Attribution of the Impacts of the 2024 Extreme Floods in Rio Grande do Sul, Brazil, to Climate Change

Extreme flood events are among the most damaging climate-related hazards, with significant human and socio-economic impacts. Understanding the extent to which anthropogenic climate change influences both the physical characteristics and impacts of such events is important for supporting policymakers in risk management and adaptation, informing loss and damage mechanisms, and raising public awareness of the impacts of climate change. Here, we apply a circulation-analogue attribution approach to quantify the impacts of climate change on flooding, extending the use of dynamical analogues from hazard attribution to impact analysis. The framework is designed to work with limited data, making it particularly relevant for data-scarce regions, including much of the Global South.

In late April and early May 2024, extreme flooding affected large parts of the state of Rio Grande do Sul in southern Brazil, being the largest floods ever observed along several regional rivers. The event caused at least 183 fatalities and affected more than 2.3 million people, making it one of the most severe climate-related disasters in Brazil’s history. Weekly rainfall totals exceeded 300 mm across much of the state and 500 mm locally.

In this study, we assess the influence of anthropogenic climate change on the socio-economic impacts of this extreme flood event using a three-step attribution framework. First, we attribute the total event rainfall to climate change by identifying dynamical analogues—events with similar large-scale atmospheric circulation—in single-model initial-condition large ensembles under factual and counterfactual climate conditions. Second, the resulting precipitation signals are used to force a hydrological flood model to quantify climate-induced changes in flood magnitude and spatial extent. Finally, we evaluate the associated socio-economic impacts based on the climate-attributed flood signal.