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Flooding in in Porto Alegre, Rio Grande do Sul on May 5, 2024. Image by Ricardo Stuckert / PR.
Between 24 April and 4 May 2024 over 420 mm of rain fell in Brazil’s southernmost state Rio Grande do Sul, leading to more than 90% of the state being affected by flooding.
The floods displaced more than 80,000 people, led to over 150,000 being injured and, on the 29th of May, to 169 fatalities with 44 people still missing (Governo do Estado de Rio Grande do Sul, 2024). Essential services were also disrupted, leaving 418,200 households without electricity and over a million consumer units without water. Dozens of municipalities lost telephone and internet services. Municipalities in Rio Grande do Sul that were severely affected by floods, and inundations are classified with risk levels ranging from ‘Medium’ to ‘Very High’ for geo-hydrological disasters on the AdaptaBrasil platform. Cities such as Porto Alegre, Eldorado do Sul, Canoas, Guaíba, Novo Hamburgo, Estrela, and Encantado, for example, are characterised by the highest degrees of Hazard and Exposure.
Researchers from Brazil, the United Kingdom, Sweden, the Netherlands, and the US collaborated to answer the question of whether and to what extent human-induced climate change altered the likelihood and intensity of the rainfall that caused the flooding. They also investigated the role of the El Niño Southern Oscillation (ENSO).
Rainfall in Southern Brazil (comprising the states of Paraná, Santa Catarina, and Rio Grande do Sul) is characterised by a subtropical climate (transition between tropical and temperate climate) with a continuous supply of moisture from the Atlantic Ocean and the Amazon region thus no distinct rainy seasons exist. Rainfall varies from year to year depending on large scale climate phenomena such as ENSO.
To capture the nature of the extreme rainfall that resulted in extreme flooding across Rio Grande do Sul, two event definitions are analysed in this study: the 4- and 10-day rainfall accumulations, averaged over the state of Rio Grande do Sul. The 4-day window captures the most severe single event in which record rainfall fell across several consecutive days, while the 10-day window (encompassing 26th April – 5th May, inclusive) captures the succession of heavy rainfall events, including the very wet individual days either side of the major 4-day peak (figure 1).
Figure 1: Accumulated rainfall over Rio Grande do Sul, the southernmost state of Brazil, in late April and early May 2024. The longer 10-day period (top) represents a succession of 3 rainfall events, the shorter 4-day period (bottom) covers the single largest multi-day pulse of rainfall. Data from MSWEP.
Main findings
- The unprecedented 2024 April-May floods in Rio Grande do Sul have affected over 90% of the state, an area equivalent to the UK, displacing 581,638 people and causing 169 deaths. While Rio Grande do Sul is often perceived as a well-off region, it still has significant pockets of poverty and marginalisation.