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

=== 3.4.2 Freshwater Resources (Quantity and Quality) ===

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<span id="water-availability"></span>
==== 3.4.2.1 Water availability ====

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Working Group II of AR5 concluded that about 80% of the world’s population already suffers from serious threats to its water security, as measured by indicators including water availability, water demand and pollution (Jiménez Cisneros et al., 2014) <sup>[[#fn:r374|374]]</sup> . UNESCO (2011) <sup>[[#fn:r375|375]]</sup> concluded that climate change can alter the availability of water and threaten water security.

Although physical changes in streamflow and continental runoff that are consistent with climate change have been identified (Section 3.3.5), water scarcity in the past is still less well understood because the scarcity assessment needs to take into account various factors, such as the operations of water supply infrastructure and human water use behaviour (Mehran et al., 2017) <sup>[[#fn:r376|376]]</sup> , as well as green water, water quality and environmental flow requirements (J. Liu et al., 2017) <sup>[[#fn:r377|377]]</sup> . Over the past century, substantial growth in populations, industrial and agricultural activities, and living standards have exacerbated water stress in many parts of the world, especially in semi-arid and arid regions such as California in the USA (AghaKouchak et al., 2015 <sup>[[#fn:r378|378]]</sup> ; Mehran et al., 2015) <sup>[[#fn:r379|379]]</sup> . Owing to changes in climate and water consumption behaviour, and particularly effects of the spatial distribution of population growth relative to water resources, the population under water scarcity increased from 0.24 billion (14% of the global population) in the 1900s to 3.8 billion (58%) in the 2000s. In that last period (2000s), 1.1 billion people (17% of the global population) who mostly live in South and East Asia, North Africa and the Middle East faced serious water shortage and high water stress (Kummu et al., 2016) <sup>[[#fn:r380|380]]</sup> .

Over the next few decades, and for increases in global mean temperature less than about 2°C, AR5 concluded that changes in population will generally have a greater effect on water resource availability than changes in climate. Climate change, however, will regionally exacerbate or offset the effects of population pressure (Jiménez Cisneros et al., 2014) <sup>[[#fn:r381|381]]</sup> .

The differences in projected changes to levels of runoff under 1.5°C and 2°C of global warming, particularly those that are regional, are described in Section 3.3.5. Constraining warming to 1.5°C instead of 2°C might mitigate the risks for water availability, although socio-economic drivers could affect water availability more than the risks posed by variation in warming levels, while the risks are not homogeneous among regions ( ''medium confidence'' ) (Gerten et al., 2013; Hanasaki et al., 2013; Arnell and Lloyd-Hughes, 2014; Schewe et al., 2014; Karnauskas et al., 2018) <sup>[[#fn:r382|382]]</sup> . Assuming a constant population in the models used in his study, Gerten et al. (2013) <sup>[[#fn:r383|383]]</sup> determined that an additional 8% of the world population in 2000 would be exposed to new or aggravated water scarcity at 2°C of global warming. This value was almost halved – with 50% greater reliability – when warming was constrained to 1.5°C. People inhabiting river basins, particularly in the Middle East and Near East, are projected to become newly exposed to chronic water scarcity even if global warming is constrained to less than 2°C. Many regions, especially those in Europe, Australia and southern Africa, appear to be affected at 1.5°C if the reduction in water availability is computed for non-water-scarce basins as well as for water-scarce regions. Out of a contemporary population of approximately 1.3 billion exposed to water scarcity, about 3% (North America) to 9% (Europe) are expected to be prone to aggravated scarcity at 2°C of global warming (Gerten et al., 2013) <sup>[[#fn:r384|384]]</sup> . Under the Shared Socio-Economic Pathway (SSP)2 population scenario, about 8% of the global population is projected to experience a severe reduction in water resources under warming of 1.7°C in 2021–2040, increasing to 14% of the population under 2.7°C in 2043–2071, based on the criteria of discharge reduction of either &gt;20% or &gt;1 standard deviation (Schewe et al., 2014) <sup>[[#fn:r385|385]]</sup> . Depending on the scenarios of SSP1–5, exposure to the increase in water scarcity in 2050 will be globally reduced by 184–270 million people at about 1.5°C of warming compared to the impacts at about 2°C. However, the variation between socio-economic levels is larger than the variation between warming levels (Arnell and Lloyd-Hughes, 2014) <sup>[[#fn:r386|386]]</sup> .

On many small islands (e.g., those constituting SIDS), freshwater stress is expected to occur as a result of projected aridity change. Constraining warming to 1.5°C, however, could avoid a substantial fraction of water stress compared to 2°C, especially across the Caribbean region, particularly on the island of Hispaniola (Dominican Republic and Haiti) (Karnauskas et al., 2018) <sup>[[#fn:r387|387]]</sup> . Hanasaki et al. (2013) <sup>[[#fn:r388|388]]</sup> concluded that the projected range of changes in global irrigation water withdrawal (relative to the baseline of 1971–2000), using human configuration fixing non-meteorological variables for the period around 2000, are 1.1–2.3% and 0.6–2.0% lower at 1.5°C and 2°C, respectively. In the same study, Hanasaki et al. (2013) <sup>[[#fn:r389|389]]</sup> highlighted the importance of water use scenarios in water scarcity assessments, but neither quantitative nor qualitative information regarding water use is available.

