Editing IPCC:AR6/WGIII/Chapter-17 (section)

==== 17.3.3.2 Water-Energy-Food Nexus ====

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This section addresses the links between water, energy and food in the context of sustainable development and the associated synergies and trade-offs, with links to related chapters. The focus outline includes scoping and the relationship with the SDGs, general climate change impacts on global water resources, energy-system impacts and the relationship to renewables, enabling strategies, trade-offs and cross-sectoral implications (see also Chapter 12), nexus-management tools and strategies, and a box with examples from India and South Africa.

The continually increasing pressures on natural resources, such as land and water, due to the rising demands from increases in population and living standards, which also require more energy, emphasises the need to integrate sustainable planning and exploitation ( [[#Bleischwitz--2018|Bleischwitz et al. 2018]] ).

The water-energy-food nexus (WEFN) is at the epicentre of these challenges, which are of global relevance and are the focus of policies and planning at all levels and sectors of global society. The nexus between water, energy and food (Zhang et al. 2018) is tight and complex, and needs careful attention and deciphering across spatio-temporal scales, sectors and interests to balance proper management and trade-offs and to pursue sustainable development ( [[#Biggs--2015|Biggs et al. 2015]] ; [[#Dai--2018|Dai et al. 2018]] ; [[#Hamiche--2016|Hamiche et al. 2016]] ). The WEFN touches upon the majority of the UN’s SDGs, such as SDG 2, SDG 6, SDG 7 and SDGs 11–15 ( [[#Bleischwitz--2018|Bleischwitz et al. 2018]] ), and deals with basic commodities, thus guaranteeing the basic livelihoods of the global population.

The task of gaining an improved understanding of WEFN processes across disciplines such as the natural sciences, economics, the social sciences and politics has been further exacerbated by climate change, population growth and resource depletion. In light of the system of interlinkages involved, the WEFN concept essentially also covers land ( [[#Ringler--2013|Ringler et al. 2013]] ) and climate ( [[#Brouwer--2018|Brouwer et al. 2018]] ; [[#Sušnik--2018|Sušnik et al. 2018]] ), and can be further assessed in light of the relevant economic, ecological, social and SDG aspects ( [[#Fan--2019a|Fan et al. 2019a]] ). The nexus approach was introduced in the early 2010s, when it was argued that advantages could be gained by adopting a nexus approach with regard to cross-sectoral and human–nature dependencies and by taking externalities into account ( [[#Hoffmann--2011|Hoffmann 2011]] ). Hence, within the nexus, obvious trade-offs exist with competing interests, such as water availability versus food production.

Climate change is projected to impact on the distribution, magnitude and variability of global water resources. A yearly increase in precipitation of 7% globally is expected by 2100 in a high-emissions scenario (RCP8.5), although with significant inter-model, inter-regional and inter-temporal differences ( [[#Giorgi--2019|Giorgi et al. 2019]] ). Similarly, extreme events related to the water balance, such as droughts and extreme precipitation, are projected to shift in the future (RCP4.5) towards 2100: for example, the number of consecutive dry days is projected to increase in the Mediterranean region, southern Africa, Australia and the Amazon ( [[#Chen--2014|Chen et al. 2014]] ). In impact terms, an increase of 20–30% in global water use is expected by 2050 due to the industrial and domestic demand for water. Already 4 billion people experience severe water scarcity for at least one month per year ( [[#WWAP-UNESCO--2019|WWAP-UNESCO 2019]] ).

Globally, climate change has been shown to cause increases of 4%, 8% and 10% in the share of population being exposed to water scarcities under the 1.5°C, 2°C and 3°C scenarios for global warming respectively (RCP8.5) ( [[#Koutroulis--2019|Koutroulis et al. 2019]] ). At the same time, climate change is projected to cause a general increase in extreme events and climate variability, placing a substantial burden on society and the economy ( [[#Hall--2014|Hall et al. 2014]] ). Other than the human influence on the global hydro-climate, human activities have been shown to surpass even the impact of climate change in low to moderate emission scenarios of the water balance ( [[#Haddeland--2014|Haddeland et al. 2014]] ). Similar conclusions have been found by ( [[#Destouni--2013|Destouni et al. 2013]] ; [[#Koutroulis--2019|Koutroulis et al. 2019]] ).

