BACKGROUND: Climate change can directly impact temperature-related excess deaths and might subsequently change the seasonal variation in mortality. In this study, we aimed to provide a systematic and comprehensive assessment of potential future changes in the seasonal variation, or seasonality, of mortality across different climate zones. METHODS: In this modelling study, we collected daily time series of mean temperature and mortality (all causes or non-external causes only) via the Multi-Country Multi-City Collaborative (MCC) Research Network. These data were collected during overlapping periods, spanning from Jan 1, 1969 to Dec 31, 2020. We projected daily mortality from Jan 1, 2000 to Dec 31, 2099, under four climate change scenarios corresponding to increasing emissions (Shared Socioeconomic Pathways [SSP] scenarios SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). We compared the seasonality in projected mortality between decades by its shape, timings (the day-of-year) of minimum (trough) and maximum (peak) mortality, and sizes (peak-to-trough ratio and attributable fraction). Attributable fraction was used to measure the burden of seasonality of mortality. The results were summarised by climate zones. FINDINGS: The MCC dataset included 126 809 537 deaths from 707 locations within 43 countries or areas. After excluding the only two polar locations (both high-altitude locations in Peru) from climatic zone assessments, we analysed 126 766 164 deaths in 705 locations aggregated in four climate zones (tropical, arid, temperate, and continental). From the 2000s to the 2090s, our projections showed an increase in mortality during the warm seasons and a decrease in mortality during the cold seasons, albeit with mortality remaining high during the cold seasons, under all four SSP scenarios in the arid, temperate, and continental zones. The magnitude of this changing pattern was more pronounced under the high-emission scenarios (SSP3-7.0 and SSP5-8.5), substantially altering the shape of seasonality of mortality and, under the highest emission scenario (SSP5-8.5), shifting the mortality peak from cold seasons to warm seasons in arid, temperate, and continental zones, and increasing the size of seasonality in all zones except the arid zone by the end of the century. In the 2090s compared with the 2000s, the change in peak-to-trough ratio (relative scale) ranged from 0·96 to 1·11, and the change in attributable fraction ranged from 0·002% to 0·06% under the SSP5-8.5 (highest emission) scenario. INTERPRETATION: A warming climate can substantially change the seasonality of mortality in the future. Our projections suggest that health-care systems should consider preparing for a potentially increased demand during warm seasons and sustained high demand during cold seasons, particularly in regions characterised by arid, temperate, and continental climates. FUNDING: The Environment Research and Technology Development Fund of the Environmental Restoration and Conservation Agency, provided by the Ministry of the Environment of Japan.

Barcelona Institute for Global Health Barcelona Spain

Center for Climate Change Adaptation National Institute for Environmental Studies Tsukuba Japan

Center for Environmental and Respiratory Health Research University of Oulu Oulu Finland; Medical Research Center Oulu Oulu University Hospital and University of Oulu Oulu Finland; Department of Public Health University of Helsinki Helsinki Finland

Center for Environmental and Respiratory Health Research University of Oulu Oulu Finland; Medical Research Center Oulu Oulu University Hospital and University of Oulu Oulu Finland; Finnish Meteorological Institute Helsinki Finland

Climate Research Foundation Madrid Spain; Spanish Consortium for Research and Public Health Instituto de Salud Carlos 3 Madrid Spain

Department of Epidemiology Instituto Nacional de Saúde Dr Ricardo Jorge Lisbon Portugal

Department of Global Health Policy Graduate School of Medicine The University of Tokyo Tokyo Japan

Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine London UK

Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine London UK; Department of Economics Ca' Foscari University of Venice Venice Italy

Department of Statistics Computer Science and Applications G Parenti University of Florence Florence Italy

Environmental Health Department National Institute of Health Porto Portugal; EPIUnit Instituto de Saúde Pública Universidade do Porto Porto Portugal; Laboratório para a Investigação Integrativa e Translacional em Saúde Populacional Porto Portugal

Institute of Atmospheric Physics Czech Academy of Sciences Prague Czech Republic; Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic

Institute of Environmental Assessment and Water Research Spanish Council for Scientific Research Barcelona Spain

Institute of Social and Preventive Medicine University of Bern Bern Switzerland; Oeschger Center for Climate Change Research University of Bern Bern Switzerland

National Institute of Environmental Health Chinese Center for Disease Control and Prevention Beijing China; School of Public Health and Social Work Queensland University of Technology Brisbane QLD Australia

School of Epidemiology and Public Health Faculty of Medicine University of Ottawa Ottawa ON Canada; Environmental Health Science and Research Bureau Health Canada Ottawa ON Canada

School of the Environment Yale University New Haven CT USA

School of Tropical Medicine and Global Health Nagasaki University Nagasaki Japan

School of Tropical Medicine and Global Health Nagasaki University Nagasaki Japan; Department of Global Health Policy Graduate School of Medicine The University of Tokyo Tokyo Japan

Spanish Consortium for Research and Public Health Instituto de Salud Carlos 3 Madrid Spain; Department of Statistics and Computational Research Universitat de València València Spain

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