Western North American extreme heat virtually impossible without human-caused climate change

A rapid analysis of the June 2021 Pacific Northwest heatwave concludes that the event was virtually impossible without human-caused climate change. The study estimates the event as a 1-in-1000-year occurrence in the current climate, noting it was approximately 2°C hotter than it would have been before the industrial revolution. Projections indicate that with 2°C of global warming, such events could occur every 5 to 10 years.

• The June 2021 heatwave in the region between 45–52 ºN and 119–123 ºW was virtually impossible without human-caused climate change. The observed temperatures were so extreme that they fell far outside historical ranges, making the event's rarity difficult to quantify, though the most realistic statistical analysis estimates it as a 1 in 1000 year event in the current climate.
• Assuming the heatwave was a very low probability event not caused by new nonlinearities, an event of this rarity (1 in 1000 years) would have been at least 150 times rarer without human-induced climate change. Additionally, the heatwave was approximately 2°C hotter than it would have been at the start of the industrial revolution, when global mean temperatures were 1.2°C lower.
• In a future scenario with 2°C of global warming—which could be reached as early as the 2040s at current emission levels—this event would be another degree hotter and its frequency would increase from once every 1000 years to roughly every 5 to 10 years.
• The heatwave was linked to a record-breaking 'heat dome' or Omega-blocking high pressure system. Other contributing factors may have included a very dry spring in the Western U.S. reducing evaporative cooling, anticyclonic subsidence, and downslope winds, though the exact combined influence of these factors remains unclear.
• The event caused significant public health impacts, including spikes in sudden deaths, emergency calls, and hospital visits for heat-related illnesses, particularly in populations unprepared for such heat due to a lack of air-conditioning. Current mortality estimates of several hundred deaths are considered an underestimate.

APA

World Weather Attribution (2021). Western North American extreme heat virtually impossible without human-caused climate change. https://www.worldweatherattribution.org/western-north-american-extreme-heat-virtually-impossible-without-human-caused-climate-change/

Chicago

World Weather Attribution. Western North American extreme heat virtually impossible without human-caused climate change. 2021. https://www.worldweatherattribution.org/western-north-american-extreme-heat-virtually-impossible-without-human-caused-climate-change/.

Wikipedia

{{cite report |author=World Weather Attribution |title=Western North American extreme heat virtually impossible without human-caused climate change |date=7 July 2021 |url=https://www.worldweatherattribution.org/western-north-american-extreme-heat-virtually-impossible-without-human-caused-climate-change/ |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{worldweatherattribution2021western, author = {{World Weather Attribution}}, title = {{Western North American extreme heat virtually impossible without human-caused climate change}}, institution = {World Weather Attribution}, year = {2021}, month = jul, url = {https://www.worldweatherattribution.org/western-north-american-extreme-heat-virtually-impossible-without-human-caused-climate-change/}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }