Mortality impacts of rainfall and sea-level rise in a developing megacity

Rainfall and flooding frequently disrupt the lives of urban residents worldwide, posing substantial public health risks^1,2. Rapid urbanization is exposing larger and more vulnerable populations to flooding³, while climate change intensifies rainfall patterns^4,5 and rising sea levels impair drainage systems^6-11. Despite the growing recognition and urgency of these hazards, the health impacts of rainfall remain poorly understood, and those of sea-level rise are entirely unquantified. Here we estimate the mortality consequences of rainfall in one of the world's largest cities-Mumbai, India. We integrate high-resolution data on rainfall, tides and mortality to analyse how unmanaged rainfall, and its interaction with tidal dynamics, contribute to urban health risks. We find that rainfall causes more than 8% of Mumbai's deaths during the monsoon season, and that more than 80% of this burden is borne by slum residents. Children face the biggest increase in mortality risk from rainfall, and women face a greater risk than men. We also demonstrate that mortality risk from rainfall increases sharply during high tides and use this relationship to evaluate how rising sea levels could amplify rainfall-induced mortality in the absence of adaptation. Our findings reveal that the mortality impacts of rainfall are an order of magnitude larger than is documented by official statistics^12,13, highlighting the urgent need for investment in improved drainage, sanitation and waste-management infrastructure.