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Anthropogenic contributions to the 2021 Pacific Northwest heatwave
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  • Emily Bercos-Hickey,
  • Travis Allen O'Brien,
  • Michael F Wehner,
  • Likun Zhang,
  • Christina M Patricola,
  • Huanping Huang,
  • Mark Risser
Emily Bercos-Hickey
Lawrence Berkeley National Laboratory

Corresponding Author:[email protected]

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Travis Allen O'Brien
Indiana University Bloomington
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Michael F Wehner
Lawrence Berkeley National Laboratory (DOE)
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Likun Zhang
University of Missouri
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Christina M Patricola
Iowa State University
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Huanping Huang
Lawrence Berkeley National Laboratory
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Mark Risser
Lawrence Berkeley National Laboratory
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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.