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

=== 4.3.5 Observed Impacts on Freshwater Ecosystems ===

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The loss and degradation of freshwater ecosystems have been widely documented, and SRCCL assessed with ''medium confidence'' the loss of wetlands since the 1970s (Olsson et al., 2020).

The links between air and water temperatures and ecological processes in freshwater ecosystems are well recognised. Increasing temperatures affect wetlands by influencing biophysical processes, affecting feeding and breeding habits and species’ distribution ranges, including their ability to compete with others. Increased temperatures can also cause deoxygenation in the lower depths of the water columns and throughout the entire water column if heating destabilises the water column. Under extreme heat, often associated with minimal rainfall or water flows, the drying of shallower areas and the migration or death of individual organisms can occur ( [[#Dell--2014|Dell et al., 2014]] ; [[#Miller--2014|Miller et al., 2014]] ; [[#Scheffers--2016|Scheffers et al., 2016]] ; [[#Szekeres--2016|Szekeres et al., 2016]] ; [[#Myers--2017|Myers et al., 2017]] ; [[#FAO--2018a|FAO, 2018a]] ) ( ''high confidence'' ). A global systematic review of studies since 2005 shows that climate change is a critical direct driver of freshwater ecosystems impacts through increasing temperatures or declining rainfall, for example, by causing physiological stress or death (thermal stress, dehydration or desiccation), limiting food supplies, or resulting in migration of animals to other feeding or breeding areas, and possibly increased competition with animals already present in those migrating locations {Diaz et al. 2019; Dziba et al. 2018} . Other drivers include land use changes, water pollution, extraction of water, drainage and conversion, and invasive species, which to varying extents interact synergistically with climate change or are exacerbated due to climate change ( [[#Finlayson--2017|Finlayson et al., 2017]] ; [[#Ramsar%20Convention--2018|Ramsar Convention, 2018]] ).

The Global Wetland Outlook ( [[#Ramsar%20Convention--2018|Ramsar Convention, 2018]] ) reported that between 1970 and 2015, the area of freshwater wetlands declined by approximately 35% ( [[#Davidson--2018|Davidson and Finlayson, 2018]] ), with high levels of the overall percentage of threatened species recorded in Madagascar and Indian Ocean islands (43%); in Europe (36%); in the tropical Andes (35%); and New Zealand (41%) ( [[#Ramsar%20Convention--2018|Ramsar Convention, 2018]] ). Where long-term data are available, only 13% of the wetlands recorded in and around the year 1700 remained by 2000. However, these data may overestimate the rate of loss ( [[#Davidson--2014|Davidson, 2014]] ) ( ''limited evidence, medium agreement'' ). Many wetland-dependent species have seen a long-term decline, with the Living Planet Index showing that 81% of populations of freshwater species are in decline and others being threatened by extinction ( [[#Davidson--2018|Davidson and Finlayson, 2018]] ; [[#Darrah--2019|Darrah et al., 2019]] ; [[#Diaz--2019|Diaz et al., 2019]] ) ( ''high confidence'' ).

Temperature changes lead to changes in the distribution patterns of freshwater species. Poleward and up-elevation range shifts due to warming temperatures tend to ultimately lead to reduced range sizes. Freshwater species in the tropics are particularly vulnerable ( [[#Jezkova--2016|Jezkova and Wiens, 2016]] ; [[#Sheldon--2019|Sheldon, 2019]] ). Systematic shifts towards higher elevation and upstream were found for 32 stream fish species in France ( [[#Comte--2013|Comte and Grenouillet, 2013]] ). In North America, for the bull trout ( ''Salvelinus confluentus'' ) a reduction in the number of occupied sites was documented in a watershed in Montana ( [[#Eby--2014|Eby et al., 2014]] ). Other impacts include disruption of seasonal movements of migratory waterbirds that regularly visit freshwater ecosystems, with adverse impacts on their feeding and breeding ( [[#Finlayson--2006|Finlayson et al., 2006]] ; [[#Bussière--2015|Bussière et al., 2015]] ). Keystone species, such as the beaver ( ''Caster Canadensis'' ) in North America, have been moving into new areas as the vegetation structure has changed in response to higher temperatures enabling shrubs to establish in the Arctic and alpine tundra ecosystems ( [[#Jung--2016|Jung et al., 2016]] ). Increased occurrence and intensity of algal blooms have occurred due to the interactive effects of thermal extremes and low dissolved oxygen concentrations in water ( [[#Griffith--2020|Griffith and Gobler, 2020]] ) ( [[#4.2.7|Section 4.2.7]] ). A global review found that almost 90% of all studies reviewed documented a decline in salmonid populations in North America and Europe, and identified knowledge gaps elsewhere ( [[#Myers--2017|Myers et al., 2017]] ). Another review ( [[#Pecl--2017|Pecl et al., 2017]] ) found declines in Atlantic salmon in Finland and poleward shift in coastal fish species, while another review ( [[#Scheffers--2016|Scheffers et al., 2016]] ) noted hybridisation between freshwater species like invasive rainbow trout ( ''Oncorhynchus mykiss'' ) and native cutthroat trout ( ''O. clarkia'' ).

