Temporal Evolution and Intensification of Extreme Heatwaves in Brazil’s capitals: Insights from the XHW Index (1950–2024)

The increase of heatwaves has been observed due to the influence of a changing climate on the planet. Among populations, this phenomenon is associated with a rise in various types of cardiovascular diseases, particularly among vulnerable groups such as the elderly and individuals with pre-existing conditions. This presents a growing concern for healthcare systems and urban management. Although the literature predominantly highlights these events on the European continent, it is known that heatwaves are not confined to this region. Thus, the objective of this study is to evaluate the temporal evolution of the phenomenon through an index developed by LAMMOC/UFF to detect extreme heatwaves in the 26 Brazilian capitals and the Federal District. For this purpose, ERA5 reanalysis data from January 1950 to September 2024 were utilized. The index was calculated hourly and aggregated on a daily and monthly basis. Initially, the Mann-Kendall test was employed to assess the trends in the time series. It was observed that 19 of the analyzed cities exhibited a positive trend, two showed no trend (Florianópolis and Fortaleza), and six coastal cities (primarily in northeastern region, except for Teresina and Recife) located in a region with barotropic conditions did not show occurrences of extreme heat waves during the study period. Subsequently, a correlation matrix between the time series was analyzed, along with clustering, identifying three distinct groups: one less affected by extreme heat waves, a second intermediate group with a positive trend, and a third group that exhibited the most significant positive trends. The time series were then divided, applying a clustering technique, into three distinct periods: 1950–1974 (P1), 1975–1999 (P2), and 2000–2024 (P3). This allowed the calculation of the average accumulated values for these three periods across all evaluated cities to observe the percentage differences. For instance, from P1 to P3, in the southeastern region, the cities of Rio de Janeiro and São Paulo presented an approximate 613% and 643% increase in extreme heatwave occurrences, while in the south region, Porto Alegre had an increase of 557%. In the midwest, Campo Grande and Cuiabá presented an increase of 983% and 671% and in the northeast Recife and Teresina presented an increase of 217% and 257%. Furthermore, the third cluster displayed the highest trends and averages, encompassing most cities of the Northern region. Therefore, cities such as Macapá, Rio Branco, Manaus and Belém experienced an increase of  3154%, 1339%, 1100% and 1028%, respectively. Notably, it was observed that this third cluster predominantly comprised cities in the northern region of the country, situated within the Amazon biome. These findings call for further investigation into the relationship between this trend and factors such as increasing deforestation and forest fires in the region. These alarming results highlight the urgent challenges in environmental, healthcare, and urban management driven by climate change and extreme events, emphasizing the need for investments in municipal health, resilience, and climate mitigation and adaptation strategies.