The remarkable Western U.S. heatwave in March of this year produced the largest temperature anomaly recorded in the western United States in 170 years of instrumental observations at +11.5°F above the mid-20th-century baseline, with roughly 30% of active U.S. weather stations setting new monthly records. Only the Dust Bowl summers of the 1930s approach that figure, and those occurred without the background thermodynamic contribution caused by the greenhouse warming effect.
In the Special Report below, Berkeley Earth documents these extremes in detail, examining the event in light of the instrumental record and past extreme events. But the finding we consider most consequential is less about this event specifically and more about what it may indicate about the distribution of future extremes.
March 2023 was an anomalously cold March in the western U.S. March 2026 was a record-shattering warm one. That juxtaposition — extreme cold and extreme heat within the same short window — is consistent with a widening of year-to-year temperature variance, not simply a shift in the mean. If that signal is real, standard return-period frameworks, which are largely built on a mean-shift assumption, will systematically underestimate the probability of future extremes in both directions.
This is an open and ongoing question that warrants additional investigation. Formal testing of whether the variance parameter in extreme-value models for western U.S. spring temperatures is stationary or trending is, in our view, a first-order research priority. We are publishing this as the first in what we intend to be an ongoing series of special report analyses. Additional findings from March 2026 can be found in our March 2026 Temperature Update as well as in the materials from our March 2026 Press Briefing.
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March 2026 Temperature Report
March 2026 Press Briefing Presentation
