Unprecedented 21st century heat across the Pacific Northwest of North America

Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict severe socioeconomic and biological consequences. In summer 2021, the Pacific Northwest region of North America (PNW) experienced a 2-week-long extreme heatwave, contributing to record-breaking summer temperatures. Using tree-ring records, the authors show that summer temperatures in 2021, as well as the rate of summertime warming during recent decades, are unprecedented within the context of the last millennium for the PNW. Absent significant mitigation to reduce anthropogenic emissions below intermediate levels (SSP2-4.5), climate model projections indicate a rapidly increasing risk that the PNW will regularly experience 2021-like extreme summer temperatures, with a roughly 50% chance of yearly occurrence by 2050. The 2021 conditions provide a benchmark and impetus for communities in historically temperate climates to account for extreme heat impacts in adaptation strategies.