Review status: this preprint is currently under review for the journal ESD.
Rapid attribution analysis of the extraordinary heatwave on the Pacific Coast of the US and Canada June 2021
Sjoukje Y. Philip1,,Sarah F. Kew1,,Geert Jan van Oldenborgh1,2,,Faron S. Anslow3,Sonia I. Seneviratne4,Robert Vautard5,Dim Coumou1,6,Kristie L. Ebi7,Julie Arrighi8,9,10,Roop Singh8,Maarten van Aalst8,9,11,Carolina Pereira Marghidan9,Michael Wehner12,Wenchang Yang13,Sihan Li14,Dominik L. Schumacher4,Mathias Hauser4,Rémy Bonnet5,Linh N. Luu1,Flavio Lehner15,16,Nathan Gillett17,Jordis Tradowsky18,19,Gabriel A. Vecchi13,20,Chris Rodell21,Roland B. Stull21,Rosie Howard21,and Friederike E. L. Otto14Sjoukje Y. Philip et al.Sjoukje Y. Philip1,,Sarah F. Kew1,,Geert Jan van Oldenborgh1,2,,Faron S. Anslow3,Sonia I. Seneviratne4,Robert Vautard5,Dim Coumou1,6,Kristie L. Ebi7,Julie Arrighi8,9,10,Roop Singh8,Maarten van Aalst8,9,11,Carolina Pereira Marghidan9,Michael Wehner12,Wenchang Yang13,Sihan Li14,Dominik L. Schumacher4,Mathias Hauser4,Rémy Bonnet5,Linh N. Luu1,Flavio Lehner15,16,Nathan Gillett17,Jordis Tradowsky18,19,Gabriel A. Vecchi13,20,Chris Rodell21,Roland B. Stull21,Rosie Howard21,and Friederike E. L. Otto14
Received: 05 Nov 2021 – Accepted for review: 11 Nov 2021 – Discussion started: 12 Nov 2021
Abstract. 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 hospital visits for heat-related illnesses and emergency calls. 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 and modeling, the occurrence of a heatwave with maximum daily temperatures (TXx) as observed in the area 45° N–52° N, 119° W–123° W, was found to be virtually impossible without human-caused climate change. The observed temperatures were so extreme that they lie far outside the range of historically observed temperatures. This makes it hard to quantify with confidence how rare the event was. In the most realistic statistical analysis, which uses the assumption that the heatwave was a very low probability event that was not caused by new nonlinearities, the event is estimated to be about a 1 in 1000 year event in today’s climate. With this assumption and combining the results from the analysis of climate models and weather observations, an event, defined as daily maximum temperatures (TXx) in the heatwave region, as rare as 1 in a 1000 years would have been at least 150 times rarer without human-induced climate change. Also, this heatwave was about 2 °C hotter than a 1 in 1000-year heatwave that at the beginning of the industrial revolution would have been (when global mean temperatures were 1.2 °C cooler than today). Looking into the future, in a world with 2 °C of global warming (0.8 °C warmer than today), a 1000-year event would be another degree hotter. It would occur roughly every 5 to 10 years in such global warming conditions.
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.
In June 2021, the Pacific northwest U.S. and Canada experienced record temperatures far exceeding those previously observed. This attribution study found that the event would have been virtually impossible without human-induced climate change. Assuming no nonlinear interactions, we estimated the likelihood of the event to be about 1 in 1000 years. Such heatwaves would have been at least 150 times rarer without human-induced climate change, and will become more common as warming continues.
In June 2021, the Pacific northwest U.S. and Canada experienced record temperatures far...