An obvious consequence of the impact of climate change on future hydro-climatic patterns is the fact that the energy system is projected to experience vast impacts through climate change ( [[#Fricko--2016|Fricko et al. 2016]] ; Van Vliet et al. 2016a; [[#van%20Vliet--2016b|van Vliet et al. 2016b]] ) (Chapter 6). In the short run, where fossil fuel sources make up a significant share of the global energy grid, climate impacts related to water availability and water temperatures will affect thermoelectric power generation, which relies mainly on water cooling ( [[#Larsen--2019|Larsen and Drews 2019]] ; [[#Pan--2018|Pan et al. 2018]] ); water is also used for pollution and dust control, cleaning, and so on ( [[#Larsen--2019|Larsen et al. 2019]] ). Currently, 98% of electricity generation relies on thermoelectric power (81%) and hydropower (17%) ( [[#van%20Vliet--2016a|van Vliet et al. 2016a]] ).

Of these thermoelectric sources, the vast majority employ substantial amounts of water for cooling purposes, although there is a trend currently towards implementing more hybrid or drier forms of cooling ( [[#Larsen--2019|Larsen et al. 2019]] ).

The renewable-energy conversion technologies that are currently dominant globally and are projected to remain so are less vulnerable to water deficiencies than fossil-based technologies, since no cooling is used. These renewable-energy conversion sources include, for example, wind, solar PV and wave energy. The implementation of such sources will, in the longer run, have the potential to reduce water usage by the energy sector substantially ( [[#Lohrmann--2019|Lohrmann et al. 2019]] ). Also, an increasing share of renewables within desalination, as well as improved irrigation efficiencies, have been shown to potentially improve the inter-sectorial WEFN water balance ( [[#Lohrmann--2019|Lohrmann et al. 2019]] ; [[#Caldera--2020|Caldera and Breyer 2020]] ). Some less dominant renewable-energy technologies do use water for cooling, such as geothermal energy and concentrating solar power (CSP), if wet cooling is employed. Despite the general detachment from water resources, wind and solar PV, for example, are highly dependent on climate change patterns, including variability depending on future energy-storage capacities and on-/off-grid solutions ( [[#Schlott--2018|Schlott et al. 2018]] ). Furthermore, regardless of whether or not they are based on renewables, climate change will affect energy usage across sectors, such as heating and cooling in the building stock. The energy systems in question need to be able to handle variations and extremes in demand ( [[#Larsen--2020|Larsen et al. 2020]] ).

For the 2080s compared to 1971–2000, an increase of 2.4% to 6.3% in the global gross hydropower potential, from the hydrological side alone, is seen across all scenarios ( [[#van%20Vliet--2016a|van Vliet et al. 2016a]] ) (Chapter 6). Alongside the global increase in hydropower potential, the global mean water-discharge cooling capacity, which also relates to water temperatures, experiences a decrease of 4.5% to 15% across the scenarios. In very general and global terms, when combined, these changes support the shift towards sources of renewable energy, including hydropower, in the energy mix. When it comes to ensuring stability in the management of the electricity grid, hydro-climatological extremes have the potential to pose vast difficulties in certain regions and/or seasons depending on the nature of the energy mix (Van Vliet et al. 2016c). Van Vliet et al. (2016b) showed significant reductions in both thermoelectric and hydropower electricity capacities, exemplified by the 2003 European drought, which resulted in reductions of 4.7% and 6.6%, respectively.

