Latitudinal changes in moist heatwaves across the Northern Hemisphere during 1950–2023

The intensification of moist heat extremes under global warming represents a critical hazard to human health and ecosystem stability. However, the distinct spatiotemporal evolutionary patterns of moist heatwaves (MHWs; relative humidity ≥40%) versus extreme moist heatwaves (EMHWs; relative humidity ≥70%) across different latitudinal zones remain insufficiently understood. Utilizing ERA5 reanalysis data, this study presents a comprehensive analysis of the latitudinal characteristics of MHWs and EMHWs over the Northern Hemisphere land during 1950–2023. By contrasting historical (1950–1979) and recent (1994–2023) periods, we quantify changes in event number, affected area, and duration. MHW occurrence exhibits a pronounced nonlinear acceleration, with the strongest increases concentrated in continental monsoon regions. Although tropical regions show substantial increases in both affected area and duration, mid-latitude regions exhibit particularly prominent changes, characterized by marked increases in MHW event number, affected area, and duration. Emerging MHW hotspots are evident across the Mediterranean coast, Northeast China, and the southeastern United States. Due to the stringent formation conditions of EMHWs, these events rarely occur in high-latitude regions. Despite relatively limited changes in their broad spatial distribution, EMHWs have intensified substantially in terms of event number and duration. Their spatial expansion is predominantly concentrated in the low-to-mid latitude monsoon regions of the Northern Hemisphere, while event number has accelerated markedly in recent decades, particularly over the Indian subcontinent and Indochina Peninsula. Mean EMHW duration has also increased substantially over West Africa, coastal India, and East Asia. Importantly, many of the regions experiencing the strongest EMHW intensification coincide with densely populated areas, highlighting increasing exposure to hazardous humid-heat stress under continued global warming.