Reduction of the uncertainty of flood hazard analyses under a future climate by integrating multiple SSP-RCP scenarios

Abstract. The uncertainty due to small number of ensemble members is a major source of uncertainty in future climate predictions. While the uncertainty due to the above in general circulation models (GCMs) projections can be mitigated by increasing the number of simulation ensembles, only a limited number of large-ensemble experiments are available in CMIP6 future scenario experiments. Here we propose a method that increases the sample ensemble size in evaluations of future hazard, by integrating multiple SSP-RCPs for a time period corresponding to a specific increase in temperature from the preindustrial level (i.e., X °C warming). The success of the method was assessed by investigating whether the uncertainty due to small number of ensemble members could be reasonably reduced. First, the similarity in the spatial distributions of flood hazard projection at the same warming level was determined for different SSP-RCP scenarios. Under a 2 °C warming, all SSP-RCPs had a similar distribution with respect to the change ratio of the flood magnitude. Additionally, we showed that the uncertainty due to the different SSP-RCPs (5 %–10 %) was smaller than the differences between different warming levels such as between 2 °C and 3 °C (around 20 %–30 %), which suggests that differences among SSP-RCPs as to future flood discharge change are relatively small. These results suggested that integrating SSP-RCPs to increase the ensemble size was a reasonable approach, reducing unbiased variance among GCMs in about 70 % of land grid points comparing to the result using SSP5-RCP8.5 alone.