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

==== 2.3.3.5 Extreme Weather Events and Freshwater Systems ====

<div id="h3-5-siblings" class="h3-siblings"></div>

Since AR5, numerous drastic short-term responses have been observed in lakes and rivers, to both expected seasonal extreme events and unexpected supra-seasonal extremes extending over multiple seasons. Consequences for ecosystem functioning are not well understood ( [[#Bogan--2015|Bogan et al., 2015]] ; [[#Death--2015|Death et al., 2015]] ; [[#Stockwell--2020|Stockwell et al., 2020]] ) ''.'' Increasing frequencies of severe floods and droughts attributed to climate change are major threats for river ecosystems ( [[#Peters--2016|Peters et al., 2016]] ; [[#Alfieri--2017|Alfieri et al., 2017]] ). While extreme floods cause massive physical disturbance, moderate floods can have positive effects, providing woody debris that contributes to habitat complexity and diversity, flushing fine sediments, dissolving organic carbon and providing important food sources from terrestrial origins ( [[#Peters--2016|Peters et al., 2016]] ; [[#Talbot--2018|Talbot et al., 2018]] ). Droughts reduce river habitat diversity and connectivity, threatening aquatic species, especially in deserts and arid regions ( [[#Bogan--2015|Bogan et al., 2015]] ; [[#Death--2015|Death et al., 2015]] ; [[#Ledger--2015|Ledger and Milner, 2015]] ; [[#Jaric--2019|Jaric et al., 2019]] ).

Rivers already under stress from human activities such as urban development and farming on floodplains are prone to reduced resilience to future extreme events ( ''medium confidence'' ) ( [[#Woodward--2016|Woodward et al., 2016]] ; [[#Talbot--2018|Talbot et al., 2018]] ). Thus, the potential for floods to become catastrophic for ecosystem services is exacerbated by LULCC ( [[#Peters--2016|Peters et al., 2016]] ; [[#Talbot--2018|Talbot et al., 2018]] ). However, biota can recover rapidly from extreme flood events if river geomorphology is not greatly altered. If instream habitat is strongly affected, recovery, if it occurs, takes much longer, resulting in a decline of biodiversity ( ''medium confidence'' ) ( [[#Thorp--2010|Thorp et al., 2010]] ; [[#Death--2015|Death et al., 2015]] ; [[#Poff--2018|Poff et al., 2018]] ).

However, not all extreme events will have a biological impact, depending, in particular, on the timing, magnitude and frequency of events and the antecedent conditions ( [[#Bailey--2016|Bailey and van de Pol, 2016]] ; [[#Stockwell--2020|Stockwell et al., 2020]] ; [[#Jennings--2021|Jennings et al., 2021]] ; [[#Thayne--2021|Thayne et al., 2021]] ). For instance, an extreme wind event may have little impact on phytoplankton in a lake that was fully mixed prior to the event. Conversely, the effects of a storm on phytoplankton communities may compound when lakes have not yet recovered from a previous storm or if periods of drought alternate with periods of intense precipitation ( ''limited evidence'' ) ( [[#Leonard--2014|Leonard et al., 2014]] ; [[#Stockwell--2020|Stockwell et al., 2020]] ).

In summary, extreme events (heat waves, storms and loss of ice) affect lakes in terms of water temperature, water level, light, oxygen concentrations and nutrient dynamics, which, in turn, affect primary production, fish communities and GHG emissions ( ''high confidence'' ). These impacts are modified by levels of solar radiation, wind speed and precipitation ( [[#Woolway--2020a|Woolway et al., 2020a]] ). Droughts have a negative impact on water quality in streams and lakes by increasing water temperature, salinity, the frequency of algal blooms and contaminant concentrations, and reducing concentrations of nutrients and dissolved oxygen ( ''medium confidence'' ) ( [[#Peters--2016|Peters et al., 2016]] ; [[#Alfieri--2017|Alfieri et al., 2017]] ; [[#Woolway--2020a|Woolway et al., 2020a]] ). Understanding how these pressures subsequently cascade through freshwater ecosystems will be essential for future projections of their resistance and resilience towards extreme events ( [[#Leonard--2014|Leonard et al., 2014]] ; [[#Stockwell--2020|Stockwell et al., 2020]] ). See Table SM2.1 for specific examples of observed changes.

<div id="2.3.3.6 " class="h3-container"></div>

<span id="projected-changes-in-physical-characteristics-of-lakes-and-rivers"></span>