Selected timeseries of heatwave days (a); and length of longest heatwave (b). Decadal trends in heatwave days (c); and length of longest heatwave (d) commencing yearly between 1950 and 2000 and truncating in 2017. All values are calculated for the global observational dataset Berkeley Earth (see “Methods”). Closed circles in (c) and (d) indicate when trends commencing in that year are NOT statistically significant at the 5% level (see “Methods”). Regions displayed are Central North America (CNA), East Asia (EAS), Mediterranean (MED), North Asia (NAS), North Australia (NAU), North Europe (NEU), West North America (WNA), and the global average (Wor).