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

=== 4.7.6 Trade-Offs and Synergies between Water-Related Adaptation and Mitigation ===

<div id="h2-49-siblings" class="h2-siblings"></div>

In AR5, there was ''medium evidence'' and ''high agreement'' that some adaptation and mitigation measures can lead to maladaptive outcomes, such as a rise in GHG emissions, while further exacerbating water scarcity leading to increased vulnerability to climate change, now or in the future ( [[#Noble--2014|Noble et al., 2014]] ). In addition, SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#IPCC--2018a|IPCC, 2018a]] ) and SRCCL ( [[#IPCC--2019b|IPCC, 2019b]] ) reiterated the challenge of trade-offs that may undermine sustainable development. Conversely, adaptation, when framed and implemented appropriately, can synergistically reduce emissions and enhance sustainable development.

Different mitigation pathways can either increase or decrease water withdrawals or water consumption (or both, or either) depending on the specific combination of mitigation technologies deployed ( ''high confidence'' ) ( [[#Fricko--2016|Fricko et al., 2016]] ; [[#Jakob--2016|Jakob and Steckel, 2016]] ; [[#Mouratiadou--2016|Mouratiadou et al., 2016]] ; [[#Fujimori--2017|Fujimori et al., 2017]] ; [[#Parkinson--2019|Parkinson et al., 2019]] ). For example, the impacts of climate change mitigation on future global water demand depend largely on assumptions regarding socioeconomic and water policy conditions and range from reduction of 15,000 km 3 to an increase of more than 160,000 km 3 by the end of century ( [[#Mouratiadou--2016|Mouratiadou et al., 2016]] ). This section assesses some of the mitigation and adaptation measures from a water trade-off and synergy lens.

Solar pumps for irrigation are increasingly introduced where conventional energy is not available ( [[#Senthil%20Kumar--2020|Senthil Kumar et al., 2020]] ) or supply is intermittent or expensive ( [[#Shah--2018|Shah et al., 2018]] ), for example, in Africa ( [[#Schmitter--2018|Schmitter et al., 2018]] ), Europe ( [[#Rubio-Aliaga--2016|Rubio-Aliaga et al., 2016]] ) and South Asia ( [[#Sarkar--2017|Sarkar and Ghosh, 2017]] ). Solar pumps can replace diesel and electric pumps ( [[#Rajan--2020|Rajan et al., 2020]] ), potentially reduce 8–11% of India’s carbon emissions (~45.3–62.3 MMT of CO 2 ) attributable to groundwater pumping while also boosting agricultural productivity ( [[#Gupta--2019|Gupta, 2019]] ). However, in the absence of incentives to deter groundwater over-exploitation ( [[#Shah--2018|Shah et al., 2018]] ), solar pumps may exacerbate groundwater depletion ( [[#Closas--2017|Closas and Rap, 2017]] ; [[#Gupta--2019|Gupta, 2019]] ) ( ''low evidence, medium agreement'' ).

In many places, treatment and reuse of wastewater from urban residential and industrial sources may be the principal supply option under acute water scarcity ( [[#US%20EPA--2017|US EPA, 2017]] ) and help reduce other freshwater withdrawals ( [[#Tram%20Vo--2014|Tram Vo et al., 2014]] ; [[#Diaz-Elsayed--2019|Diaz-Elsayed et al., 2019]] ). While reuse may recover valuable nutrients, capture energy as methane, and save water, effluent containing heavy metals may degrade land and surface and groundwater quality and pose a salinisation risk in semiarid regions ( ''medium evidence, high agreement'' ). Agricultural reuse of poor-quality wastewater will become increasingly necessary, but treatment is energy-intensive and may contribute to further GHG emissions ( [[#Qadir--2014|Qadir et al., 2014]] ; [[#Salgot--2018|Salgot and Folch, 2018]] ) (Box 4.5).

Desalination of seawater or brackish water is an adaptation measure in many coastal water-scarce regions ( [[#Hanasaki--2016|Hanasaki et al., 2016]] ; [[#Jones--2019|Jones et al., 2019]] ). Solar desalination is developing rapidly, and it lessens the carbon footprint of conventional, fossil-fuel-powered desalinisation plants ( [[#Pouyfaucon--2018|Pouyfaucon and García-Rodríguez, 2018]] ) (also see Box 4.5). However, the desalinisation process is energy-intensive ( [[#Caldera--2018|Caldera et al., 2018]] ); it ejects brine that is difficult to manage inland, has high salinity and other contaminants ( [[#Wilder--2016|Wilder et al., 2016]] ) ( ''medium evidence, high agreement'' ) (Box 4.5).

Negative-emission technologies, such as direct air capture (DAC) of CO 2 , could reduce emissions up to 3 GtCO 2 /year by 2035, equivalent to 7% of 2019 global emissions. However, they can increase net water consumption by 35 km 3 yr –1 in 2050 ( [[#Fuhrman--2020|Fuhrman et al., 2020]] ) under the low-overshoot emissions scenario. According to other estimates, capturing 10 Gt CO 2 could translate to water losses of 10–100 km 3 , depending on the technology deployed and climatic conditions (temperate vs. tropical) (Chapter 12, WGIII). Some DAC technologies that include solid sorbents also produce water as a by-product, but not in quantities that can offset total water losses ( [[#Beuttler--2019|Beuttler et al., 2019]] ; [[#Fasihi--2019|Fasihi et al., 2019]] ) ( ''medium confidence'' ).

