Extreme heat and mortality in the State of Rio de Janeiro in the 2023/24 season: attribution to climate change and ENSO

Abstract. During the 2023/24 season, the State of Rio de Janeiro experienced unprecedented maximum temperatures, resulting in a substantial increase in human mortality. This study aims to analyze the contribution of global warming to changes in the distribution of annual maximum temperatures and their subsequent impact on mortality rates. Our analysis of extreme temperatures reveals that a non-stationary model, in which the location parameter shifts linearly as a function of El Niño-Southern Oscillation (ENSO) and/or climate change, provides a significantly better fit to the data than its stationary counterpart. The northern region of the State exhibited the strongest response to climate change, while ENSO effects were most pronounced in the eastern region. Events as likely as the 2023 heatwave were estimated about 2 °C colder in pre-industrial times. Under a 2 °C global warming scenario, the probability of experiencing 2023-like daytime temperatures increases by at least a factor of three. These findings highlight climate change as the primary driver of extreme temperature intensification, with ENSO acting as a secondary but significant factor in the eastern region. As global warming approaches 2 °C, Rio de Janeiro is projected to experience heatwaves of that magnitude every four years approximately. Climate change has contributed to one in three heat-related deaths recorded during the peak of the event. Without adaptation measures, global warming would further increase the death toll during extreme events of the same frequency as the 2023/24 heat wave.