Attributing compound flood impact from tropical cyclone Idai in Mozambique to climate and population change

Impacts of extreme weather events are a complex interplay of compounding climatic and socio-economic drivers. In this paper, we attribute the effect of climate and population change on the number of people exposed to flooding from tropical cyclone (TC) Idai, making landfall in Mozambique in 2019. We apply an extended storyline attribution framework based on a globally-applicable model chain to simulate compound flooding and exposed population as a proxy for impact under factual and counterfactual climate and population conditions. This enables impact attribution to both drivers, and disaggregation across flood depth categories and socio-economic groups. Under the selected storyline experiments, 84% of the exposed population is attributable to the combined effect of climate and population change since 1975, with population change alone accounting for 80% and climate change alone for 9%. Climate change increases flood extent and flood depth, whereas population change alters the number of people living in the flooded area. These drivers change the distribution of people exposed to flooding, leading to different attribution assessments per flood depth category. Our results indicate that the effect of climate change is not evenly distributed across socio-economic groups, which is masked when spatially aggregating the attribution assessment.