A record-breaking heatwave event occurred in western North America in the summer of 2021. Previous studies on a similar event's uniqueness and return period are typically estimated based on extreme value analysis and statistics of selected stations or area-mean temperature data. The extreme statistics results are no longer valid if moved to another location. Further, the detailed evolution of spatial extent and intensity through its duration is not quantified and compared to other historical heatwave events to claim its rarity and severity. By applying an event detection and tracking algorithm to heatwaves, we first construct an archive for all the events over the region since 1979 based on daily maximum temperature data from reanalysis. It documented the daily mean temperature anomalies, affected areas, and durations of all individual heatwave events. It is, therefore, possible to objectively compare and rank the magnitudes of all historical heatwave events by examining the total integrated effects from the intensity, impact region, and duration. With the global mean warming background increase of about 0.4 °C from 1979 to 2000 to 2000–2021, we found that the mean integrated magnitude and size of the top 100 heatwave events increased significantly by 155 ± 88%. The dominant relative contributions to this integrated severity measure increase are +78 ± 39% from the mean affected area, followed by +54 ± 21% from duration and +7 ± 4% from the intensity. With the above analyzed general historical increasing trend of heatwave event metrics, the severity of the 2021 summer heatwave over the region was due to a combination of its record-breaking event-mean spatial coverages (2090383 km2), highest normalized daily maximum temperature anomalies and long duration (11 days, in the top 4% of all heatwave events). This integrated severity measure of the event was more than double of any other historical heatwave event and about 17 times larger than the mean severity of all heatwave events over the region.
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