Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.5.5 Projected Risks to Freshwater Ecosystems ===

<div id="h2-31-siblings" class="h2-siblings"></div>

AR5 concluded that climate change is projected to be an important stressor on freshwater ecosystems in the second half of the 21st century, especially under high-warming scenarios of RCP6.0 and RCP8.5 ( ''high confidence'' ), even though direct human impacts will continue to be the dominant threat ( [[#Settele--2014|Settele et al., 2014]] ). Rising water temperatures are also projected to cause shifts in freshwater species distribution and worsen water quality problems ( ''high confidence'' ), especially in those systems that already experience high anthropogenic loading of nutrients ( [[#Settele--2014|Settele et al., 2014]] ).

Changes in precipitation and temperatures are projected to affect freshwater ecosystems and their species through, for example, direct physiological responses from higher temperatures or drier conditions or a loss of habitat for feeding or breeding ( [[#Settele--2014|Settele et al., 2014]] ; [[#Knouft--2017|Knouft and Ficklin, 2017]] ; [[#Blöschl--2019b|Blöschl et al., 2019b]] ). In addition, increased water temperatures could lead to shifts in the structure and composition of species assemblages following changes in metabolic rates, body size, timing of migration, recruitment, range size and destabilisation of food webs. A review of the impact of climate change on biodiversity and functioning of freshwater ecosystems found that under all scenarios, except the one with the lowest GHG emission scenario, freshwater biodiversity is expected to decrease proportionally to the degree of warming and precipitation alteration ( [[#Settele--2014|Settele et al., 2014]] ) '''(''' ''medium evidence'' , ''high agreement'' ''')''' .

These are several examples of such projected changes. Due to higher water temperatures, changes in macroinvertebrates and fish are projected under all future warming scenarios ( [[#Mantyka-Pringle--2014|Mantyka-Pringle et al., 2014]] ). Decreased abundance of many fish species, such as salmonids, under higher temperatures, is also projected, although the effects between species are variable ( [[#Myers--2017|Myers et al., 2017]] ). Poleward and shifts of freshwater species are projected as they try to stay within preferred cooler environmental conditions ( [[#Pecl--2017|Pecl et al., 2017]] ). Other anticipated changes include physiological adjustments with impacts on morphology with some species shrinking in body size because large surface-to-volume ratios are generally favoured under warmer conditions ( [[#Scheffers--2016|Scheffers et al., 2016]] ) and changes in species communities and food webs as a consequence of increases in metabolic rates in response to increased temperatures with the flow-on effects for many ecosystem processes ( [[#Woodward--2010|Woodward et al., 2010]] ). Changes in the seasonality of flow regimes and variability ( [[#Blöschl--2019b|Blöschl et al., 2019b]] ) and more intermittent flows ( [[#Pyne--2017|Pyne and Poff, 2017]] ) are also projected and could result in decreased food chain lengths through the loss of large-bodied top predators ( [[#Sabo--2010|Sabo et al., 2010]] ) and changes in nutrient loadings and water quality ( [[#Woodward--2010|Woodward et al., 2010]] ). The impacts on freshwater systems in drylands are projected to be more severe ( [[#Jaeger--2014|Jaeger et al., 2014]] ; [[#Gudmundsson--2016|Gudmundsson et al., 2016]] ). Changes to snow and glacier melting, including the complete melting of some glaciers ( [[#Leadley--2014|Leadley et al., 2014]] ; [[#Kraaijenbrink--2017|Kraaijenbrink et al., 2017]] ), are projected to reduce water availability and cause declines in biodiversity in high altitudes through local extirpations and species extinctions in regions of high endemism. Lake nutrient dynamics are expected to change, for example, at 2°C warming, and net increase in CH 4 emissions by 101–183% in hypereutrophic lakes and 47–56% in oligotrophic lakes in Europe are projected ( [[#Sepulveda-Jauregui--2018|Sepulveda-Jauregui et al., 2018]] ). Similarly, under the high-GHG emission scenario, lake stratification is projected to begin 22.0 ± 7.0 d earlier and end 11.3 ± 4.7 d later by the end of this century ( [[#Woolway--2021|Woolway et al., 2021]] ). While overall future trends on climate change on freshwater species and habitats are largely negative, evidence indicates that different species are projected to respond at different rates, with interactions between species expected to be disrupted and which may result in novel biological communities and rapid change in ecological processes and functions ( [[#Pecl--2017|Pecl et al., 2017]] ).

These impacts are expected to be most noticeable where significant air temperature increases are projected, leading to local or regional population extinctions for cold-water species because of range shrinking, especially under the RCP4.5, 6.0 and 8.5 scenarios ( [[#Comte--2017|Comte and Olden, 2017]] ). The consequences for freshwater species are projected to be severe with local extinctions as the freshwater ecosystems dry. In the Americas, under all scenarios that have been examined, the risk of extinction of freshwater species is projected to increase above that already occurring levels due to biodiversity loss caused by pollution, habitat modification, over-exploitation and invasive species ( [[#IPBES--2019|IPBES, 2019]] ). Freshwater ecosystems are also at risk of abrupt and irreversible change, especially those in the higher latitudes and altitudes with significant changes in species distributions, including those induced by melting permafrost systems ( [[#Moomaw--2018|Moomaw et al., 2018]] ; [[#IPBES--2019|IPBES, 2019]] ).

While changes in the species distribution across freshwater ecosystems are projected, the extent of change and the ability of individual species or populations to adapt are not widely known. Species that cannot move to more amenable habitats may become extinct, whereas those who migrate may relocate. An unknown outcome could be establishing novel ecosystems with new assemblages of species, including invasive alien species, in response to changes in the environment with the prospect of irreversible changes in freshwater ecosystems ( [[#Moomaw--2018|Moomaw et al., 2018]] ).

In summary, changes in precipitation and temperatures are projected to affect all types of freshwater ecosystems and their species. Under all scenarios, except the one with the lowest GHG emission scenario, freshwater biodiversity is expected to decrease proportionally to the degree of warming and precipitation change ( ''medium evidence, high agreement'' ).

<div id="4.5.6" class="h2-container"></div>

<span id="projected-risks-to-water-related-conflicts"></span>