The daily timing of a given temperature has shifted by over an hour since 1980

Climate change manifests not only as changes in daily mean temperatures but also as shifts in the time of day at which equivalent temperatures occur. These changes influence how people experience heat, when they can work safely, and when energy is needed to stay cool. We analyze historical analogues in the daily temperature cycle by comparing equivalent hourly temperatures since the 1980s. On a global average, temperatures characteristic of the morning warming period occur roughly 15 minutes earlier per decade, while those in the afternoon cooling period occur more than 20 minutes later per decade. For example, temperatures that occurred at 10 AM in the 1980s now occur at 9 AM, with even greater shifts in the afternoon. We extend this analysis to wet-bulb globe temperature (WBGT) to assess how shifts in timing translate to human-perceived heat stress, and find that these shifts are even larger in magnitude (minutes per decade). Across IPCC AR6 land regions, timing shifts scale with regional warming rates and diurnal temperature range, as stronger warming and smaller diurnal temperature ranges produce larger temporal displacements of equivalent temperatures. If sustained, these shifts would displace the time of day at which equivalent temperatures occur by more than three hours by 2100 relative to the 1980s, persisting under a pathway broadly consistent with recent emissions trends (SSP2-4.5) but slowing and eventually stopping under strong mitigation (SSP1-2.6). The timing changes perturb ecological cues, increase human heat exposure, and displace energy demand in ways not captured by means or extremes, underscoring the value of time-of-day metrics for characterizing climate change impacts.