Anthropogenic Contributions to the 2021 Pacific Northwest Heatwave

Abstract

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 global warming caused a ∼0.8°C–1°C increase in heatwave temperatures. Future warming would lead to a ∼5°C increase in heatwave temperature by the end of the 21st century.

The unprecedented Pacific Northwest heatwave of June 2021

“…Despite recent progress in extreme event attribution science 22 , this is a difficult question to answer for extremely rare events. One common approach is to quantify the change in the return period of an event; however, the validity of this method is uncertain for events this rare [23][24][25] . Of the papers published on this event thus far, estimates of the current return period of this event vary from 1 in 10,000-100,000 years 24 , 1 in 1000 years 25 , to 1 in 200 years; 10 further research on methods to estimate the expected occurrence of such unprecedented extremes in the context of a limited observational record and a changing climate are critically needed.…”

“…It is clear, however, that anthropogenic warming of the planet contributed to the severity of this event 23,25,26 -mean global temperatures for 2010-2019 were around 1.1 °C warmer than in 1850-1900, with the Intergovernmental Panel of Climate Change (IPCC) attributing this increase entirely to human activities 27 . Land temperatures have warmed faster than the global mean (1.5 °C since 1850-1900), and the rate of warming has increased over the past 70 years 27 , increasing the probability of record-shattering extremes 28 .…”

Heatwaves Similar to the Unprecedented One in Summer 2021 Over Western North America Are Projected to Become More Frequent in a Warmer World

“…A rapid attribution study (Philip et al., 2021) revealed that climate change made a similar heatwave to the 2021 WNA one 150 times more likely, and the heatwave would be impossible without the influence of anthropogenic climate change. Global warming could largely shorten the return period and increase the reoccurrence of a heatwave similar to the 2021 WNA one in the future (Bartusek et al., 2022; Bercos‐Hickey et al., 2022). It makes such a previously impossible heatwave nearly a once‐in‐decade event in a climate 2°C warmer than the pre‐industrial era under the Shared Socioeconomic Pathways 245 (SSP2‐4.5) or SSP5‐8.5 scenario (Bartusek et al., 2022; Philip et al., 2021).…”

Sixth Oregon climate assessment

“…The simplest and most straightforward way to interpret the effect of anthropogenic climate change on the severity of the June 2021 event is to assume that it elevated temperatures by an amount equivalent to the mean increase in temperature since anthropogenic climate change began. A recent study estimated that this heat wave was about two degrees Fahrenheit warmer than it would have been without human influence on the climate (Bercos-Hickey et al 2022), consistent with the increase in mean temperature. An event of this magnitude is likely to occur once in 1000 to 100,000 years (McKinnon andSimpson 2022, Thompson et al 2022).…”

“…During the last week of June, 2021, an exceptional heat wave with no precedent in the modern observational record occurred across Oregon and the Pacific Northwest (Bercos-Hickey et al 2022, Vescio and Bair 2022. The all-time high temperature records at multiple weather stations were broken by several degrees.…”