Climate and impact attribution of compound flooding induced by tropical cyclone Idai in Mozambique

Abstract

In this study, we investigate the effect of climate change on tropical cyclone (TC) induced compound flooding and impacts for TC Idai, making landfall in Mozambique in 2019. TCs are one of the most damaging extreme events and are challenging to attribute using conventional, probabilistic methods. We develop a storyline attribution framework including a state-of-the-art modelling chain that combines hydrological, coastal, flood and impact models to simulate the changes in flooding and its impact under factual and counterfactual scenarios, with a plausible range of the climate trend removed from TC rainfall, maximum wind speed and sea level rise (SLR). For the case of TC Idai, we find that climate trends in SLR and wind-driven storm surge lead to the largest increase in flood damage of 6 %–27 %, compared to the counterfactual, while causing the smallest increase in flood volume and flood extent of 0.2 %–0.8 % and 0.1 %–0.6 %, respectively. Climate trends in rainfall lead to the largest increase in flood volume and flood extent of 5 %–19 % and 1 %–4 %, respectively, but account for the smallest increase in flood damage of 2 %–8 %. Changes in all drivers combined lead to about the same increase in flood volume and flood extent as the rain-only scenario (5 %–19 % and 1 %–5 %, respectively) but the largest increase in flood damage of 8 %–35 %. A non-linear relationship between flood hazard and flood damage results in a stronger climate footprint on TC impacts than hazards. Assessing the combination of all climate change-affected flood drivers is crucial for obtaining a comprehensive view on the effect of climate change. The attribution framework presented in this paper is applicable for TC-prone regions across the globe and can be applied in data-scarce, often highly impacted and vulnerable areas which are currently underrepresented in attribution studies.