The energy sector is vulnerable to production losses caused mainly by heatwaves and droughts, whereas coastal and fluvial floods are also responsible for a large relative share of the energy sector’s vulnerability, as assessed by ( [[#Forzieri--2018|Forzieri et al. 2018]] ) for Europe in 2100. In total, heatwaves and droughts will be responsible for 94% of the damage costs to the European energy system compared to 40% today. Similarly, ( [[#Craig--2018|Craig et al. 2018]] ) show that, despite potentially minor spatio-temporally aggregated differences for various energy-system components, such as demand, thermoelectric power, wind, and so on, the aggregated impact of climate change across these components will cause a significant impact on the energy system, as currently exemplified by the USA. In terms of investments and management, it is important to unravel these cross-component relations in light of the projected nature of the future climate.

In the ongoing transition towards renewable sources of energy (see also Chapters 3, 4 and 6), the impact of the hydro-climate on energy production continues to be highly relevant ( [[#Jones--2016|Jones and Warner 2016]] ). As the shares of thermoelectric energy production in the energy grid go down along with the introduction of thermoelectric cooling technologies using smaller amounts of water, new energy sources and technologies are being introduced, and existing sources scaled up. Of these, hydropower, wind and solar energy are the key energy sources currently and will be in the near future, making up 2.5% and 1.8% of the total global primary energy supply in 2017 respectively ( [[#IEA--2019|IEA 2019]] ). Wind and solar energy are directly independent of water in themselves, but are dependent on atmospheric conditions related to processes that also drive the water balance and circulation. Hydropower, on the other hand, is directly influenced by and dependent on the supply of water, while at the same time being an essential counter-component to seasonality and climatological variation, as well as to current and future demand curves and diurnal variations, as against wind and solar energy ( [[#De%20Barbosa--2017|De Barbosa et al. 2017]] ).

Furthermore, policy instruments in power-system management, here exemplified by hydropower in a climate-change scenario, have been shown to enhance energy production during droughts ( [[#Gjorgiev--2018|Gjorgiev and Sansavini 2018]] ). The significant influence of variation in the planning of renewable energy for the 21st century has also been highlighted by ( [[#Bloomfield--2016|Bloomfield et al. 2016]] ). At the same time, the integration of renewables must account for lower thermoelectric efficiencies and capacities due to increases in temperature ( [[#van%20Vliet--2016a|van Vliet et al. 2016a]] ), power-plant closures during extreme weather events due to a lack of cooling capacity ( [[#Forzieri--2018|Forzieri et al. 2018]] ), and further efficiency reductions and penalties following the implementation of CCS technologies in the effort to reach the GHG mitigation targets ( [[#Byers--2015|Byers et al. 2015]] ). However, more recent studies find more promising amounts of water being used for energy conversion ( [[#IEAGHG--2020|IEAGHG 2020]] ; [[#Magneschi--2017|Magneschi et al. 2017]] ).

The extraction, distribution and wastewater processes of anthropogenic water-management systems similarly use vast amounts of energy, making the proper management of water essential to reduce energy usage and GHG emissions ( [[#Nair--2014|Nair et al. 2014]] )Chapter 11). One study reports that the water sector accounts for 5% of total US GHG emissions ( [[#Rothausen--2011|Rothausen and Conway 2011]] ). Within the WEFN, there is an obvious trade-off between water availability and food production, competing demands that pose a risk to the supply of the basic commodities of food, energy and water in line with the SDGs ( [[#Bleischwitz--2018|Bleischwitz et al. 2018]] ; [[#Gao--2019|Gao et al. 2019]] ), all of which have the potential for inter-sectorial or inter-regional conflicts ( [[#Froese--2019|Froese and Schilling 2019]] ). Currently, 24% of the global population live in regions with constant water-scarce food production, and 19% experience occasional water scarcities ( [[#Kummu--2014|Kummu et al. 2014]] ). To counterbalance the demand for food and comestibles in regions that experience constant or intermittent supplies, transportation is needed, which in itself requires suitable infrastructure, energy supplies, a well-functioning trading environment and support policies. Of the 2.6 billion people who experience constant or occasional water scarcities in food production, 55% rely on international trade, 21% on domestic trade and the remainder on water stocks ( [[#Kummu--2014|Kummu et al. 2014]] ).

