Editing IPCC:AR6/WGI/Chapter-11 (section)

=== 11.5.4 Detection and Attribution, Event Attribution ===

<div id="h2-37-siblings" class="h2-siblings"></div>

There are very few studies focused on the attribution of long-term changes in floods, but there are studies on changes in flood events. Most of the studies focus on flash floods and urban floods, which are closely related to intense precipitation events ( [[#Hannaford--2015|Hannaford, 2015]] ). In other cases, event attribution focused on runoff using hydrological models, and examples include river basins in the UK ( [[#11.4.4|Section 11.4.4]] ; [[#Schaller--2016|Schaller et al., 2016]] ; [[#Kay--2018|Kay et al., 2018]] ), the Okavango River in Africa ( [[#Wolski--2014|Wolski et al., 2014]] ), and the Brahmaputra River in Bangladesh ( [[#Philip--2019|Philip et al., 2019]] ). Findings about anthropogenic influences vary between different regions and basins. For some flood events, the probability of high floods in the current climate is lower than in a climate without an anthropogenic influence ( [[#Wolski--2014|Wolski et al., 2014]] ), while in other cases anthropogenic influence leads to more intense floods ( [[#Cho--2016|Cho et al., 2016]] ; [[#Pall--2017|Pall et al., 2017]] ; [[#van%20der%20Wiel--2017|van der Wiel et al., 2017]] ; [[#Philip--2018a|Philip et al., 2018a]] ; [[#Teufel--2019|Teufel et al., 2019]] ). Factors such as land-cover change and river management can also increase the probability of high floods ( [[#Ji--2020|Ji et al., 2020]] ). These, along with model uncertainties and the lack of studies overall, suggest a ''low confidence'' in general statements to attribute changes in flood events to anthropogenic climate change. A few individual regions have been well studied, which allows for ''high confidence'' in the attribution of increased flooding in these cases. For example, flooding in the UK following increased winter precipitation ( [[#Schaller--2016|Schaller et al., 2016]] ; [[#Kay--2018|Kay et al., 2018]] ) can be attributed to anthropogenic climate change ( [[#Schaller--2016|Schaller et al., 2016]] ; [[#Vautard--2016|Vautard et al., 2016]] ; [[#Yiou--2017|Yiou et al., 2017]] ; [[#Otto--2018b|Otto et al., 2018b]] ).

Attributing changes in heavy precipitation to anthropogenic activities ( [[#11.4.4|Section 11.4.4]] ) cannot be readily translated to attributing changes in floods to human activities, because precipitation is only one of the multiple factors, albeit an important one, that affect floods. For example, [[#Teufel--2017|Teufel et al. (2017)]] showed that, while human influence increased the odds of the flood-producing rainfall for the 2013 Alberta flood in Canada, it was not detected to have influenced the probability of the flood itself. [[#Schaller--2016|Schaller et al. (2016)]] showed that human influence on the increase in the probability of heavy precipitation translated linearly into an increase in the resulting river flow of the Thames in the UK in winter 2014, but its contribution to the inundation was inconclusive.

[[#Gudmundsson--2021|Gudmundsson et al. (2021)]] compared the spatial pattern of the observed regional trends in high river flows (&gt;90th percentile) over 1971–2010 with that simulated by global hydrological models. The hydrological models were driven by outputs of climate model simulations under all historical forcing and pre-industrial forcing conditions. They found complex spatial patterns of extreme river flow trends. They also found the observed spatial patterns of trends can be reproduced only if anthropogenic climate change is considered, and that simulated effects of water and land management cannot reproduce the observed spatial pattern of trends. As there is only one study and multiple caveats associated with the study, including relatively poor observational data coverage, there is ''low confidence'' about human influence on the changes in high river flows on the global scale.

In summary there is ''low confidence'' in the human influence on the changes in high river flows on the global scale. In general, there is ''low confidence'' in attributing changes in the probability or magnitude of flood events to human influence because of a limited number of studies, differences in the results of these studies and large modelling uncertainties.

<div id="11.5.5" class="h2-container"></div>

<span id="future-projections"></span>