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

=== 4.4.6 Projected Changes in Groundwater ===

<div id="h2-24-siblings" class="h2-siblings"></div>

AR5 concluded that the range of projected future changes in groundwater storage was large, from statistically significant declines to increases due to several uncertainties in existing models ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). AR6 ( [[#Douville--2021|Douville et al., 2021]] ) concluded with ''high confidence'' that projected increases in precipitation alone cannot ensure an increase in groundwater storage under a warming climate unless unsustainable trends in groundwater extraction are also reversed.

Projected impacts of climate change on groundwater systems are commonly simulated using models at local to global scales ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ). The relations between climate change and groundwater are more complex than those embedded in current numerical models ( [[#Cuthbert--2019b|Cuthbert et al., 2019b]] ). For instance, groundwater systems register effects of drought with several years of lag effect, and aquifer response times to changes in hydraulic forcing also vary across aquifers ( [[#Cuthbert--2019a|Cuthbert et al., 2019a]] ). For instance, long groundwater response times can buffer drought impacts and lengthen recovery times to sustained drought events ( [[#Van%20Lanen--2013|Van Lanen et al., 2013]] ; [[#Opie--2020|Opie et al., 2020]] ).

Global total and non-renewable groundwater withdrawals are projected to increase from 952 km 3 year –1 (2010) to 1621 km 3 year –1 (2099) and from 304 km 3 year –1 (2010) to 597 km 3 year –1 (2099), respectively ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ). At the same time, groundwater depletion is projected to increase from approximately 204 (± 30) km 3 year –1 in 2000 to 427 (± 56) km 3 year –1 by 2099 ( [[#Wada--2016|Wada, 2016]] ). Much of the projected depletion is a function of increased future abstraction of groundwater for irrigation and increased ET ( [[#Condon--2020|Condon et al., 2020]] ) in a warmer climate. For example, the projected doubling of average water use by 2050 in Tunisia is attributed partly (3.8–16.4%) to climate change and mainly to socioeconomic policies ( [[#Guermazi--2019|Guermazi et al., 2019]] ). Similarly, groundwater depletion in the Bengal Basin and North China Plain is more due to irrigation development than climate change per se ( [[#Leng--2015|Leng et al., 2015]] ; [[#Kirby--2016|Kirby et al., 2016]] ).

A recent synthesis of modelling studies conducted in various climates showed that out of 33 studies, 21 reported a decrease in the projected groundwater recharge or storage, eight reported an increase and the rest showed no substantial change ( [[#Amanambu--2020|Amanambu et al., 2020]] ). A global-scale multi-model ensemble study projected decreasing recharge in southern Chile, Brazil, central continental USA, the Mediterranean and East China, but consistent and increasing recharge for northern Europe and East Africa ( [[#Reinecke--2021|Reinecke et al., 2021]] ). In continental Spain, a modelling study ( [[#Pulido-Velazquez--2018|Pulido-Velazquez et al., 2018]] ) projected significant reductions in groundwater recharge in the central and southeast region but a small and localised increase in east and northeastern areas. In subarctic Alaska, increased contribution of glacier melts to streamflow and aquifer recharge under a warming climate is projected ( [[#Liljedahl--2017|Liljedahl et al., 2017]] ). In contrast, over the Iranian and Anatolia Plateaus, groundwater recharge is projected to reduce by ~77% in the spring season (March–May) due to a decrease in snowfall ( [[#Wu--2020|Wu et al., 2020]] ). Overall, several recent studies of climate change impacts on groundwater in different parts of the world have concluded that projected groundwater recharge could either increase or decrease, and results are often uncertain ( ''high confidence'' ) ( [[#Meixner--2016|Meixner et al., 2016]] ; [[#Zaveri--2016|Zaveri et al., 2016]] ; [[#Hartmann--2017|Hartmann et al., 2017]] ; [[#Mehran--2017|Mehran et al., 2017]] ; [[#Tillman--2017|Tillman et al., 2017]] ; [[#Kahsay--2018|Kahsay et al., 2018]] ; [[#Herbert--2019|Herbert and Döll, 2019]] ).

