Editing IPCC:AR6/WGII/Chapter-14 (section)

==== 14.5.3.2 Projected Impacts and Risks ====

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Climate change is projected to amplify current trends in water resource impacts, potentially reducing water supply security, impairing water quality and increasing flood hazards to varying degrees across North America ( ''high confidence'' ). Examples are presented in Table 14.3.

'''Table 14.3 |''' Selected projected water resource impacts in North America

{| class="wikitable"
|-
! Climate drivers and processes

! Examples of future risks and impacts

! Location (see Figure 14.1)

! References

|-
| Warming-induced reductions in mountain snow and glacial mass

| Projected decreases in annual and late-summer streamflow from high-elevation reaches of snow-fed rivers, affecting stream ecology and water supplies ( ''high confidence'' )

| US-NW, US-SW, CA-BC, CA-PR

| [[#Jost--2012|Jost et al. (2012)]] ; [[#Solander--2018|Solander et al. (2018)]] ; [[#Bonsal--2019|Bonsal et al. (2019)]] ; Milly and Dunne (2020)

|-
| Earlier seasonal snowmelt runoff

| Greater winter/early spring flooding risks and reduced summer surface water availability, intensifying seasonal mismatch with water demands ( ''high confidence'' );

increased challenges for balancing multi-purpose reservoir objectives (e.g., flood management, water supply, ecological protection and hydropower) ( ''high confidence'' )

| US-NW, US-SW, CA-BC, CA-PR

| Cohen et al. (2015); Dettinger et al. (2015); [[#Bonsal--2019|Bonsal et al. (2019)]] ; [[#Bonsal--2020|Bonsal et al. (2020)]] ; [[#RMJOC--2020|RMJOC (2020)]] ; [[#Bureau%20of%20Reclamation--2021d|Bureau of Reclamation (2021d)]]

|-
| Earlier seasonal snowmelt runoff

| Possible reductions in water supply security ( ''medium confidence'' ); reduced viability of some small-scale irrigation systems ( ''medium confidence'' )

| US-SW

| [[#Medellin-Azuara--2015|Medellin-Azuara et al. (2015)]] ; [[#Ullrich--2018|Ullrich et al. (2018)]] ; [[#Bai--2019|Bai et al. (2019)]] ; Milly and Dunne (2020); [[#Ray--2020|Ray et al. (2020)]] ; [[#Bureau%20of%20Reclamation--2021b|Bureau of Reclamation (2021b)]] ; [[#Bureau%20of%20Reclamation--2021a|Bureau of Reclamation (2021a)]] ; [[#Bureau%20of%20Reclamation--2021c|Bureau of Reclamation (2021c)]]

|-
| Changes in seasonal timing and/or total annual runoff

| Impacts on electric power generation ( ''medium confidence'' ) varying by location and type of generation

| US-SW, US-NW, CA-QC

| [[#Haguma--2014|Haguma et al. (2014)]] ; [[#Bartos--2015|Bartos and Chester (2015)]] ; Guay et al. (2015); [[#Turner--2019b|Turner et al. (2019b)]] ; [[#RMJOC--2020|RMJOC (2020)]] ; [[#Bureau%20of%20Reclamation--2021d|Bureau of Reclamation (2021d)]]

|-
| Changes in seasonal timing and/or total annual runoff

| Impacts on urban water supplies

| CA-QC

| [[#Foulon--2019|Foulon and Rousseau (2019)]]

|-
| Warming-related increased imbalance between renewable surface water supplies and consumptive water demands

| Greater pressures on groundwater resources, possible increased aquifer depletion, reduced baseflow into surface streams and reduced long-term water supply sustainability ( ''medium confidence'' )

| US-SW, US-SP, US-SE, MX-N, MX-NW

| [[#Bauer--2015|Bauer et al. (2015)]] ; [[#Molina-Navarro--2016|Molina-Navarro et al. (2016)]] ; [[#Russo--2017|Russo and Lall (2017)]] ; Brown et al. (2019b); [[#Nielsen-Gammon--2020|Nielsen-Gammon et al. (2020)]] ; [[#Bureau%20of%20Reclamation--2021b|Bureau of Reclamation (2021b)]]

|-
| Warming-related drought amplification

| Reduced water availability for human uses and ecological functioning ( ''medium'' to ''high confidence'' ) varying by location; increased evaporative losses from reservoirs

| Widespread especially:

US-SW, US-NP, US-SP, CA-PR, MX-NW, MX-N

| [[#Prein--2016|Prein et al. (2016)]] ; [[#Dibike--2017|Dibike et al. (2017)]] ; [[#Lall--2018|Lall et al. (2018)]] ; [[#Paredes-Tavares--2018|Paredes-Tavares et al. (2018)]] ; Martinez-Austria et al. (2019); [[#Tam--2019|Tam et al. (2019)]] ; [[#Martin--2020b|Martin et al. (2020b)]] ; Milly and Dunne (2020); [[#Overpeck--2020|Overpeck and Udall (2020)]] ; [[#Williams--2020|Williams et al. (2020)]] ; [[#Bureau%20of%20Reclamation--2021b|Bureau of Reclamation (2021b)]]

|-
| Heavier and/or prolonged rainfall events

| Flooding, infrastructure and property damage ( ''medium'' to ''high confidence'' ) varying by location; increased erosion and debris flows with impacts on public safety, reservoir sedimentation and stream ecology (hazards amplified in watersheds affected by wildfires)

| Widespread especially:

