Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.4.1.7 Freshwater Reservoirs ====

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===== 8.4.1.7.1 Glaciers =====

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Previous assessments have concluded that recent warming has led to a reduction in low-elevation snow cover ( ''high confidence'' ) (SROCC), permafrost ( ''high confidence'' ) (SROCC), and glacier mass ( ''high to very high confidence'' ) (AR5; SROCC). The SROCC noted that these declines are projected to continue almost everywhere over the 21st century ( ''high confidence'' ), with complete glacier loss expected in regions with only small glaciers ( ''very high confidence'' ). The SROCC supported the AR5 finding that glacier recession would continue even without further changes in climate. The SROCC concluded that cryosphere changes had already altered the seasonal timing and volume of runoff ( ''very high confidence'' ), which in turn had affected water resources and agriculture ( ''medium confidence'' ), and projected peak water runoff had already been reached before 2019 in some of the glacier regions considered.

( [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] provides detailed assessment of glacier observations and projections (Figures 9.20 and 9.21, and [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] ). Here, a summary of their key findings is presented. Since SROCC, the coordinated Glacier Model Intercomparison Project (GlacierMIP; Box 9.3; [[#Marzeion--2020|Marzeion et al., 2020]] ) has advanced modelling efforts. Global glacier volumes will substantially decline in coming decades regardless of emissions scenario; under a high-emissions scenario some areas will lose nearly all of their glacier mass ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1.3|Section 9.5.1.3]] ). The projected global glacier mass loss over 2015 – 2100 is 29,000 ± 20,000 Gt for SSP1-2.6 to 58,000 ± 30,000 Gt for SSP5-8.5 ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] ). Because of their lagged response to warming, glaciers will continue to lose mass for decades even if global temperature is stabilized ( ''very high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] ).

Global glacier mass loss projections show a scenario-dependent geographic partitioning of when peak in runoff occurs ( [[#Marzeion--2020|Marzeion et al., 2020]] ), consistent with previous studies ( [[#Radić--2014|Radić et al., 2014]] ; [[#Huss--2018|Huss and Hock, 2018]] ; [[#Hock--2019b|Hock et al., 2019b]] ). Under a low-emissions scenario ( [[#Marzeion--2020|Marzeion et al., 2020]] ) all regions exhibit runoff in the decades prior to 2050. Under a high-emissions scenario however, low- and mid-latitude regions show peak runoff before approximately 2060, whereas Arctic regions peak in later decades around 2070 – 2090. Antarctic glacier losses will not have peaked by the end of the century in the high-emissions scenario. Globally, peak runoff of 2.5 to 3 mm yr <sup>–1</sup> sea level equivalent occurs around 2090 ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Regional projections are presented in detail in [[IPCC:Wg1:Chapter:Chapter-9#9.5.1%20|Section 9.5.1]] and Figure 9.21, and briefly summarized below.

'''Himalaya and Central Asia:''' Glaciers in the Himalayas feed ten of the world’s most important river systems and are critical water sources for nearly two billion people ( [[#Wester--2019|Wester et al., 2019]] ). However, they are some of the most vulnerable ‘water towers’ ( [[#Immerzeel--2020|Immerzeel et al., 2020]] ) that are projected to experience volume losses of approximately 30 to 100% by 2100 depending on global emissions scenarios ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Under mid-range emissions scenarios glaciers in this region are projected to reach peak runoff during the period 2020 to 2040 ( [[#Marzeion--2020|Marzeion et al., 2020]] ).

'''Alaska, Yukon, British Columbia:''' Post-AR5 but pre-SROCC projections indicated a potential 70 ± 10% reduced volume of glacier ice in western Canada relative to 2005 (Clarke et al. , 2015) , with few glaciers remaining in the Interior and Rockies regions and maritime glaciers in north-western British Columbia surviving only in a diminished state. Recent global projections support these earlier findings, showing that glacier mass in western Canada and the USA may reduce by 50% under low-emissions scenarios and be completely lost under the highest emissions and most sensitive glacier model combinations (Figure 9.21; Marzeion et al. , 2020) . Arctic Canada and Alaskan glaciers are projected to experience more modest mass loss (0–60% depending on region, scenario, and model; Marzeion et al., 2020) .