[[#Wu--2020|Wu et al. (2020)]] reported a projected increase in future groundwater storage in the semiarid regions of northwest India, North China Plain, the Guarani Aquifer in South America and Canning Basin in Australia due to significant increases in projected precipitation, but the models do not consider local hydrogeological characteristics. However, the projected irrigation expansion could negate this positive gain in groundwater storage ( [[#Sishodia--2018|Sishodia et al., 2018]] ; [[#Wu--2020|Wu et al., 2020]] ). In drylands (e.g., playas in the southwestern USA), where focused groundwater recharge processes dominate, greater recharge is projected to occur from the increased number of significant runoff-generating extreme precipitation events in the future ( [[#McKenna--2018|McKenna and Sala, 2018]] ). Overall, an emerging body of studies have projected amplification of episodic recharge in the tropics and semiarid regions due to extreme precipitation under global warming ( ''medium confidence'' ).

Climate change is also projected to impact groundwater-dependent ecosystems and groundwater quality negatively ( ''medium confidence'' ). Projected increase in precipitation intensity and storms can contaminate groundwater by mobilising contaminants such as chemical fertilisers, pesticides and antibiotics and leaching of human waste from pit latrines into groundwater ( [[#Amanambu--2020|Amanambu et al., 2020]] ; [[#Lall--2020|Lall et al., 2020]] ). By 2050, environmentally critical streamflow is projected to be affected in 42–79% of the world’s watersheds. The majority of these watersheds currently experience intensive groundwater use, and changes in critical streamflow are projected to negatively impact aquatic ecosystems ( [[#de%20Graaf--2019|de Graaf et al., 2019]] ). Using a global synthesis of 9404 data points from 32 countries across six continents, [[#McDonough--2020|McDonough et al. (2020)]] report increases in DOC concentrations in groundwater following projected changes in precipitation and temperature. For example, hotspots of high DOC concentration (increases of up to 45%) are associated mainly with increased temperatures in the wettest quarter of the year in the southeastern USA under RCP8.5 scenarios.

The projected rise in sea levels can lead to saline intrusion into aquifers in low-lying areas and small islands and threaten coastal ecosystems and livelihood resilience; for example, in already vulnerable countries like Bangladesh and vulnerable ecosystems like the mangrove forest of Sundarbans ( [[#Befus--2020|Befus et al., 2020]] ; [[#Dasgupta--2020|Dasgupta et al., 2020]] ; [[#Shamsudduha--2020|Shamsudduha et al., 2020]] ). However, hydrogeological properties, aquifer settings and impacts of over-abstraction are more important determinants of salinisation of coastal aquifers than slowly rising sea levels ( [[#Michael--2013|Michael et al., 2013]] ; [[#Taylor--2013a|Taylor et al., 2013a]] ). The projected contribution of global groundwater depletion to sea level rise is expected to increase from 0.57 (± 0.09) mm year –1 in 2000 to 0.82 (± 0.13) mm year –1 by 2050, driven by a growing trend in groundwater extraction ( [[#Wada--2016|Wada, 2016]] ). However, several uncertainties around model parametrisation remain ( [[#Wada--2017|Wada et al., 2017]] ).

There are several knowledge gaps in our understanding of the global-scale sensitivity of groundwater systems to climate change and resulting feedbacks ( [[#Maxwell--2016|Maxwell and Condon, 2016]] ; [[#Cuthbert--2019a|Cuthbert et al., 2019a]] ). There are process uncertainties in groundwater recharge simulation due to the potential impact of atmospheric CO 2 on vegetation and resulting changes in ET ( [[#Reinecke--2021|Reinecke et al., 2021]] ). There are uncertainties in impact models due to poor representation of recharge pathways (diffuse compared to focused) and inability to adequately capture feedbacks among climate, land use and groundwater systems ( [[#Meixner--2016|Meixner et al., 2016]] ). Finally, there are gaps in long-term observational data, especially in less-developed countries ( [[#Amanambu--2020|Amanambu et al., 2020]] ), making it challenging to evaluate the performance of impact models ( [[#Gleeson--2020|Gleeson et al., 2020]] ).

In summary, groundwater abstraction is projected to deplete the long-term, non-renewable storage as withdrawals are projected to increase significantly in all major aquifers worldwide ( ''medium evidence, high agreement'' ). In the tropics and semiarid regions, growing precipitation intensification under global warming may enhance the resilience of groundwater through increased episodic recharge ( ''medium confidence'' ). However, in the semiarid areas, over-abstraction continues to be a threat to groundwater storage and can nullify the benefits of increased future recharge.

<div id="4.4.7" class="h2-container"></div>

<span id="projected-changes-in-water-quality"></span>