Have there been previous heat extremes greater than the June 2021 western North America event?

In June 2021 western North America experienced a record-breaking heatwave, outside the distribution of previously observed temperatures. Our research asks whether other regions across the world have experienced so far outside their natural variability - and have there been greater heat extremes.  

In our novel assessment of heat extremes we characterise the relative intensity of an event as the number of standard deviations from the mean, finding the western North America heatwave is remarkable, outside four single deviations. Throughout the globe, where we have reliable data, only 5 other heatwaves were found to be more extreme since 1960. We can also identify regions which, by chance, have not had a recent extreme heatwave, and may be less prepared for future events. 

Using extreme value analysis the western North America heat extreme has been shown to be outside the previous distribution of extremes for the region. We can test if this is unique, or if previous events show similar. 

By assessing the numbers of regions globally exceeding various thresholds, in terms of standard deviation from the mean, we can show that extremes appear to increase in line with changes to the mean-state of the distribution of the climate, and projected increase in extremes aligns with projected warming.