A Millennial Heatwave: Pacific Northwest Experiences Hottest Summer of 2021

The Pacific Northwest experienced a two-week-long heat wave in 2021, making it a record-breaking summer (SN: 7/7/21). However, tree ring data from the region's forests has revealed that it was also the hottest summer of the last millennium.

Researchers reported in npj Climate and Atmospheric Science on February 17 that the average temperature for the summer of 2021 broke the previous record set in 2015 by 0.4 degrees Celsius. Additionally, it was unprecedented since the year 950.

The average temperature in the Pacific Northwest from June through August was 3.6 degrees Celsius warmer than the average summer temperature worldwide for the years 1951 through 1980, which is a common benchmark used for comparison. According to modern meteorological records for the area, such an extremely high average temperature has only a 1-in-25,000 chance of occurring in any given year.

Starting in late June 2021, some spots in the Pacific Northwest experienced daytime temperatures of nearly 50°C (or about 122°F). The extreme heat caused wildfires and increased the rates of heat-related deaths in the region.

To compare record-setting summer temperatures with those in earlier years before meteorologists compiled weather data, researchers looked at tree ring samples from dozens of trees at 17 sites in the region. The team measured the widths and densities of the portions of tree rings formed in summer months. By statistically blending data from the 17 sites, the team created a year-by-year profile of the entire region's average summertime temperatures from 950 to 2021.

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