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12 Nov 2021
12 Nov 2021

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. Otto14 Sjoukje Y. Philip et al.
  • 1Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
  • 2Atmospheric, Oceanic and Planetary Physics, University of Oxford, UK
  • 3Pacific Climate Impacts Consortium, University of Victoria, Victoria, V8R4J1, Canada
  • 4Institute for Atmospheric and Climate Science, Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland
  • 5Institut Pierre-Simon Laplace, CNRS, Sorbonne Université, Paris, France
  • 6Institute for Environmental Studies (IVM), VU Amsterdam, The Netherlands
  • 7Center for Health and the Global Environment, University of Washington, Seattle WA, USA
  • 8Red Cross Red Crescent Climate Centre, The Hague, the Netherlands
  • 9Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Enschede, the Netherlands
  • 10Global Disaster Preparedness Center, American Red Cross, Washington DC, USA
  • 11International Research Institute for Climate and Society, Columbia University, New York, USA
  • 12Lawrence Berkeley National Laboratory, Berkeley, California USA
  • 13Department of Geosciences, Princeton University, Princeton, 08544, USA
  • 14School of Geography and the Environment, University of Oxford, UK
  • 15Department of Earth and Atmospheric Sciences, Cornell University, USA
  • 16Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, USA
  • 17Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Victoria, BC, Canada
  • 18Deutscher Wetterdienst, Regionales Klimabüro Potsdam, Potsdam, Germany
  • 19Bodeker Scientific, Alexandra, New Zealand
  • 20The High Meadows Environmental Institute, Princeton University, Princeton, 08544, USA
  • 21Department of Earth, Ocean, and Atmospheric Sciences, The University of British Columbia, Vancouver, Canada
  • These authors contributed equally to this work.
  • deceased, 12 October 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.

Sjoukje Y. Philip et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esd-2021-90', Anonymous Referee #1, 14 Dec 2021
    • AC1: 'Reply on RC1', Sjoukje Philip, 01 Feb 2022
  • RC2: 'Comment on esd-2021-90', Anonymous Referee #2, 23 Dec 2021
    • AC2: 'Reply on RC2', Sjoukje Philip, 01 Feb 2022

Sjoukje Y. Philip et al.

Sjoukje Y. Philip et al.


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Short summary
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.