Rapid attribution analysis of the extraordinary heatwave on the Pacific Coast of the US and Canada June 2021

Abstract

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.

An anomalous atmospheric river linked to the late June 2021 western North America heatwave

“…Some of this water vapour accumulated under a ridge of high pressure and may have formed a positive feedback loop to enhance the heatwave development over western NA during late June and early July 2021. A rapid attribution analysis has pointed out that that the occurrence of this extreme heatwave is virtually impossible without human-caused climate change 41 . Our results provide evidence that moisture must be considered when assessing heatwave development in a changing climate.…”

“…S4), the extreme heatwave started in Washington State and spread rapidly southward into Oregon and northward across BC and Alberta in western Canada. It generated some of the highest temperatures ever recorded in this region, including the new Canadian record of 49.6°C measured at Lytton, BC on June 29, 2021 41 .…”

“…These processes could be essential for the heat dome development, i.e., the build-up of an exceptionally hot air mass when high pressure aloft prevented warm air below from rising and the subsidence associated with the high pressure caused further warming by compression 40 . This heat dome was responsible for the extreme heatwave in western North America from late June through mid-July 2021 15,41 . Canada can be attributed jointly to the AR-induced moisture transport from the North Pacific Ocean and the local evaporation/evapotranspiration under the heat dome.…”

Analysis of community deaths during the catastrophic 2021 heat dome

“…Rapid attribution analyses found that this historic event would have been virtually impossible without anthropogenic climate change. 1 Hundreds of local temperature records were broken, often on consecutive days at the same location. For example, the small town of Lytton in the Fraser Canyon broke national temperature records on June 27 (46.6°C), June 28 (47.5°C), and June 29 (49.6°C) before being destroyed by wildfire on June 30.…”

Extreme heat in New Zealand: a synthesis

“…Whether focusing on communities living in hot, tropical climates or those in cooler, mid-latitude climates, there exists clear evidence of the manifold risks associated with extreme high temperatures, including for human health (Gasparrini et al 2015; Vicedo-Cabrera et al 2021), animal welfare (Bryant et al 2007;Dunn et al 2014) and economic productivity (Steffen et al 2019). Similarly stark evidence shows both the frequency and intensity of extreme heat events are increasing worldwide, in response to anthropogenic climate change (Vogel et al 2019;Harrington 2021a; Philip et al 2021). Despite this globally ubiquitous evidence base, extreme heat remains chronically understudied in New Zealand.…”