The relevance of high-forcing 8.5 scenarios for natural hazard assessment: a multi-site hydraulic modelling approach to flood impact

Introduction: Radiative forcing scenarios constitute an axis of climate change projection and mitigation planning. The 2026 IPCC proposal to retire 8.5 high-forcing pathways (RCP8.5, SSP5-8.5) from the forthcoming AR7, as progress in climate policy makes them unlikely, raises a critical question for natural hazard assessment. In hazard and risk analysis, low-probability but plausible worst-case scenarios retain fundamental value: they underpin spatial planning, emergency response, risk cartography and stress-testing frameworks. This study evaluates the consequences of withdrawing 8.5 scenarios for flood hazard estimation, testing whether their exclusion entails a loss of relevant information.

Methods: Daily precipitation projections from regional and global climate models under RCP4.5/RCP8.5 (AR5) and SSP2-4.5/SSP5-8.5 (AR6) were used to drive hydraulic flood simulations in HEC-RAS across six sites spanning contrasting climatic and geographic places: Bursa (Turkey), Toowoomba (Australia), Barahona (Dominican Republic), Río Santa (Peru), Toledo (Spain) and Rio Grande do Sul (Brazil). Modelled flood extent, flow velocity, flow depth and hazard were intersected in a Geographical Information System (GIS) environment with synthetic commercial and residential assets carrying assigned economic values. Joint Research Center vulnerability curves were applied to these assigned values in order to model the economic losses. These methods enabled a comparative economic estimation of the impact of retaining versus retiring high-forcing scenarios.

Results: An average of 17% difference in inundated area was found between the 4.5 and 8.5 analyzed scenarios. Flow velocity remained similar with a 1.7% change, while economic losses were 23% higher in 8.5 scenarios than in 4.5. Preliminary results indicate that the difference concentrates disproportionately in the high-impact tail, where the most severe losses occur.

Conclusions: Retiring 8.5 scenarios removes the upper bound of the hazard space, systematically biasing flood risk estimates downward. Given the asymmetric cost of underestimating low-probability, high-impact events, retaining high-forcing pathways as climate stress scenarios remains essential for robust natural hazard assessment.