evidence: Important natural factors include El Niño and the Hunga Tonga eruption. The Hunga Tonga–Hunga Ha’apai eruption in 2022 has been suggested to explain some of the 2023 extreme warmth. Unlike more common sulfur-rich volcanic eruptions, the Hunga Tonga eruption injected large amounts of water vapour into the stratosphere and thereby could enhance tropospheric warming5. However, its net contribution to global temperature remains to be determined, including careful consideration of the associated aerosol responses6. [...] Overall, about 42% of the globe experienced heat (defined as more than two standard deviations) in 2023, surpassing the 2016 record of about 32% by a large margin (Fig. 1c). Extreme heat (defined as three standard deviations exceedance) occupied about 13% of the global surface in 2023, almost doubling the 2016 area of about 7%. Area fractions measured for global land or ocean only exhibit similar percentages to the global results (not shown), indicating the strong land-ocean connection. Although individual basin surface temperatures will be affected by region-specific natural variabilities such as the Atlantic Multidecadal Variability for the North Atlantic12, strong warming over most of the global ocean basins would not occur in the same year without systematic large-scale influences.