Understanding the spatial and temporal variations in the magnitude of these trends is critical because of the intrinsic coupling between climate extremes and ecological and socioeconomic systems. In particular, the last two decades show a substantial increase in extreme summer temperatures worldwide 7, 8, 9, 10, 11, 12. The latest Intergovernmental Panel on Climatic Change (IPCC) consensus reports that global mean temperatures over the first two decades of the 21st century were 0.99 (0.84–1.10) ☌ higher than pre-Industrial levels (1850–1900 CE), and they are projected to exceed 1.5 ☌ above pre-Industrial levels by 2040 6. Surface air temperatures and the frequency and intensity of extreme heat events are increasing globally and are consequences of anthropogenic climate change 1, 2, 3, 4, 5. The 2021 summer temperatures experienced across the PNW provide a benchmark and impetus for communities in historically temperate climates to account for extreme heat-related impacts in climate change adaptation strategies. In the absence of committed efforts to curtail anthropogenic emissions below intermediate levels (SSP2–4.5), climate model projections indicate a rapidly increasing risk of the PNW regularly experiencing 2021-like extreme summer temperatures, with a 50% chance of yearly occurrence by 2050. Here, we use tree-ring records to show that summer temperatures in 2021, as well as the rate of summertime warming during the last several decades, are unprecedented within the context of the last millennium for the PNW. In summer 2021, the Pacific Northwest region of North America (PNW) experienced a 2-week-long extreme heatwave, which contributed to record-breaking summer temperatures. Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict severe socioeconomic and biological consequences.
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