'''Andes:''' [[#Huss--2018|Huss and Hock (2018)]] concluded that peak glacier mass was reached prior to 2019 for 82–95% of the glacier area in the tropical Andes. This is consistent with more recent global model simulations that show mass loss rates from low latitude glaciers that universally decline from the start of simulations in 2015, regardless of emissions scenario ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Peak runoff in low-latitude Andean glacier-fed rivers has therefore already passed ( [[#Frans--2015|Frans et al., 2015]] ; [[#Polk--2017|Polk et al., 2017]] ) but in the Southern Andes may occur in the latter half of the century under high-emissions scenarios ( [[#Marzeion--2020|Marzeion et al., 2020]] ).

In summary, glaciers are projected to continue to lose mass under all emissions scenarios ( ''very high confidence'' ). Runoff from glaciers is projected to peak at different times in different places, with maximum rates of glacier mass loss in low latitude regions taking place in the next few decades in all scenarios ( ''high confidence'' ). While runoff from small glaciers will typically decrease because of glacier mass depletion, runoff from larger glaciers will increase with increasing global warming until glacier mass is similarly depleted, after which runoff peaks and then declines and which tends to occurs later in basins with larger glaciers and higher ice-cover fractions ( ''high confidence'' ). Glaciers in the Arctic and Antarctic will continue to lose mass through the latter half of the century and beyond ( ''high co'' ''nfidence'' ).

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===== 8.4.1.7.2 Seasonal snow cover =====

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The AR5 assessed as ''very likely'' that the amount and seasonal duration of Northern Hemisphere (NH) snow cover will reduce under global warming (AR5 Sections 11.3.4.2 and 12.4.6.2). Changes in the total amount of water in the snow cover (snow water equivalent) are less certain because of the competing influences of temperature and precipitation.

As snow cover is assessed in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.3.3|Section 9.5.3.3]] ), only an overview of that assessment is provided here. Changes in seasonality of snow cover are assessed in Box 8.2. The continued consistency of reported results across all generations of model projections, along with improvements in process understanding, has increased confidence in snow cover projections since AR5.

In summary, based on the results of Chapter 9, it is now ''virtually certain'' that future NH snow cover extent and duration will continue to decrease with global warming. While most studies have focused on the NH, process understanding suggests with ''high confidence'' that these results apply to the Southern Hemisphere (SH) as well. There is ''high confidence'' in snowmelt occurring earlier in the year. Changes to the timing and amount of snowmelt will have a strong influence on all the other aspects of the water cycle in regions with seasonal snow, including run-off, soil moisture, and evapotranspiration.

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===== 8.4.1.7.3 Wetlands and lakes =====

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The AR5 did not include specific projections for wetlands and lakes. The SRCCL and SROCC provided some discussion of wetlands projections. For coastal wetlands, SRCCL noted the importance of sea level rise for increased saltwater intrusion, although projections of coastal wetland area with sea level rise are inconclusive. Some studies project substantial decreases ( [[#Spencer--2016|Spencer et al., 2016]] ) while others indicate possible increases ( [[#Schuerch--2018|Schuerch et al., 2018]] ). SRCCL also noted the general expectation for decreases in water resources, including wetlands, in areas of decreased rainfall due to increased evaporation.

Local studies of inland wetlands project decreases in a range of environments including mountain ( [[#Lee--2015|Lee et al., 2015]] ), mid- to high latitude (D. [[#Zhao--2018|]] [[#Zhao--2018|]] [[#Zhao--2018|Zhao et al., 2018]] ), and prairie (Sofaer et al. , 2016) regions. In addition to affecting wetland extent and density, changes in flooding can also affect the connectivity between wetlands and rivers (Karim et al. , 2016). Despite a number of uncertainties underlying the general response of wetlands to climate change, there are multiple ways climate change may cause considerable stress on both inland and coastal wetlands (Junk et al. , 2013; Moomaw et al. , 2018).

Widespread changes are also projected for lakes ( [[#Woolway--2020|Woolway et al., 2020]] ), including changes in lake temperature ( [[#Fang--1999|Fang and Stefan, 1999]] ; [[#Sahoo--2016|Sahoo et al., 2016]] ), ice ( [[#Sharma--2019|Sharma et al., 2019]] ), evaporation (W. [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ), and stability and mixing ( [[#Woolway--2019|Woolway and Merchant, 2019]] ). Note that lake ice is also considered in [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] of this Report. To date, CO <sub>2</sub> -induced lake acidification, analogous to ocean acidification, has not been the focus of many studies but may occur with continued emissions ( [[#Phillips--2015|Phillips et al., 2015]] ). While glacier lakes in general increase with melting glaciers ( [[#Linsbauer--2016|Linsbauer et al., 2016]] ; [[#Colonia--2017|Colonia et al., 2017]] ; [[#Magnin--2020|Magnin et al., 2020]] ) no clear projections are currently available (see discussion in Chapter 9). Projections of lake level means and variability show substantial changes for individual lakes ( [[#Bucak--2017|Bucak et al., 2017]] ; [[#Li--2021|Li et al., 2021]] ) but can be sensitive to methodology, due to the competing processes involved ( [[#Notaro--2015|Notaro et al., 2015]] ). Projected changes to wetlands and lakes due to climate change will occur in the context of widespread and continuing human-caused conversion and degradation of wetlands (e.g, [[#Davidson--2014|Davidson, 2014]] ), and where water withdrawals have a large impact on lake levels (e.g., [[#Micklin--2016|Micklin, 2016]] ).

