2.2.6.1 Water security

Water security encompasses a wide set of issues, including water scarcity (which is affected by demand as well as supply), water quality, water hazards, access to water, and governance (Caretta et al., 2022). A key challenge for water is the difficulty in long-term planning for adaptation, due to large uncertainties in regional climate changes, particularly precipitation. The potential for tipping points may make this worse in some cases, if the existence of a potential tipping point adds an additional element of uncertainty in regional precipitation or evapotranspiration, or in the timing of global changes.  

AMOC collapse is simulated to change patterns of precipitation and water availability worldwide (Jackson et al., 2015), with reduced annual mean precipitation in Europe, northern South America, central Africa and southern Asia, and increased annual mean precipitation in southern North America, north-eastern South America, southern Africa and western Australia. Decreased precipitation could reduce water security by increasing the risk of water scarcity. Simulated rainfall reductions in the growing season in the British Isles would have very large negative impacts on crop yields (Ritchie et al., 2020).

Sea level rise as accelerated by ice sheet collapse can result in groundwater salinisation, having secondary impacts upon water and food security. Mean sea level rise affects the water table height, while coastal flooding events directly salinate freshwater (Magnan, 2022). Water salinisation impacts on coastal ecosystems, drinking water supply and also water supply for agriculture (Mazhar et al., 2022). Such changes could be compounded by drying patterns also projected under climate change.

Bangladesh is one nation with extreme vulnerability to sea level rise, where salinisation is posing a risk to both water and food security (Chen and Mueller, 2018; Barbour et al., 2022). Khanom (2016) reports that the intrusion of saline water occurs 15km inland, increasing up to 160km in the dry season, although other factors such as water abstraction and rainfall also impact saline water incursion (IPCC, 2019).

It is estimated that around 200,000 people are displaced annually in Bangladesh from the effects of salinisation on reducing agricultural productivity (Hauer et al., 2020), many moving to other regions of Bangladesh (Chen and Mueller, 2018). For example, certain crops are no longer produced due to intolerance of salinated soils, including oilseed, sugarcane and jute (Khanom, 2016). The same study indicates that rice cultivation is more appropriate under increasing salinity, and others suggest a move towards aquaculture production would increase resilience and reduce threats to food security (Hauer et al., 2020). However, it is unclear how sustainable these levels of adaptation are under extreme sea level rise.

Increasing salinisation of soils, surface water and groundwater aquifers, in part due to rising sea levels, reduces availability of freshwater resources (IPCC, 2019). The salinisation of groundwater due to sea level rise may result in the uninhabitability of atoll island nations in the coming decades, before inundation would force abandonment (Bailey et al., 2016). Impacts from rising sea levels in atoll nations are compounded by reduced precipitation, also associated with climate change impacts (Bailey et al., 2016, Hauer et al., 2020). Delta regions are also susceptible to vulnerability from salinisation of groundwater resources, including in Bangladesh. In the present day, traces of salt in drinking water in coastal regions of Bangladesh raise health concerns, for example having consequences on maternal health during pregnancy (Khan et al., 2011). Adaptation approaches, such as rainwater harvesting, are currently used in Bangladesh to provide safe drinking water (Rahman et al., 2017), and limits to the approaches are not well understood. The effectiveness of adaptation approaches under more rapid and extreme sea level rise, such as those associated with ice-sheet disintegration, are not well researched, but limitations likely apply.

Water quality in Arctic rivers and lakes is reduced by thawing permafrost releasing contaminants (Schäfer et al., 2020). Permafrost thawing also leads to damage to infrastructure including pipelines (Hjort et al., 2018; Hjort et al., 2022), which can reduce access to fresh water.

Amazon dieback is projected to lead to reduced river flows (Stickler et al., 2013; Lapola et al., 2018) which could potentially increase water scarcity.

A very rough indication of potential water-related impacts of some tipping points can be obtained by considering projected rates of increase in impacts with global warming and applying these to the estimate of the increase in global warming due to the group tipping points from Wang et al., (2023), as described in section 2.2.5. These tipping points were: release of CO₂ and/or CH₄ from permafrost thaw, marine methane hydrate destabilisation, Amazon forest dieback; and increased shortwave radiative forcing from Arctic sea ice loss. 

For example, Gosling and Arnell (2016) projected the global exposure to increased water scarcity to be over 1 billion people at 3°C global warming and 1,161m people at 4°C global warming, with very large uncertainties. Assuming a linear relationship between people exposed and the level of global warming, the Wang et al., (2023) estimate that 3°C global warming would be increased by 0.13°C (0.06-0.23°C) due to the above group of tipping points would imply an increase of 13.8m (6.4m to 24.4m) people exposed to increased water scarcity. 

Similarly, Alfieri et al., (2016) projected the population exposed to river flooding to be 97m and 211m at 2°C and 4°C global warming respectively, again with large uncertainties. Again assuming a linear relationship with warming, this suggests an exposure of 154m people to river flooding at 3°C global warming, increasing by 7.4m (3.4-13.1m) with the Wang et al., (2023) estimate of increased warming due to the collective effect of the above tipping points. 

As noted in section 2.2.5, these estimates do not account for dynamical tipping behaviour or interactions between tipping points, and do not include other tipping points such as rapid ice sheet loss, boreal forest loss or AMOC collapse, for which quantitative estimates could not be made. This is therefore not a comprehensive analysis of the effect of tipping points on water-related climate impacts. Rather, it illustrates their potential to have substantial impacts on water scarcity and flooding. Further research is required to provide a more comprehensive assessment.