Earth’s forests face grave challenges in the Anthropocene, including hotter droughts increasingly associated with widespread forest die-off. But despite the vital importance of forests to global ecosystem services, their fates in a warming world remain highly uncertain. In particular, Earth’s historical forests—large-tree communities with dominants established before circa 1880—are disproportionately vital in supporting biodiversity and in the cycling of carbon and water, and maybe particularly vulnerable to dieback and mortality from hotter droughts. Critically missing is quantitative determination of hotter-drought climatic drivers at globally-distributed, ground-based, tree-mortality sites. We established a precisely geo-referenced global database documenting climate-induced mortality events spanning all tree-supporting biomes from 154 studies since 1970. Here we quantify a lethal global hotter-drought fingerprint from these tree-mortality sites across 675 locations encompassing 1,303 database plots. Frequency of these lethal climate conditions accelerates under projected warming, up 140% by +4℃. Our database provides initial footing for further community-developed, quantitative, ground-based monitoring of global tree mortality, immediately enabling critical predictive model validation and improved remote-sensing of mortality. This global fingerprint of lethal hotter-drought confirms many of Earth’s forests are increasingly imperiled by further warming.