Editing IPCC:AR6/WGII/Chapter-15 (section)

==== 15.3.3.2 Impacts on Freshwater Systems ====

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Freshwater systems on small islands are exposed to dynamic climate impacts and are considered to be among the most threatened on the planet (key risk 3 in Box 15.1; [[#Settele--2014|Settele et al., 2014]] ; [[#IPCC--2018|IPCC, 2018]] ; [[#Butchart--2019|Butchart et al., 2019]] ). [[#Hoegh-Guldberg--2019|Hoegh-Guldberg et al. (2019)]] estimated that freshwater stress on small islands would be 25% less with a warming of 1.5°C or less as compared to 2.0°C. While some island regions are projected to experience substantial freshwater decline, an opposite trend is observed for some western Pacific and northern Indian Ocean islands ( [[#Holding--2016|Holding et al., 2016]] ; [[#Karnauskas--2016|Karnauskas et al., 2016]] ). Island topography and ecophysiology influence water storage capacity and rainfall response potential ( [[#Dunn--2018|Dunn et al., 2018]] ). On high volcanic and granitic islands, freshwater ecosystems are often closely connected with coastal spaces, and changes in freshwater supply from river systems have direct implications for salinity and sediment loads ( ''high confidence'' ) ( [[#Yang--2015|Yang et al., 2015]] ; [[#Zahid--2018|Zahid et al., 2018]] ). Climate impacts on streamflow patterns in tropical islands also create shifts in water supply for downstream users and habitat conditions for organisms supporting a wide range of ecosystem services ( ''high confidence'' ) ( [[#Strauch--2015|Strauch et al., 2015]] ; [[#Frazier--2019|Frazier and Brewington, 2019]] ; [[#Frauendorf--2020|Frauendorf et al., 2020]] ).

Projected changes in aridity are expected to impose freshwater stress on many small islands, especially SIDS ''(high confidence'' ).These changes are congruent with drought risk projections for Caribbean SIDS ( [[#Lehner--2017|Lehner et al., 2017]] ; [[#Taylor--2018|Taylor et al., 2018]] ) and aligned with observations from the Shared Socioeconomic Pathway (SSP) 2 scenario, where a 1°C increase in temperature (from 1.7°C to 2.7°C) could result in a 60% increase in the number of people projected to experience severe water resources stress from 2043 to 2071 ( [[#Schewe--2014|Schewe et al., 2014]] ; [[#Karnauskas--2018|Karnauskas et al., 2018]] ). In the Mediterranean region, freshwater resources will decline by 10–30% ''(medium confidence)'' ( [[#Koutroulis--2016|Koutroulis et al., 2016]] ; [[#Kumar--2020|Kumar et al., 2020]] ). For example, analysis of annual and seasonal streamflow data on the island of Mallorca shows a decreasing trend during spring and summer, with a reduction of up to 17% in some basins (Garcia, 2017).

The influence of climate change spans several variables for atoll islands with multiple, interacting forces that exacerbate impacts on freshwater ecosystems ( [[#Connell--2016|Connell, 2016]] ), including groundwater and freshwater resources ( [[#Warix--2017|Warix et al., 2017]] ). Analysis of groundwater resources on Roi-Namur, in the Marshall Islands, reveals that the extent of salinisation of fresh groundwater lenses varies with the scale of the overwash ( [[#Gingerich--2017|Gingerich et al., 2017]] ). [[#Alsumaiei--2018|Alsumaiei and Bailey (2018)]] estimated an 11–36% reduction in the fresh groundwater lens volume of the small atoll islands (area &lt; 0.6 km²) of the Maldives due to SLR. Small overwash events lead to saline conditions that last for up to 3 months ( [[#Oberle--2017|Oberle et al., 2017]] ).

SLR undermines the long-term persistence of freshwater-dependent ecosystems on islands ( [[#Goodman--2012|Goodman et al., 2012]] ) and is one of the greatest threats to the goods and services these environments provide (Box 16.1; [[#Mitsch--2013|Mitsch and Hernandez, 2013]] ). [[#Hoegh-Guldberg--2019|Hoegh-Guldberg et al. (2019)]] posit that as sea level rises, managing the risk of salinisation of freshwater resources will become increasingly important. On Roi-Namur, Marshall Islands, [[#Storlazzi--2018|Storlazzi et al. (2018)]] found that the availability of freshwater is impacted by the compounding effect of SLR and coastal flooding. In other Pacific atolls, [[#Terry--2012|Terry and Chui (2012)]] showed that freshwater resources could be significantly affected by a 0.40-m SLR. Similar impacts are anticipated for some Caribbean countries (Stennett- [[#Brown--2017|Brown et al., 2017]] ). Such changes in SLR could increase salinity in estuarine and aquifer water, affecting ground and surface water resources for drinking and irrigation water ( [[#Mycoo--2018a|Mycoo, 2018a]] ) across the region ( ''high confidence'' ). SLR also affects groundwater quality ( [[#Bailey--2016|Bailey et al., 2016]] ), salinity ( [[#Gingerich--2017|Gingerich et al., 2017]] ) and water-table height ( [[#Masterson--2014|Masterson et al., 2014]] ).

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