Lawrence Berkeley National Laboratory

Abstract Daily maximum temperatures during the 2021 heatwave in the Pacific Northwest United States and Canada shattered century old records. Multiple causal factors, including anthropogenic climate change, contributed to these high temperatures, challenging traditional methods of attributing human influence. We demonstrate that the observed 2021 daily maximum temperatures are far above the bounds of Generalized Extreme Value distributions fitted from historical data. Hence, confidence in Granger causal inference statements about the human influence on this heatwave is low. Alternatively, we present a more conditional hindcast attribution study using two regional models. We performed ensembles of simulations of the heatwave to investigate how the event would have changed if it had occurred without anthropogenic climate change and with future warming. We found that global warming caused a ∼0.8°C–1°C increase in heatwave temperatures. Future warming would lead to a ∼5°C increase in heatwave temperature by the end of the 21st century. Plain Language Summary While it is clear that global warming causes heatwaves to be warmer, the unique meteorological conditions behind the 2021 Pacific Northwest heatwave tax our ability to make quantitative estimates of the human contribution. We discuss why there is low confidence in traditional estimates of the human contribution to this heatwave's temperatures and present an alternative, albeit more highly constrained estimate that human activities caused a ∼0.8°C–1°C increase in the observed daily maximum temperatures. Additional future warming would lead to a ∼5°C increase in the heatwave by the end of the 21st century. Key Points The Pacific Northwest (PNW) heatwave was unprecedented in the observational record over a large area Statistical and global climate models fail to inform about human influence on the PNW heatwave, as it is a far outlier Hindcast attribution methods can provide limited and conditional information about the human influence on the PNW heatwave