Contribution of Climate Change to Record-Breaking Heat and Mortality in Rio de Janeiro State

In November 2023, the daily maximum temperatures (TXs) in the state of Rio de Janeiro reached their highest levels in the 1971-2023 period, and caused a substantial increase in mortality, particularly among the elderly. These temperatures occurred in the context of ongoing global warming and during the El Niño phase of the El Niño-Southern Oscillation (ENSO). This study evaluates the role of climate change and ENSO in modulating the probability of occurrence and intensity of extreme heat in this Brazilian state using non-stationary extreme value theory, as well as their impact on mortality.

We find that non-stationary models, in which the location parameter varies with global warming and/or ENSO, provide more accurate fits for annual maximum TXs than stationary models. The analysis reveals spatial differences in the influence of these drivers: the northern region is most affected by climate change, while ENSO only exerts its influence in the eastern region. Furthermore, we detect that climate change has made 2023-like events ~2°C warmer than in the pre-industrial climate, when such a high TX would have been extremely unlikely. For stations in the eastern parts of the state, the best fit is obtained with a multivariate non-stationary fit, so both global warming and El Niño contributed to increasing the probability of occurrence of the observed 2023 TX. Nevertheless, in none of the stations the ENSO effect overwhelms that of climate change. As global warming approaches 2°C, Rio de Janeiro must prepare for extreme temperature events of similar magnitude to occur on average every four years.

According to our results, these human-intensified events could have a considerable impact on mortality rates without appropriate adaptation measures. Specifically, climate change has contributed to one in three heat-related deaths during the peak of the 2023 event, increasing the daily heat-related attributable risk by approximately 1.4 times compared to pre-industrial climate conditions. These findings underscore the urgent need for adaptive public health strategies and climate-resilient infrastructure to mitigate future mortality risks.

Acknowledgments: This work was supported by the SAFETE project, which has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 847635 (UNA4CAREER).