Rapid attribution analysis of the extraordinary heat wave on the Pacific coast of the US and Canada in June 2021

Towards the end of June 2021, temperature records were broken by several degrees Celsius in several cities in the Pacific northwest areas of the U.S. and Canada, leading to spikes in sudden deaths, and sharp increases in emergency calls and hospital visits for heat-related illnesses. Here we present a multi-model, multi-method attribution analysis to investigate to what extent human-induced climate change has influenced the probability and intensity of extreme heatwaves in this region. Based on observations, modelling and a classical statistical approach, the occurrence of a heatwave defined as the maximum daily temperatures (TXx) observed in the area 45 \dg N--52 \dg N, 119 \dg W--123 \dg W, was found to be virtually impossible without human-caused climate change. The observed temperatures were so extreme that they lay far outside the range of historical temperature observations. This makes it hard to state with confidence how rare the event was. 
Using a statistical analysis that assumes that the heatwave is part of the same distribution as previous heatwaves in this region, led to a first order estimation of the event frequency of the order of once in 1000 years under current climate conditions. Using this assumption and combining the results from the analysis of climate models and weather observations, we found that such a heatwave event would be at least 150 times less common without human-induced climate change. Also, this heatwave was about 2\dg C  hotter than a 1 in 1000-year heatwave would have been in 1850--1900, when global mean temperatures were 1.2\dg C cooler than today. Looking into the future, in a world with 2\dg C of global warming (0.8\dg C warmer than today), a 1000-year event would be another degree hotter. 
Our results provide a strong warning: our rapidly warming climate is bringing us into uncharted territory with significant consequences for health, well-being, and livelihoods. Adaptation and mitigation are urgently needed to prepare societies for a very different future.