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• 3 °C or more below normal
• within 3 °C of normal
• 3 to 6 °C above
• 6 °C or more above
• in a heatwave

Sorted by how far the latest high is above the city’s normal for the date. The last column counts the days this year that were part of a heatwave, with the 1991–2020 average by the same date in brackets.

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• More and hotter. It is virtually certain that hot extremes, including heatwaves, have become more frequent and more intense across most land regions since the 1950s, and human-caused climate change is the main driver (IPCC AR6, 2021, statement A.3.1).
• Every half degree counts. Every additional 0.5 °C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heatwaves (IPCC AR6, statement B.2.2).
• Impossible without warming. The late-June 2021 heatwave in the Pacific Northwest of the US and Canada, which set Canada’s record of 49.6 °C at Lytton, was virtually impossible without human-caused climate change (Philip et al. 2022).

Heat-related deaths are counted only afterwards, by comparing deaths with what would be expected without the heat. Estimates for Europe, where daily death records are best:

• Summer 2003: more than 70,000 additional deaths in 16 European countries (Robine et al. 2008).
• Summer 2022, then Europe’s hottest on record: an estimated 61,672 heat-related deaths in 35 countries between 30 May and 4 September (range 37,643–86,807; Ballester et al. 2023).
• 2023: an estimated 47,690 heat-related deaths (range 28,853–66,525); without the adaptation made since 2000, the toll would have been about 80% higher (Gallo et al. 2024).
• Summer 2010 in Russia and eastern Europe was hotter and covered a wider area than 2003; such “mega-heatwaves” likely broke 500-year summer records over about half of Europe (Barriopedro et al. 2011).