Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.4.2.3 Groundwater ====

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Working Group II of AR5 concluded that the detection of changes in groundwater systems, and attribution of those changes to climatic changes, are rare, owing to a lack of appropriate observation wells and an overall small number of studies (Jiménez Cisneros et al., 2014) <sup>[[#fn:r420|420]]</sup> .

Since AR5, the number of studies based on long-term observed data continues to be limited. The groundwater-fed lakes in northeastern central Europe have been affected by climate and land-use changes, and they showed a predominantly negative lake-level trend in 1999–2008 (Kaiser et al., 2014) <sup>[[#fn:r421|421]]</sup> .

WGII AR5 concluded that climate change is projected to reduce groundwater resources significantly in most dry subtropical regions ( ''high confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r422|422]]</sup> .

In some regions, groundwater is often intensively used to supplement the excess demand, often leading to groundwater depletion. Climate change adds further pressure on water resources and exaggerates human water demands by increasing temperatures over agricultural lands (Wada et al., 2017) <sup>[[#fn:r423|423]]</sup> . Very few studies have projected the risks of groundwater depletion under 1.5°C and 2°C of global warming. Under 2°C of warming, impacts posed on groundwater are projected to be greater than at 1.5°C ( ''low confidence'' ) (Portmann et al., 2013; Salem et al., 2017) <sup>[[#fn:r424|424]]</sup> .

Portmann et al. (2013) <sup>[[#fn:r425|425]]</sup> indicated that 2% (range 1.1–2.6%) of the global land area is projected to suffer from an extreme decrease in renewable groundwater resources of more than 70% at 2°C, with a clear mitigation at 1.5°C. These authors also projected that 20% of the global land surface would be affected by a groundwater reduction of more than 10% at 1.5°C of warming, with the percentage of land impacted increasing at 2°C. In a groundwater-dependent irrigated region in northwest Bangladesh, the average groundwater level during the major irrigation period (January–April) is projected to decrease in accordance with temperature rise (Salem et al., 2017) <sup>[[#fn:r426|426]]</sup> .

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