of the West was dismal; in many areas, it was the worst ever recorded.
“The heat dome put an exclamation point on the worst winter in a century,” said Still. “It was the worst possible way to end a winter that was already worse than normal.”
