4.     Whither flood attribution?

We have proposed our view that explicit inclusion of flooding in extreme event attribution is necessary, but complicated by challenges at several levels. In all cases, hydrological/hydrodynamic modelling has to be included, as no existing dataset includes flood events under the factual and counterfactual climate conditions needed for the attribution. An area of advancement will therefore be to tackle these challenges in an expanded probabilistic attribution framework (Section 3.1). This can benefit from the huge progress recently made towards generating historical and near-real-time flood information. Such a framework should include propagation of uncertainties over the additional steps of hydrological and flood modelling, and we expect that uncertainties will grow considerably compared to attribution of precipitation. For most basins and river segments, hydrology is altered (Grill et al., 2019). Here, other drivers of flood occurrence need consideration, and future research should establish and test novel frameworks for attribution. We have illustrated the broad ideas of a multi-driver framework for conditional attribution (Section 3.2). As this framework enables isolating the hydrologic from the climatic drivers, it could also be adapted to include and isolate other climatic drivers, to link the occurrence of the event to specific climate mechanisms. For example, thermodynamic versus dynamic drivers (Diffenbaugh et al., 2017), teleconnections with climatic oscillations, or sea surface temperatures. In the future, efforts could also be made towards a framework to also enable attribution of compound fluvial and coastal floods (Zscheischler & Lehner, 2022).
An interesting advantage offered by an attribution framework that includes representation of past hydrological change, is that with relatively small additional effort, it could also include simulations with possible future changes in land-cover and possible river/flood management measures, in addition to possible climatic scenarios. This would directly connect flood attribution with adaptation and with the policies of flood management (Lahsen & Ribot, 2022; Osaka & Bellamy, 2020).
Besides flood managers, flood attribution could benefit from collaborations with fluvial geomorphologists and paleoflood hydrologists. While dealing with the last decades or century makes the attribution more tractable, historical records or sedimentary evidence for extreme events can expand the limited window of modern observations, and inform on rare high-magnitude events that occurred in the past (Wilhelm et al., 2018). Usually, the offset between the past setting (land-use and river morphology) and the present requires assessment, which may be a challenge for heavily engineered Anthropocene rivers. Despite the uncertainty associated with discharge estimates from paleoflood studies, their inclusion could benefit flood risk attribution as they provide the precedent and the synoptic conditions for such extremes to have occurred in the past (St. George & Mudelsee, 2019). Besides, paleo flood information could enable attribution of older historical floods (Blöschl et al., 2020).