Lakes have been warming, as shown by an increasing trend of summer surface water temperatures between 1985 and 2009 of 0.34°C per decade ( [[#O’Reilly--2015|O’Reilly et al., 2015]] ). However, responses of individual lakes to warming were very dependent on local characteristics ( [[#O’Reilly--2015|O’Reilly et al., 2015]] ), with warming enhancing the impacts of eutrophication in some instances ( [[#Sepulveda-Jauregui--2018|Sepulveda-Jauregui et al., 2018]] ). For example, temperature increases led to lower oxygen concentrations in eutrophic coastal wetlands due to phytoplankton and microbial respiration ( [[#Jenny--2016|Jenny et al., 2016]] ) and stimulated algal blooms ( [[#Michalak--2016|Michalak, 2016]] ) and affected the community structure of fish and other biotas ( [[#Mantyka-Pringle--2014|Mantyka-Pringle et al., 2014]] ; [[#Poesch--2016|Poesch et al., 2016]] ).

Rising temperatures have a strong impact in the arctic zone, where the southern limit of permafrost is moving north and leading to changes in the landscape ( [[#Arp--2016|Arp et al., 2016]] ; [[#Minayeva--2018|Minayeva et al., 2018]] ). Thawing of the permafrost leads to increased erosion and runoff and changes in the geomorphology and vegetation of arctic peatlands ( [[#Nilsson--2015|Nilsson et al., 2015]] ; [[#Sun--2018b|Sun et al., 2018b]] ). Permafrost thawing has led to the expansion of lakes in the Tibetan Plateau ( [[#Li--2014|Li et al., 2014]] ). As northern high-latitude peatlands store a large amount of carbon, permafrost thawing can increase methane and carbon dioxide emissions ( [[#Schuur--2015|Schuur et al., 2015]] ; [[#Moomaw--2018|Moomaw et al., 2018]] ). This represents a major gap in our understanding of the rates of change and their consequences for freshwater ecosystems.

The extent of past degradation due to multiple drivers is important, as climate change is expected to interact synergistically and cumulatively with these ( [[#Finlayson--2006|Finlayson et al., 2006]] ), exacerbate existing problems for wetland managers and potentially increase emissions from carbon-rich wetland soils ( [[#Finlayson--2017|Finlayson et al., 2017]] ; [[#Moomaw--2018|Moomaw et al., 2018]] ). Freshwater ecosystems are also under extreme pressure from changes in land use and water pollution, with climate change exacerbating these, such as the further decline of snow cover ( [[#DeBeer--2016|DeBeer et al., 2016]] ) and increased consumptive use of fresh water, and leading to the decline, and possibly extinction, of many freshwater-dependent populations ( ''high confidence'' ). Thus, differentiating between the impacts of multiple drivers is needed, especially given the synergistic and cumulative nature of such impacts, which remains a knowledge gap.

In summary, climate change is one of the key drivers of the loss and degradation of freshwater ecosystems and the unprecedented decline and extinction of many freshwater-dependent populations. The predominant key drivers are changes in land use and water pollution ( ''high confidence'' ).

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