Rapid Increase in Tropical Humid Heat Stress and Its Predictability in a Warming World

Abstract Anthropogenic activities are pushing key Earth system components beyond safe thresholds, disrupting economies, ecosystems, and societies. Yet, climate action remains largely rhetorical, and sustainable development has stalled. Here, we show an abrupt increase in global potential productivity loss from humid heat stress since the mid‐1980s, with India contributing over half of the total increase. This coincides with Indo‐Pacific sea surface temperatures exceeding C and rapid population growth. The loss has continued and recently crossed a critical transition, suggesting a shift to a new state. On interannual timescales, strong correlation with ENSO indicates that such losses are predictable up to 6 months in advance, offering a window for adaptive planning. These findings reveal that accounting for humid heat stress significantly raises the estimated socioeconomic costs of climate change and highlights a feedback loop where climate impacts undermine the development pathways that contribute to those very impacts.