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).