Rapid attribution analysis of the extraordinary heatwave on the Pacific Coast of the US and Canada June 2021

The report concludes that the June 2021 Pacific Northwest heatwave was virtually impossible without human-caused climate change, estimating it as a 1-in-1000 year event in the current climate. The analysis indicates the event was approximately 2°C hotter than it would have been in a pre-industrial climate and would have been at least 150 times rarer without anthropogenic warming. Future projections suggest that in a world with 2°C of global warming, such an event would occur every 5 to 10 years and be another degree hotter. Meteorological drivers included an 'omega-block' high-pressure system and dry antecedent soil conditions, while natural variability modes like ENSO and PDO played little to no role.

• In the most realistic statistical analysis, the heatwave is estimated to be a 1-in-1000 year event in the current climate, though it was at least 150 times rarer before human-induced climate change.
• 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 than in 2021.
• If global warming reaches 2°C, an event of this magnitude would occur roughly every 5 to 10 years and would be an additional 1°C hotter than the 2021 event.
• The event was characterized by an 'omega-block' high-pressure system with a 500 hPa geopotential height of approximately 5980 m, which was unprecedented for western North America between 1948 and June 2021.
• Anomalously dry antecedent conditions from March through June 2021, particularly in Western Oregon, likely amplified temperatures by reducing latent cooling from evapotranspiration.
• Natural variability modes, specifically the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), were found to have little to no influence on the heatwave.
• The heatwave caused significant human and infrastructure impacts, including several hundred extra deaths in Canada, power outages, and the destruction of the village of Lytton, where a record temperature of 49.6 °C was measured on June 29.

APA

Philip, S. Y., Kew, S. F., van Oldenborgh, G. J., Yang, W., Vecchi, G. A., Anslow, F. S., Li, S., Seneviratne, S. I., Luu, L. N., Arrighi, J., Singh, R., van Aalst, M., Hauser, M., Schumacher, D. L., Marghidan, C. P., Ebi, K. L., Bonnet, R., Vautard, R., Tradowsky, J., ... Otto, F. E. L. (2021). Rapid attribution analysis of the extraordinary heatwave on the Pacific Coast of the US and Canada June 2021. World Weather Attribution. https://www.worldweatherattribution.org/wp-content/uploads/NW-US-extreme-heat-2021-scientific-report-WWA.pdf

Chicago

Philip, Sjoukje Y., Sarah F. Kew, Geert Jan van Oldenborgh, Wenchang Yang, Gabriel A. Vecchi, Faron S. Anslow, Sihan Li, et al. Rapid attribution analysis of the extraordinary heatwave on the Pacific Coast of the US and Canada June 2021. World Weather Attribution, 2021. https://www.worldweatherattribution.org/wp-content/uploads/NW-US-extreme-heat-2021-scientific-report-WWA.pdf.

Wikipedia

{{cite report |last1=Philip |first1=Sjoukje Y. |last2=Kew |first2=Sarah F. |last3=van Oldenborgh |first3=Geert Jan |last4=Yang |first4=Wenchang |last5=Vecchi |first5=Gabriel A. |last6=Anslow |first6=Faron S. |last7=Li |first7=Sihan |last8=Seneviratne |first8=Sonia I. |last9=Luu |first9=Linh N. |display-authors=etal |title=Rapid attribution analysis of the extraordinary heatwave on the Pacific Coast of the US and Canada June 2021 |publisher=World Weather Attribution |date=2021 |url=https://www.worldweatherattribution.org/wp-content/uploads/NW-US-extreme-heat-2021-scientific-report-WWA.pdf |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{philip2021rapid, author = {Philip, Sjoukje Y. and Kew, Sarah F. and van Oldenborgh, Geert Jan and Yang, Wenchang and Vecchi, Gabriel A. and Anslow, Faron S. and Li, Sihan and Seneviratne, Sonia I. and Luu, Linh N. and Arrighi, Julie and Singh, Roop and van Aalst, Maarten and Hauser, Mathias and Schumacher, Dominik L. and Marghidan, Carolina Pereira and Ebi, Kristie L and Bonnet, Rémy and Vautard, Robert and Tradowsky, Jordis and Coumou, Dim and Lehner, Flavio and Wehner, Michael and Rodell, Chris and Stull, Roland and Howard, Rosie and Gillett, Nathan and Otto, Friederike E L}, title = {{Rapid attribution analysis of the extraordinary heatwave on the Pacific Coast of the US and Canada June 2021}}, institution = {World Weather Attribution}, year = {2021}, url = {https://www.worldweatherattribution.org/wp-content/uploads/NW-US-extreme-heat-2021-scientific-report-WWA.pdf}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }