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

=== 11.5.2 Observed Trends ===

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The SREX ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ) assessed ''low confidence'' for observed changes in the magnitude or frequency of floods at the global scale. This assessment was confirmed by AR5 ( [[#Hartmann--2013|Hartmann et al., 2013]] ). The SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) found increases in flood frequency and extreme streamflow in some regions, but decreases in other regions. While the number of studies on flood trends has increased since AR5, and there were also new analyses after the release of SR1.5 ( [[#Berghuijs--2017|Berghuijs et al., 2017]] ; [[#Blöschl--2019|Blöschl et al., 2019]] ; [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ), hydrological literature on observed flood changes is heterogeneous, focusing at regional and sub-regional basin scales, making it difficult to synthesize at the global and sometimes regional scales. The vast majority of studies focus on river floods using streamflow as a proxy, with limited attention to urban floods. Streamflow measurements are not evenly distributed over space, with gaps in spatial coverage, and their coverage in many regions of Africa, South America, and parts of Asia is poor (e.g., [[#Do--2017|Do et al., 2017]] ), leading to difficulties in detecting long-term changes in floods ( [[#Slater--2017|Slater and Villarini, 2017]] ). See also [[IPCC:Wg1:Chapter:Chapter-8#8.3.1.5|Section 8.3.1.5]] .

Peak flow trends are characterized by high regional variability and lack overall statistical significance of a decrease or an increase over the globe as a whole. Of more than 3500 streamflow stations in the USA, central and Northern Europe, Africa, Brazil, and Australia, 7.1% stations showed a significant increase, and 11.9% stations showed a significant decrease in annual maximum peak flow during 1961–2005 ( [[#Do--2017|Do et al., 2017]] ). This is in direct contrast to the global and continental scale intensification of short-duration extreme precipitation ( [[#11.4.2|Section 11.4.2]] ). There may be some consistency over large regions (see [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ), in high streamflows (&gt;90th percentile), including a decrease in some regions (e.g., in the Mediterranean) and an increase in others (e.g., northern Asia), but gauge coverage is often limited. On a continental scale, a decrease seems to dominate in Africa ( [[#Tramblay--2020|Tramblay et al., 2020]] ) and Australia ( [[#Ishak--2013|Ishak et al., 2013]] ; [[#Wasko--2019|Wasko and Nathan, 2019]] ), an increase in the Amazon ( [[#Barichivich--2018|Barichivich et al., 2018]] ), and trends are spatially variable in other continents (Q. [[#Zhang--2015b|]] [[#Zhang--2015|Zhang et al., 2015]] b ; [[#Bai--2016|Bai et al., 2016]] ; [[#Do--2017|Do et al., 2017]] ; [[#Hodgkins--2017|Hodgkins et al., 2017]] ). In Europe, flow trends have large spatial differences ( [[#Hall--2014|Hall et al., 2014]] ; [[#Mediero--2015|Mediero et al., 2015]] ; [[#Kundzewicz--2018|Kundzewicz et al., 2018]] ; [[#Mangini--2018|Mangini et al., 2018]] ), but there appears to be a pattern of increase in north-western Europe, and a decrease in southern and eastern Europe in annual peak flow during 1960–2000 ( [[#Blöschl--2019|Blöschl et al., 2019]] ). In North America, peak flow has increased in north-east USA and decreased in south-west USA ( [[#Peterson--2013b|Peterson et al., 2013b]] ; [[#Armstrong--2014|Armstrong et al., 2014]] ; [[#Mallakpour--2015|Mallakpour and Villarini, 2015]] ; [[#Archfield--2016|Archfield et al., 2016]] ; [[#Burn--2016|Burn and Whitfield, 2016]] ; [[#Wehner--2017|Wehner et al., 2017]] ; [[#Neri--2019|Neri et al., 2019]] ). There are important changes in the seasonality of peak flows in regions where snowmelt dominates, such as northern North America ( [[#Burn--2016|Burn and Whitfield, 2016]] ; [[#Dudley--2017|Dudley et al., 2017]] ) and Northern Europe ( [[#Blöschl--2017|Blöschl et al., 2017]] ), corresponding to strong winter and spring warming.

In summary, the seasonality of floods has changed in cold regions where snowmelt dominates the flow regime in response to warming ( ''high confidence'' ). There is ''low confidence'' about peak flow trends over past decades on the global scale '','' but there are regions experiencing increases, including parts of Asia, Southern South America, north-east USA, north-western Europe, and the Amazon, and regions experiencing decreases, including parts of the Mediterranean, Australia, Africa, and south-western USA.

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