In summary, there is ''medium confidence'' that inland wetland extent will decrease in regions of projected precipitation decrease and evaporation increase, and ''high confidence'' that sea level rise will increase saltwater intrusion into coastal wetlands. However, there is ''low agreement'' on the influence of sea level rise on the extent of coastal wetlands. Regarding lakes, there is ''high confidence'' for temperature increases and ice decreases, based on both projections and physical expectations, and ''low confidence'' for non-homogeneous decreases in mixing, given there is currently ''limited'' ''evidence'' .

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===== 8.4.1.7.4 Groundwater =====

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Groundwater projections were not assessed in AR5. Groundwater processes are not explicitly included in most current CMIP6 models and so must be calculated separately with hydrologic models (e.g., R.G. Taylor et al. , 2013; Cuthbert et al. , 2019a) . A range of factors are important in assessing groundwater projections, including the mean difference between precipitation and evaporation, the intensity of precipitation (R.G. [[#Taylor--2013|Taylor et al., 2013]] a), and in changes in snow ( [[#Tague--2009|Tague and Grant, 2009]] ), glaciers ( [[#Gremaud--2009|Gremaud et al., 2009]] ), and permafrost ( [[#Okkonen--2011|Okkonen and Kløve, 2011]] ). Climate impacts on groundwater are occurring in the context of severe and growing human-caused groundwater depletion (WGII; [[#Konikow--2005|Konikow and Kendy, 2005]] ; [[#Rodell--2018|Rodell et al., 2018]] ; [[#Bierkens--2019|Bierkens and Wada, 2019]] ), and water scarcity issues ( [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ). Climate-related changes to the water cycle can influence water demand (for example, precipitation decreases in an irrigated area), and anthropogenic groundwater depletion can influence the water cycle through interactions with surface energy fluxes, surface water, and vegetation ( [[#Cuthbert--2019a|Cuthbert et al., 2019a]] ), although uncertainties in estimates of future groundwater depletion are large ( [[#Smerdon--2017|Smerdon, 2017]] ; [[#Bierkens--2019|Bierkens and Wada, 2019]] ) . Some aspects of groundwater change will be irreversible, including the increase of saltwater intrusion into coastal aquifers with sea level rise ( [[#Werner--2009|Werner and Simmons, 2009]] ), and depletion of fossil aquifers and aquifers with very long recharge times ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ).

Globally, two modelling studies have shown substantial decreases in groundwater in regions including the Mediterranean, north-eastern Brazil and south-western Africa, with less clarity for other regions ( [[#Döll--2009|Döll, 2009]] ; Portmann et al. , 2013) . Recent regional-scale analyses of the impact of water cycle changes on groundwater recharge (e.g., Meixner et al. , 2016; Tillman et al. , 2017; Shrestha et al. , 2018 ) suggest changes in both seasonality and spatial distribution, which are amplified under a higher greenhouse-gas emissions scenario (i.e., RCP 8.5 compared to RCP4.5). Seasonality changes are linked to increases during wet winter periods and declines during dry summer periods. Changes in spatial distribution are linked with increases in more humid regions and declines in more arid locations. Uncertainty in projections of groundwater were found to be substantially influenced by the conceptual and numerical models employed to estimate groundwater recharge ( [[#Meixner--2016|Meixner et al., 2016]] ; [[#Hartmann--2017|Hartmann et al., 2017]] ). Accordingly, current research on estimating water cycles change on groundwater includes a focus on improving the numerical representation of groundwater systems ( [[#Bierkens--2015|Bierkens et al., 2015]] ; [[#Döll--2016|Döll et al., 2016]] ).

In summary, based on known limitations in current modelling, no confident assessment of groundwater projections is made here, although important climate-related changes in groundwater recharge are expected. In many environments, such climate-related impacts are expected to occur in the context of substantial human groundwater withdrawals depleting groundwater storage.

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