The summer of 2021 was the hottest in the Pacific Northwest in millennia


Average summer temperatures in the region that year were unmatched since at least 950 AD.

A two-week heatwave in the Pacific Northwest in 2021 helped make that summer a record for the region. Now, tree-ring data from the region’s forests show that the summer of 2021 was also the region’s hottest in a millennium.

The average summer temperature that year not only beat the previous record set in 2015 by 0.4 degrees Celsius. It was also unprecedented since 950 the researchers reported on February 17 npj Climate and Atmospheric Science .

The average temperature in the Pacific Northwest from June to August was a whopping 3.6 degrees warmer than the 1951-1980 global summer average. This global value of 15.2 °C is the common benchmark used for comparison.

Based on modern meteorological records for the region, the scientists also found that such an unusually high average temperature has only a 1 in 25,000 chance of occurring in any given year.

As of late June 2021, daytime temperatures in some locations in the Pacific Northwest have reached nearly 50°C (or about 122°F). The extreme heat has fueled wildfires and increased heat-related deaths in the region, said Karen Heeter, a dendrochronologist at Columbia University’s Lamont-Dougherty Earth Observatory in Palisades, New York.

To see how record summer temperatures compared to years before meteorologists collected weather data, Heather and her colleagues looked at tree-ring samples collected from dozens of trees at 17 locations in the Pacific Northwest. Core samples were taken from living trees between the early 1990s and 2021. The team found that two core samples recorded temperatures as high as 950 AD.

The team measured the width and density of parts of the tree rings that formed during the summer months. Generally, for conifers in the Pacific Northwest, the higher the temperature of the growing season, the wider the tree ring and the denser the wood formed during that period. By statistically blending data from 17 sites, the team created a year-by-year profile of average summer temperatures across the region from 950 to 2021.

Tree ring tempo

Tree-ring data in the Pacific Northwest allow researchers to reconstruct the average annual summer temperature in the region from 950 to 2021. This graph shows the distribution of these temperatures relative to the global mean summer temperature for the years 1951–1980. Summer temperatures were within this long-term average for most years. But the summer of 2021 (arrow) is about 3.6 degrees Celsius warmer—hotter than any summer in the past millennium.

Distribution of Northwestern Pacific temperature anomalies, 950–2021

Evidence of record heat in 2021 in the Pacific Northwest came as no surprise to Brian Black, a dendrochronologist at the University of Arizona in Tucson. While conducting fieldwork in the area that summer, he noticed that many of the trees in the region showed noticeable browning on the southwest side that could have taken the brutal afternoon sunlight.

“The burning of the trees was shocking,” says Black, who was not involved in the new study. The average summer temperature “was extremely far from normal.”

This anomaly, says Lauren Stachowiak, a geographer at Eastern Washington University in Cheyney who studies tree rings and lived in the area at the time, will no doubt show up in tree ring samples obtained in future studies.

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