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

=== 11.8.2 Concurrent Extremes in Coastal and Estuarine Regions ===

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Coastal and estuarine zones are prone to a number of meteorological extreme events and also to concurrent extremes (see also Section 6.8.2 in SROCC). Floods are a major climatic impact-driver in coastal regions around the world (Chapter 12), and flood occurrence may be influenced by the dependence between storm surge, extreme rainfall, and river flow, but also by sea level rise, waves and tides, as well as groundwater for estuaries. Floods with multiple drivers are often referred to as ‘compound floods’ ( [[#Wahl--2015|Wahl et al., 2015]] ; [[#Moftakhari--2017|Moftakhari et al., 2017]] ; [[#Bevacqua--2020c|Bevacqua et al., 2020c]] ).

At USA coasts, the probability of co-occurring storm surge and heavy precipitation is higher for the Atlantic/Gulf coast relative to the Pacific coast ( [[#Wahl--2015|Wahl et al., 2015]] ). Furthermore, six studied locations on the USA coast with long overlapping time series show an increase in the dependence between heavy precipitation and storm surge over the last century, leading to more frequent co-occurring storm surge and heavy precipitation events at the present day ( [[#Wahl--2015|Wahl et al., 2015]] ). Storm surge and extreme rainfall are also dependent in most locations on the Australian coasts ( [[#Zheng--2013|Zheng et al., 2013]] ) and in Europe along the Dutch coasts ( [[#Ridder--2018|Ridder et al., 2018]] ), along the Mediterranean Sea, the Atlantic coast and the North Sea ( [[#Bevacqua--2019|Bevacqua et al., 2019]] ). The probability of flood occurrence can be assessed via the dependence between storm surge and river flow ( [[#Bevacqua--2020b|Bevacqua et al., 2020b]] , c). For instance, the occurrence of a North Sea storm surge in close succession with an extreme Rhine or Meuse river discharge is much more probable due to their dependence, compared to if both events were independent ( [[#Kew--2013|Kew et al., 2013]] ; [[#Klerk--2015|Klerk et al., 2015]] ). Significant dependence between high sea levels and high river discharge are found for more than half of the available station observations, which are mostly located around the coasts of North America, Europe, Australia, and Japan ( [[#Ward--2018|Ward et al., 2018]] ). Combining global river discharge with a global storm surge model, hotspots of compound flooding have been discovered that are not well covered by observations in some regions, including Madagascar, Northern Morocco, Vietnam, and Taiwan of China ( [[#Couasnon--2020|Couasnon et al., 2020]] ). In the Dutch Noorderzijlvest area, there is more than a two-fold increase in the frequency of exceeding the highest warning level compared to the case if storm surge and heavy precipitation were independent ( [[#van%20den%20Hurk--2015|van den Hurk et al., 2015]] ). In other regions and seasons, the dependence can be insignificant (W. [[#Wu--2018|]] [[#Wu--2018|Wu et al., 2018]] ) and there can be significant seasonal and regional differences in the storm surge–heavy precipitation relationship. Assessments of flood probabilities are often not based on actual flood measurements; instead, they are estimated from its main drivers, including astronomical tides, storm surge, heavy precipitation, and high streamflow. Such single driver analyses might underestimate flood probabilities if multiple correlated drivers contribute to flood occurrence (e.g., [[#van%20den%20Hurk--2015|van den Hurk et al., 2015]] ).

Many coastal areas are also prone to the occurrence of compound precipitation and wind extremes, which can cause damage, including to infrastructure and natural environments. A high percentage of co-occurring wind and precipitation extremes are found in coastal regions and in areas with frequent tropical cyclones. Finally, the combination of extreme wave height and duration is also shown to influence coastal erosion processes ( [[#Corbella--2012|Corbella and Stretch, 2012]] ).

Aspects of concurrent extremes in coastal and estuarine environments have increased in frequency and/or magnitude over the last century in some regions. These include an increase in the dependence between heavy precipitation and storm surge over the last century, leading to more frequent co-occurring storm surge and heavy precipitation events in the present day along USA coastlines ( [[#Wahl--2015|Wahl et al., 2015]] ). In Europe, the probability of compound flooding occurrence increases most strongly along the Atlantic coast and the North Sea under strong warming. This increase is mostly driven by an intensification of precipitation extremes and aggravated flooding probability due to sea level rise ( [[#Bevacqua--2019|Bevacqua et al., 2019]] ). At the global scale and under a high-emissions scenario, the concurrence probability of meteorological conditions driving compound flooding would increase by more than 25%, on average, along coastlines worldwide by 2100, compared to the present ( [[#Bevacqua--2020c|Bevacqua et al., 2020c]] ). Sea level extremes and their physical impacts in the coastal zone arise from a complex set of atmospheric, oceanic, and terrestrial processes that interact on a range of spatial and temporal scales and will be modified by a changing climate, including sea level rise ( [[#McInnes--2016|McInnes et al., 2016]] ). Interactions between sea level rise and storm surges ( [[#Little--2015|Little et al., 2015]] ), and sea level and fluvial flooding ( [[#Moftakhari--2017|Moftakhari et al., 2017]] ) are projected to lead to more frequent and intense compound coastal flooding events as sea levels continue to rise.

In summary, there is ''medium confidence'' that, over the last century, the probability of compound flooding has increased in some locations, including along the USA coastline. There is ''high confidence'' that the occurrence and magnitude of compound flooding in coastal regions will increase in the future due to both sea level rise and increases in heavy precipitation.

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