The relations between the influence of hydro-climatic variability, socio-economic conditions and patterns of water scarcity have been addressed by ( [[#Veldkamp--2015|Veldkamp et al. 2015]] ). A key finding of this study was the ability of the hydro-climate and the socio-economy to interact, enforcing or attenuating each other, though with the former acting as the key immediate driver, and the influence of the latter emerging after six to ten years.

The trade-offs between competing demands have been investigated on a continental scale in the US Great Plains, highlighting the influence of irrigation in mitigating reductions in crop yields (Zhang et al. 2018). Despite crop-yield reductions of 50% in dry years compared to wet years, a key conclusion was that the irrigation should be counterbalanced against general water and energy savings within the context of trade-offs. In East Asia, the WEFN has been quantified, highlighting obvious trade-offs between economic growth, environmental issues and food security (White et al. 2018). This same study also highlights the concept of a virtual WEFN that includes water embodied within products that are traded and shipped. ( [[#Liu--2019|Liu et al. 2019]] ) find an urgent need for proper assessment methods, including of trade within the WEFN, due to the significant resource allocations.

Within the WEFN, the implementation of policies to achieve low stabilisation targets is strongly linked to sustainable development within the water sector with regard to water management and water conservation, indicating that additional coherence in policies affecting the water, energy and food sectors (among others) will be critical in achieving the SDGs (Chapter 7). Subsidised fertilisers, energy and crops can drive unsustainable levels of water usage and pollution in agriculture. More than half the world’s population, roughly 4.3 billion people in 2016, live in areas where the demand for water resources outstrips sustainable supplies for at least part of the year. Irrigated agriculture is already using around 70% of the available freshwater, and the large seasonal variations in water supply and the needs of different crops can create conflicts between water needs across sectors at different time scales ( [[#Wada--2016|Wada et al. 2016]] ). However, as there is little potential for increasing irrigation or expanding cropland ( [[#Steffen--2015|Steffen et al. 2015]] ), gaps in food production gaps must be closed by increasing productivity and cropping densities on currently harvested land by increasing either rain-fed yields or water-use efficiency ( [[#Alexandratos--2012|Alexandratos and Bruinsma 2012]] ).

It has been argued that applying an integrated approach to water-energy-climate-food resource management and policymaking is highly beneficial in properly addressing the co-benefits and trade-offs ( [[#Brouwer--2018|Brouwer et al. 2018]] ; [[#Howells--2013|Howells et al. 2013]] ), accommodating the SDGs ( [[#Rasul--2016|Rasul 2016]] ) and, in general, assessing enabling strategies to improve resource efficiency ( [[#Dai--2018|Dai et al. 2018]] ). For an integrated approach to analysing the WEFN, a number of modelling approaches, tools and frameworks have been proposed ( [[#Brouwer--2018|Brouwer et al. 2018]] ; [[#de%20Strasser--2016|de Strasser et al. 2016]] ; [[#Gao--2019|Gao et al. 2019]] ; [[#Larsen--2019|Larsen et al. 2019]] ; [[#Smajgl--2016|Smajgl et al. 2016]] ), often involving multi-objective calibration. Such tools enable decision-makers to evaluate the optimal water-allocation and energy-saving solutions for the specific geography in question. As an example, ( [[#Scott--2011|Scott 2011]] ) found the higher transportability of electricity, compared to water, pivotal in water-energy adaptation solutions in the USA, while arguing for the additional coordination of water and energy policies as a key instrument in balancing the trade-offs.

