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 human activities
caused a 1C increase in heatwave temperatures. Future warming would lead
to a 5C increase in heatwave temperature by the end of the 21st century.