This study is based on nationwide population and health register data in Sweden and includes all individuals registered in the Swedish Total Population Register since 1973. The cohort comprises approximately 12 million adults who have lived in Sweden during the study period. Detailed information on health outcomes, demographic characteristics, and socioeconomic conditions will be obtained through record linkage of multiple national and regional administrative and health registers. These include, among others, the National Patient Register, the Cause of Death Register, the Medical Birth Register, the Prescribed Drug Register, the Cancer Register, the National Vaccination Register, and the Longitudinal Integrated Database for Health Insurance and Labour Market Studies (LISA), as well as regional healthcare registers. Meteorological and environmental data on climate variables and climate extremes will be linked to the register data using geographical and temporal information. The study investigates the health impacts of climate extremes and climate-related environmental variables, including heatwaves, cold spells, droughts, floods, storms, wildfires, air pollution, and non-optimal temperature exposures. Outcomes of interest include all-cause and cause-specific mortality and morbidity, particularly cardiovascular, respiratory, renal, psychiatric, and pregnancy-related conditions, as well as other adverse health outcomes. Multiple epidemiological study designs will be used to evaluate both short-term and long-term health effects of climate extremes. Time series analyses and case-crossover or case time series designs will be used to estimate short-term health risks associated with transient exposures to climate-related events and environmental conditions. Cohort and survival analyses will be used to assess longer-term health impacts of climate variability and extremes. Ecological and systems-based approaches will also be applied to examine population-level trends and interactions between climatic variables, societal processes, and health outcomes. The analyses will quantify the excess risk associated with climate extremes and non-optimal environmental conditions and evaluate how these effects vary across population subgroups. Particular attention will be given to potential effect modifiers such as age, sex, socioeconomic position, geographical location, and societal processes, including economic crises and public health responses. In addition, scenario-based approaches will be used to explore potential future impacts of climate extremes and identify pathways for societal adaptation and public health preparedness.