Common to all these integrated efforts is the challenge involved in making comparisons across studies due to the combined complexities of assumptions, model codes, regions, variables, forcings, and so on. To accommodate these challenges, ( [[#Larsen--2019|Larsen et al. 2019]] ) suggest employing shared criteria and forcing data to enable cross-model comparisons and uncertainty estimates, as also highlighted by ( [[#Brouwer--2018|Brouwer et al. 2018]] ). Other limitations in current WEFN research are partial system descriptions, the failure to address uncertainties, system boundaries, and evaluation methods and metrics (Zhang et al. 2018). The lack of proper access to WEFN data and data quality has been highlighted by ( [[#D’Odorico--2018|D’Odorico et al. 2018]] ; [[#Larsen--2019|Larsen et al. 2019]] ). Furthermore, gaps have been identified between theory and end-user applications in the lack of any focus on food nutritional values as opposed to calories alone, in the understanding of water availability in relation to management practices, in integrating new energy technologies and in the resulting environmental issues ( [[#D’Odorico--2018|D’Odorico et al. 2018]] ).

Therefore, looking ahead, future fields of WEFN research should provide greater insights into all these aspects. Holistic frameworks have been put forward to facilitate methods of WEFN management by focusing on, for example, the geographical complexities with regard to transboundary challenges within hydrological catchments ( [[#de%20Strasser--2016|de Strasser et al. 2016]] ), aligning policy incentives ( [[#Rasul--2016|Rasul 2016]] ) and making synergies and trade-offs in relation to WEFN SDG targets ( [[#Fader--2018|Fader et al. 2018]] ), and so on. The roles of all levels of government in optimal WEFN management are also highlighted in ( [[#Kurian--2017|Kurian 2017]] ), especially with regard to shaping the behaviour of individuals. Furthermore, ( [[#Kurian--2017|Kurian 2017]] ) highlights the challenges involved in science and policy communicating with one another and in the provision of optimal instruments and guidelines. Engaging non-experts and end-users in scientific processes is seen as essential to capturing previous failures and successes, and to ensure that understanding the challenges is updated to help shape the research questions.

Coordination of water use across different sectors and deltas is an important factor in sustainable water management. Examples of instruments and policies that support this from India and Sub-Saharan Africa in relation to the groundwater crisis are given below. India is the world’s largest user of groundwater for irrigation, which covers more than half of the country’s total irrigated agricultural area, is responsible for 70% of food production and supports more than 50% of the population (700 million people) (Chapter 7). However, excessive extraction of groundwater is depleting aquifers across the country, and falls in the water table have become pervasive. Improved water-use efficiency in irrigated agriculture is being considered, both globally and in India, as a way of meeting future food requirements with increasingly scarce water resources ( [[#Fishman--2015|Fishman et al. 2015]] ).

The entirety of Sub-Saharan Africa has an undeveloped potential for groundwater exploitation, despite the general perception of a global groundwater crisis, this being due to the absence of services to support groundwater development ( [[#Cobbing--2020|Cobbing 2020]] ). It is estimated that most Sub-Saharan countries in Africa utilise less than 5% of their national sustainable yields ( [[#Cobbing--2019|Cobbing and Hiller 2019]] ). The initial tool for driving sustainable groundwater exploitation is a change in the narrative of a lack of resources in order to stimulate increased agricultural production and increased fulfilment of the SDGs ( [[#Cobbing--2020|Cobbing 2020]] ). Quantitative measures of actual groundwater vulnerability based on multiple indicators have been calculated by, for example, ( [[#van%20Rooyen--2020|van Rooyen et al. 2020]] ), showing that 20.4% of South Africa’s current water resources are highly vulnerable and are projected to worsen fifty years into the future.

Despite the positive perspectives regarding Sub-Saharan groundwater resources, the 2015–2017 water crisis in South Africa, including in Cape Town, clearly predicts vulnerability to climate variability ( [[#Carvalho%20Resende--2019|Carvalho Resende et al. 2019]] ), which is predicted to increase. Serving as inspiration for the future mitigation of water depletion, ( [[#Olivier--2019|Olivier and Xu 2019]] ) suggest certain governance tools to improve the diversification of water sources and the management of existing supplies.

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