Editing IPCC:AR6/WGII/Chapter-2 (section)

=== 2.3.1 Observed Changes to Hazards and Extreme Events ===

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The major climate hazards at the global level are generally well understood ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ) (WGI AR6 Interactive Atlas). Increased temperatures and changes to rainfall and runoff patterns; greater variability in temperature, rainfall, river flow and water levels; and rising sea levels and the increased frequency of extreme events means that greater areas of the world are being exposed to climate hazards outside of those to which they are adapted ( ''high confidence'' ) ( [[#Lange--2020|Lange et al., 2020]] ).

Extreme events are a natural and important part of many ecosystems, and many organisms have adapted to cope with long-term and short-term climate variability within the disturbance regime experienced during their evolutionary history ( ''high confidence'' ). However, climate changes, disturbance regime changes and the magnitude and frequency of extreme events such as floods, droughts, cyclones, heat waves and fire have increased in many regions ( ''high confidence'' ). These disturbances affect ecosystem functioning, biodiversity and ecosystem services ( ''high confidence'' ), but are, in general, poorly captured in impact models ( [[#Albrich--2020b|Albrich et al., 2020b]] ), although this should improve as higher-resolution climate models that better capture smaller-scale processes and extreme events become available ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ). Extreme events pose huge challenges for EbA ( [[#IPCC--2012|IPCC, 2012]] ). Ecosystem functionality, on which such adaptation measures rely, may be altered or destroyed by extreme episodic events ( [[#Handmer--2012|Handmer et al., 2012]] ; [[#Lal--2012|Lal et al., 2012]] ; [[#Pol--2017|Pol et al., 2017]] ).

There is ''high confidence'' that the combination of internal variability, superimposed on longer-term climate trends, is pushing ecosystems to tipping points, beyond which abrupt and possibly irreversible changes are occurring ( [[#Harris--2018a|Harris et al., 2018a]] ; [[#Jones--2018|Jones et al., 2018]] ; [[#Hoffmann--2019b|Hoffmann et al., 2019b]] ; [[#Prober--2019|Prober et al., 2019]] ; [[#Berdugo--2020|Berdugo et al., 2020]] ; [[#Bergstrom--2021|Bergstrom et al., 2021]] ). Increases in the frequency and severity of heat waves, droughts and aridity, floods, fires and extreme storms have been observed in many regions ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ; [[#Ummenhofer--2017|Ummenhofer and Meehl, 2017]] ), and these trends are projected to continue ( ''high confidence'' ) ( [[IPCC:Wg2:Chapter:Chapter-3#3.2.2.1|Section 3.2.2.1]] , Cross-Chapter Box EXTREMES this Chapter) ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Seneviratne--2021|Seneviratne et al., 2021]] ).

While the major climate hazards at the global level are generally well described with ''high confidence'' , there is less understanding about the importance of hazards on ecosystems when they are superimposed ( [[#Allen--2010|Allen et al., 2010]] ; [[#Anderegg--2015|Anderegg et al., 2015]] ; [[#Seidl--2017|Seidl et al., 2017]] ; [[#Dean--2018|Dean et al., 2018]] ), and the outcomes are difficult to quantify in future projections ( [[#Handmer--2012|Handmer et al., 2012]] ). Simultaneous or sequential events (coincident or compounding events) can lead to an extreme event or impact, even if each event is not in themselves extreme ( [[#Denny--2009|Denny et al., 2009]] ; [[#Hinojosa--2019|Hinojosa et al., 2019]] ). For example, the compounding effects of SLR, extreme coastal high tide, storm surge, and river flow can substantially increase flooding hazard and impacts on freshwater systems ( [[#Moftakhari--2017|Moftakhari et al., 2017]] ). On land, changing rainfall patterns and repeated heat waves may interact with biological factors such as altered plant growth and nutrient allocation under elevated CO 2 , affecting herbivore rates and insect outbreaks leading to the widespread dieback of some forests (e.g., in Australian eucalypt forests) ( [[#Gherlenda--2016|Gherlenda et al., 2016]] ; [[#Hoffmann--2019a|Hoffmann et al., 2019a]] ). Risk assessments typically only consider a single climate hazard with no changing variability, thereby potentially underestimating the actual risk ( [[#Milly--2008|Milly et al., 2008]] ; [[#Sadegh--2018|Sadegh et al., 2018]] ; [[#Zscheischler--2018|Zscheischler et al., 2018]] ; [[#Terzi--2019|Terzi et al., 2019]] ; [[#Stockwell--2020|Stockwell et al., 2020]] ).

Understanding impacts associated with the rapid rate of climate change is less developed and more uncertain than changes in mean climate. High climate velocity ( [[#Loarie--2009|Loarie et al., 2009]] ) is expected to be associated with distribution shifts, incomplete range filling and species extinctions ( ''high confidence'' ) ( [[#Sandel--2011|Sandel et al., 2011]] ; [[#Burrows--2014|Burrows et al., 2014]] ), although not all species are equally at risk from high velocity (see Sections 2.4.2.2, 2.5.1.3). It is generally assumed that the more rapid the rate of change, the greater the impact on species and ecosystems, but responses are taxonomically and geographically variable ( ''high confidence'' ) ( [[#Kling--2020|Kling et al., 2020]] ).

For example, strong dispersers are less at risk, while species with low dispersal ability, small ranges and long lifespans (e.g., many plants, especially trees, many amphibians and some small mammals) are more at risk ( [[#IPCC--2014b|IPCC, 2014b]] ; [[#Hamann--2015|Hamann et al., 2015]] ) . This is likely to favour generalist and invasive species, altering species composition, ecosystem structure and function ( [[#Clavel--2011|Clavel et al., 2011]] ; [[#Büchi--2014|Büchi and Vuilleumier, 2014]] ). The ability to track suitable climates is substantially reduced by habitat fragmentation and human modifications of the landscape such as dams on rivers and urbanisation ( ''high confidence'' ). Freshwater systems are particularly at risk of rapid warming, given their naturally fragmented distribution. Velocity of changes in surface temperature of inland standing waters globally was estimated as being 3.5 ± 2.3 km per decade from 1861 to 2005. From 2006 to 2099, this is projected to increase from 8.7 ± 5.5 km (representative concentration pathway, RCP2.6) to 57.0 ± 17.0 km (RCP8.5) per decade ( [[#Woolway--2020|Woolway and Maberly, 2020]] ). Although the dispersal of the aerial adult stage of some aquatic insects can surpass these climate velocities, rates of change under mid- and high-emission scenarios (RCP4.5, RCP6.0, RCP8.5) are substantially higher than the known rates of the active dispersal of many species ( [[#Woolway--2020|Woolway and Maberly, 2020]] ). Many species, both terrestrial and freshwater, are not expected to be able to disperse fast enough to track suitable climates under mid- and high-emission scenarios ( ''medium confidence'' ) (RCP4.5, RCP6.0, RCP8.5; ( [[#Brito-Morales--2018|Brito-Morales et al., 2018]] ).

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