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

=== 11.8.1 Overview ===

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The combination of two or more – not necessarily extreme – weather or climate events that occur: i) at the same time; ii) in close succession; or iii) concurrently in different regions, can lead to extreme impacts that are much larger than the sum of the impacts due to the occurrence of individual extremes alone. This is because multiple stressors can exceed the coping capacity of a system more quickly. The contributing events can be of similar types (clustered multiple events) or of different types ( [[#Zscheischler--2020|Zscheischler et al., 2020]] ). Many major weather- and climate-related catastrophes are inherently of a compound nature ( [[#Zscheischler--2019|Zscheischler et al., 2019]] ).This has been highlighted for a broad range of hazards, such as droughts, heatwaves, wildfires, coastal extremes, and floods ( [[#Westra--2016|Westra et al., 2016]] ; [[#AghaKouchak--2020|AghaKouchak et al., 2020]] ; [[#Ridder--2020|Ridder et al., 2020]] ). Co-occurring extreme precipitation and extreme winds can result in infrastructural damage ( [[#Martius--2016|Martius et al., 2016]] ); the compounding of storm surge and precipitation extremes can cause coastal floods ( [[#Wahl--2015|Wahl et al., 2015]] ); the combination of drought and heat can lead to tree mortality ( [[#11.6|Section 11.6]] ; [[#Allen--2015|Allen et al., 2015]] ); and wildfires increase occurrences of hailstorms and lightning (Y. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] a). Compound storm types consisting of co-located cyclone, front and thunderstorm systems have a higher chance of causing extreme rainfall and extreme winds than individual storm types ( [[#Dowdy--2017|Dowdy and Catto, 2017]] ). Extremes may occur at similar times at different locations ( [[#De%20Luca--2020a|De Luca et al., 2020a]] , b) but affect the same system, for instance, spatially concurrent climate extremes affecting crop yields and food prices ( [[#Singh--2018|Singh et al., 2018]] ; [[#Anderson--2019|Anderson et al., 2019]] ). Studies also show an increasing likelihood for breadbasket regions to be concurrently affected by climate extremes with increasing global warming, even between 1.5°C and 2°C of global warming (Box 11.2; [[#Gaupp--2019|Gaupp et al., 2019]] ). Concomitant extreme conditions at different locations become more probable as changes in climate extremes are emerging over an increasing fraction of the land area (Sections 11.2.3, 11.2.4, 11.8.2 and 11.8.3, and Box 11.4).

Finally, impacts may occur because of large multivariate anomalies in the climate drivers, if systems are adapted to historical multivariate climate variability ( [[#Flach--2017|Flach et al., 2017]] ). For instance, ecosystems are typically adapted to the local covariability of temperature and precipitation such that a bivariate anomaly may have a large impact, even though neither temperature nor precipitation may be extreme based on a univariate assessment ( [[#Mahony--2018|Mahony and Cannon, 2018]] ). Given that almost all systems are affected by weather and climate phenomena at multiple space-time scales ( [[#Raymond--2020|Raymond et al., 2020]] ), it is natural to consider extremes in a compound event framework. It should be noted, however, that multi-hazard dependencies can also decrease risk, for instance when hazards are negatively correlated ( [[#Hillier--2020|Hillier et al., 2020]] ). Despite this recognition, the literature on past and future changes in compound events has been limited, but is growing. This section assesses examples of types of compound events in available literature.

In summary, compound events include the combination of two or more – not necessarily extreme – weather or climate events that occur (i) at the same time, (ii) in close succession, or (iii) concurrently in different regions. The land area affected by concurrent extremes has increased ( ''high confidence'' ). Concurrent extreme events at different locations, but possibly affecting similar sectors (e.g., breadbaskets) in different regions, will become more frequent with increasing global warming, in particular above +2°C of global warming ( ''high'' ''confidence'' ).

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