Developing countries are projected to witness the highest increase in future energy demand under 2°C global warming leading to significant increases in water use for energy production ( [[#Fricko--2016|Fricko et al., 2016]] ) ( [[#4.5.2|Section 4.5.2]] ). Results from a simulation study on retrofitting coal-fired power plants built after 2000 with carbon capture and storage (CCS) technologies show an increase in global water consumption, currently at 9.66 km 3 yr –1 , by 31–50% (to 12.66 km 3 yr –1 and 14.47 km 3 yr –1 , respectively) depending on the cooling and CCS technology deployed, and hence are best deployed in locations which are not water scarce ( [[#Rosa--2020c|Rosa et al., 2020c]] ) ( ''medium confidence'' ). In Asia, the near-term mitigation scenario with high CCS deployment increases the average regional water withdrawal intensity of coal generation by 50–80% compared to current withdrawals ( [[#Wang--2019b|Wang et al., 2019b]] ). Carbon can be ‘scrubbed’ from thermoelectric power plant emissions and injected for storage in deep geological strata ( [[#Turner--2018|Turner et al., 2018]] ), but this can lead to pollution of deep aquifers ( [[#Chen--2021|Chen et al., 2021]] ) and have health consequences ( ''low confidence'' ).

Bio-energy crop with carbon capture and storage (BECCS) involves CO 2 sequestration as biofuel or forest bioenergy ( [[#Creutzig--2015|Creutzig et al., 2015]] ). BECCS has profound implications for water resources ( [[#Ai--2020|Ai et al., 2020]] ), depending on factors including the scale of deployment, land use, and other local conditions. Evaporative losses from biomass irrigation and thermal bioelectricity generation are projected to peak at 183 km 3 yr –1 in 2050 under a low overshoot scenario ( [[#Fuhrman--2020|Fuhrman et al., 2020]] ). ( [[#Senthil%20Kumar--2020|Senthil Kumar et al., 2020]] ) projected that while BECCS strategies like irrigating biomass plantations can limit global warming by the end of the 21st century to 1.5°C, this will double the global area and population living under severe water stress compared to the current baseline. Both BECCS ( [[#Muratori--2016|Muratori et al., 2016]] ) and DAC can significantly impact food prices via demand for land and water ( [[#Fuhrman--2020|Fuhrman et al., 2020]] ). The direction and magnitude of price movement will depend on future carbon prices, while vulnerable people in the Global South will be most severely affected ( ''medium evidence, high agreement'' ).

Afforestation and reforestation are considered one of the most cost-effective ways of storing carbon. An additional 0.9 billion ha of canopy cover in suitable locations could store 205 Gt of carbon ( [[#Bastin--2019|Bastin et al., 2019]] ), but this estimate is deemed unrealistic. Aggressive afforestation and reforestation efforts can result in trade-offs between biodiversity, carbon sequestration, and water use ( [[#Smith--2008|Smith et al., 2008]] ). In northern China, ecological restoration by regreening drylands resulted in several environmental and social benefits ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ) but also led to increased freshwater use in some pockets ( [[#Zhao--2020|]] [[#Zhao--2020|Zhao et al., 2020]] ). Afforestation and reforestation with appropriate broad-leaf species in temperate Europe ( [[#Schwaab--2020|Schwaab et al., 2020]] ) can offer water quality and quantity-related benefits, mitigate extreme heat, and buffer against drought ( [[#Staal--2018|Staal et al., 2018]] ). A global assessment on forest and water showed that forests influence the overall water cycle, including downstream water availability via rainfall-runoff dynamics and downwind water availability via recycled rainfall effects ( [[#Creed--2018|Creed and van Noordwijk, 2018]] ). The study concluded that afforestation and reforestation should be concentrated ( [[#Ellison--2017|Ellison et al., 2017]] ) in water-abundant locations (to offset downstream impacts) and where transpiration can potentially be captured downwind as precipitation ( [[#Creed--2019|Creed et al., 2019]] ) (Cross-Chapter Box NATURAL in Chapter 2). Overall, extensive BECCS and afforestation/reforestation deployment can alter the water cycle at regional scales ( ''high confidence'' ) (Cross-Chapter Box 5.1 in Chapter 5, WGI, ( [[#Canadell--2021|Canadell et al., 2021]] )).

On the other hand, demand-side mitigation options, such as dietary changes to more plant-based diets, reduced food waste ( [[#Aleksandrowicz--2016|Aleksandrowicz et al., 2016]] ; [[#Springmann--2018|Springmann et al., 2018]] ; [[#Kim--2020|Kim et al., 2020]] ), can reduce water use ( ''medium evidence, high agreement'' ).

In summary, many adaptation and mitigation measures have synergistic or maladaptive consequences for water use, depending on associated incentives, policies, and governance that guide their deployment. Many mitigation measures have a considerable water footprint ( ''high confidence'' ), which must be managed in socially and politically acceptable ways to reduce the water intensity of mitigation while increasing synergies with sustainable development ( ''medium evidence, high agreement'' ).

<div id="4.8" class="h1-container"></div>

<span id="enabling-principles-for-achieving-water-security-sustainable-and-climate-resilient-development-through-systems-transformations"></span>