US-SE, US-NE, US-NP, US-SP, US-SW, CA-BC, MX-CE, MX-NE, MX-SE

| [[#Feng--2016|Feng et al. (2016)]] ; [[#Emanuel--2017|Emanuel (2017)]] ; [[#Prein--2017a|Prein et al. (2017a)]] ; [[#Prein--2017b|Prein et al. (2017b)]] ; [[#Haer--2018|Haer et al. (2018)]] ; [[#Kossin--2018|Kossin (2018)]] ; Mahoney et al. (2018); [[#Thistlethwaite--2018|Thistlethwaite et al. (2018)]] ; [[#Curry--2019|Curry et al. (2019)]] ; [[#Larrauri--2019|Larrauri and Lall (2019)]] ; [[#Wobus--2019|Wobus et al. (2019)]] ; Ball et al. (2021)

|-
| Heavier and/or prolonged rainfall events

| Water quality impairment, increasing HAB events due to increased sediment and nutrient loading together with warming; greatest impacts in humid areas with extensive agriculture ( ''medium to high confidence'' ) varying by location

| US-MW, US-NE, US-SE, US-NP, US-SP, CA-ON, CA-AT, MX-NE, MX-NW

| [[#Alam--2017|Alam et al. (2017)]] ; [[#Chapra--2017|Chapra et al. (2017)]] ; Sinha et al. (2017); Ballard et al. (2019)

|-
| Increasingly variable precipitation

| Highly variable precipitation poses challenges for water management, worsening water supply and flooding risks; atmospheric river events are projected to increase variability by dominating future North American west coast precipitation ( ''medium confidence'' )

| US-SW, US-NW, CA-BC

| [[#Gershunov--2019|Gershunov et al. (2019)]] ; Huang et al. (2020)

|-
| Hotter summer season

| Evaporative losses from reservoirs are projected to increase significantly ( ''very high confidence'' )

| US-SW, US-NW, US-NP

| [[#Bureau%20of%20Reclamation--2021b|Bureau of Reclamation (2021b)]]

|}

Projected long-term reduction in water availability in the southwest US and northern Mexico (e.g., from the Colorado and Rio Grande rivers) will have substantial ecological and economic impacts given the region’s heavy water demands ( ''high confidence'' ) ( [[#Lall--2018|Lall et al., 2018]] ; [[#Paredes-Tavares--2018|Paredes-Tavares et al., 2018]] ; [[#Martinez-Austria--2019|Martinez-Austria et al., 2019]] ; [[#Milly--2020|Milly and Dunne, 2020]] ; [[#Williams--2020|Williams et al., 2020]] ). Increased water scarcity will intensify the need to address competing interests across state and national boundaries, including honouring commitments to Indigenous Peoples who have long struggled with inadequate access to their water entitlements and marginalisation in water resource planning ( [[#Mumme--1999|Mumme, 1999]] ; [[#Cozzetto--2013b|Cozzetto et al., 2013b]] ; [[#Mumme--2016|Mumme, 2016]] ; [[#McNeeley--2017|McNeeley, 2017]] ; [[#Radonic--2017|Radonic, 2017]] ; [[#Robison--2018|Robison et al., 2018]] ; [[#Curley--2019|Curley, 2019]] ; Water and Tribes Initiative, 2020; [[#Wilder--2020|Wilder et al., 2020]] ).

Increased scarcity of renewable water relative to legally allocated or desired uses may develop in many parts of North America. A detailed analysis of projected water demands (consumptive uses) and availability found increasingly frequent shortages in several watersheds across the USA ( [[#Brown--2019b|Brown et al., 2019b]] ). This might lead to maladaptive increased groundwater mining, or alternatively to policies promoting sustainable balancing of water consumption with renewable supplies, for example, by facilitating voluntary water transfers or improving enforcement of groundwater rights ( [[#Colorado%20River%20Basin%20Stakeholders--2015|Colorado River Basin Stakeholders, 2015]] ; California Natural Resources Agency et al., 2020; [[#Colorado%20Water%20Conservation%20Board--2020|Colorado Water Conservation Board, 2020]] ; [[#Pauloo--2020|Pauloo et al., 2020]] ).

Climate change is projected to reduce groundwater recharge in major southwest US aquifers (e.g., Southern High Plains, San Pedro and Wasatch Front), exacerbating their ongoing depletion due to unsustainable pumping. Other aquifers, especially those farther north, face uncertain or possibly increasing recharge ( ''medium confidence'' ) ( [[#Meixner--2016|Meixner et al., 2016]] ).

Projected changes in temperature and precipitation present direct risks to North American water quality, varying with regional and watershed contexts ( [[#Chapra--2017|Chapra et al., 2017]] ; [[#Coffey--2019|Coffey et al., 2019]] ; [[#Paul--2019a|Paul et al., 2019a]] ), and related to streamflow, population growth ( [[#Duran-Encalada--2017|Duran-Encalada et al., 2017]] ) and land-use practices ( ''medium confidence'' ) ( [[#Mehdi--2015|Mehdi et al., 2015]] ). Harmful algal blooms increase in frequency across the USA ( [[#Wells--2015|Wells et al., 2015]] ) with the highest risk projected for the Great Plains and Northeast USA, and greatest economic impacts from lost recreation value in the southeast USA ( [[#Chapra--2017|Chapra et al., 2017]] ).

The diversity of climate regimes across North America results in regional differences in water-related climate-change risks (Figure 14.4).

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