Rapid attribution analysis of the extraordinary heat wave on the Pacific coast of the US and Canada in 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 US and Canada, leading to spikes in sudden deaths and sharp increases in emergency calls and hospital visits for heat-related illnesses. Here we present a multi-model, multi-method attribution analysis to investigate the extent to which human-induced climate change has influenced the probability and intensity of extreme heat waves in this region. Based on observations, modelling and a classical statistical approach, the occurrence of a heat wave defined as the maximum daily temperature (TXx) observed in the area 45–52∘ N, 119–123∘ W, was found to be virtually impossible without human-caused climate change. The observed temperatures were so extreme that they lay far outside the range of historical temperature observations. This makes it hard to state with confidence how rare the event was. Using a statistical analysis that assumes that the heat wave is part of the same distribution as previous heat waves in this region led to a first-order estimation of the event frequency of the order of once in 1000 years under current climate conditions. Using this assumption and combining the results from the analysis of climate models and weather observations, we found that such a heat wave event would be at least 150 times less common without human-induced climate change. Also, this heat wave was about 2 ∘C hotter than a 1-in-1000-year heat wave would have been in 1850–1900, 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. 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.

Typicality of the 2021 Western North America summer heatwave

“…a 30 year time span commonly used to benchmark climatology. We conclude that the red grid points in figure 2 define the (vast) core region of the 2021 event, which results to be larger than the area investigated in [18,19].…”

“…Several new temperature records were established: 49.6 • C were measured in Lytton, which is the new absolute temperature record for 2-m temperature (T2M) for Canada. It is estimated that the heatwave has led to over 1000 directly attributable deaths and damages worth several billion USD, associated with wildfires, crop loss, infrastructural damages, and flooding due to rapid snow melt [18][19][20]. The record-breaking temperatures were the result of preconditioning combined with an anomalous atmospheric circulation.…”

“…The record-breaking temperatures were the result of preconditioning combined with an anomalous atmospheric circulation. Prior to the heatwave, the southern portion of the region was anomalously dry [18,21] and it is known that long-term drought increases the probability of occurrence of heatwaves, as a result of the drying of the soil [22][23][24], possibly leading to the onset of a cycle of heat, drought, and fire [25,26]. This was compounded by the presence of an intense atmospheric Ω-block, like the one associated with the 2010 Russian heatwave [27][28][29].…”

“…Recently, a team from the World Weather Attribution initiative has concluded, following the framework presented in [38] and using a multi-model and multi-method attribution analysis applied to daily maximum temperatures, that it is possible to attribute almost non-ambiguously the 2021 event to climate change, because its likelihood in present climate conditions (factual world) is estimated to be orders of magnitude higher than in past climate conditions (counterfactual world) [18].…”

“…The 2021 WNAHW featured a particularly acute phase between 27th June and 29th June. Hence, some studies have focused on the statistical investigation of daily temperature records [18,19] and taken advantage of the statistical framework provided by extreme value theory [2,43]. Since persistence plays a major role in determining the impact of heatwaves [9], we prefer to look into cumulative temperature anomalies.…”