When the impacts on hydropower production at 1.5°C and 2°C are compared, it is found that mean gross potential increases in northern, eastern and western Europe, and decreases in southern Europe (Jacob et al., 2018; Tobin et al., 2018) <sup>[[#fn:r390|390]]</sup> . The Baltic and Scandinavian countries are projected to experience the most positive impacts on hydropower production. Greece, Spain and Portugal are expected to be the most negatively impacted countries, although the impacts could be reduced by limiting warming to 1.5°C (Tobin et al., 2018) <sup>[[#fn:r391|391]]</sup> . In Greece, Spain and Portugal, warming of 2°C is projected to decrease hydropower potential below 10%, while limiting global warming to 1.5°C would keep the reduction to 5% or less. There is, however, substantial uncertainty associated with these results due to a large spread between the climate models (Tobin et al., 2018) <sup>[[#fn:r392|392]]</sup> .

Due to a combination of higher water temperatures and reduced summer river flows, the usable capacity of thermoelectric power plants using river water for cooling is expected to reduce in all European countries (Jacob et al., 2018; Tobin et al., 2018) <sup>[[#fn:r393|393]]</sup> , with the magnitude of decreases being about 5% for 1.5°C and 10% for 2°C of global warming for most European countries (Tobin et al., 2018) <sup>[[#fn:r394|394]]</sup> . Greece, Spain and Bulgaria are projected to have the largest reduction at 2°C of warming (Tobin et al., 2018) <sup>[[#fn:r395|395]]</sup> .

Fricko et al. (2016) <sup>[[#fn:r396|396]]</sup> assessed the direct water use of the global energy sector across a broad range of energy system transformation pathways in order to identify the water impacts of a 2°C climate policy. This study revealed that there would be substantial divergence in water withdrawal for thermal power plant cooling under conditions in which the distribution of future cooling technology for energy generation is fixed, whereas adopting alternative cooling technologies and water resources would make the divergence considerably smaller.

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<span id="extreme-hydrological-events-floods-and-droughts"></span>
==== 3.4.2.2 Extreme hydrological events (floods and droughts) ====

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Working Group II of AR5 concluded that socio-economic losses from flooding since the mid-20th century have increased mainly because of greater exposure and vulnerability ( ''high confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r397|397]]</sup> . There was ''low confidence'' due to ''limited evidence'' , however, that anthropogenic climate change has affected the frequency and magnitude of floods. WGII AR5 also concluded that there is no evidence that surface water and groundwater drought frequency has changed over the last few decades, although impacts of drought have increased mostly owing to increased water demand (Jiménez Cisneros et al., 2014) <sup>[[#fn:r398|398]]</sup> .

Since AR5, the number of studies related to fluvial flooding and meteorological drought based on long-term observed data has been gradually increasing. There has also been progress since AR5 in identifying historical changes in streamflow and continental runoff (Section 3.3.5). As a result of population and economic growth, increased exposure of people and assets has caused more damage due to flooding. However, differences in flood risks among regions reflect the balance among the magnitude of the flood, the populations, their vulnerabilities, the value of assets affected by flooding, and the capacity to cope with flood risks, all of which depend on socio-economic development conditions, as well as topography and hydro-climatic conditions (Tanoue et al., 2016) <sup>[[#fn:r399|399]]</sup> . AR5 concluded that there was ''low confidence'' in the attribution of global changes in droughts (Bindoff et al., 2013b) <sup>[[#fn:r400|400]]</sup> . However, recent publications based on observational and modelling evidence assessed that human emissions have substantially increased the probability of drought years in the Mediterranean region (Section 3.3.4).

WGII AR5 assessed that global flood risk will increase in the future, partly owing to climate change ( ''low to medium confidence'' ), with projected changes in the frequency of droughts longer than 12 months being more uncertain because of their dependence on accumulated precipitation over long periods (Jiménez Cisneros et al., 2014) <sup>[[#fn:r401|401]]</sup> .

Increases in the risks associated with runoff at the global scale ( ''medium confidence'' ), and in flood hazard in some regions ( ''medium confidence'' ), can be expected at global warming of 1.5°C, with an overall increase in the area affected by flood hazard at 2°C ( ''medium confidence'' ) (Section 3.3.5). There are studies, however, that indicate that socio-economic conditions will exacerbate flood impacts more than global climate change, and that the magnitude of these impacts could be larger in some regions (Arnell and Lloyd-Hughes, 2014; Winsemius et al., 2016; Alfieri et al., 2017; Arnell et al., 2018; Kinoshita et al., 2018) <sup>[[#fn:r402|402]]</sup> . Assuming constant population sizes, countries representing 73% of the world population will experience increasing flood risk, with an average increase of 580% at 4°C compared to the impact simulated over the baseline period 1976–2005. This impact is projected to be reduced to a 100% increase at 1.5°C and a 170% increase at 2°C (Alfieri et al., 2017) <sup>[[#fn:r403|403]]</sup> . Alfieri et al. (2017) <sup>[[#fn:r404|404]]</sup> additionally concluded that the largest increases in flood risks would be found in the US, Asia, and Europe in general, while decreases would be found in only a few countries in eastern Europe and Africa. Overall, Alfieri et al., (2017) <sup>[[#fn:r405|405]]</sup> reported that the projected changes are not homogeneously distributed across the world land surface. Alfieri et al. (2018) <sup>[[#fn:r406|406]]</sup> studied the population affected by flood events using three case studies in European states, specifically central and western Europe, and found that the population affected could be limited to 86% at 1.5°C of warming compared to 93% at 2°C. Under the SSP2 population scenario, Arnell et al. (2018) <sup>[[#fn:r407|407]]</sup> found that 39% (range 36–46%) of impacts on populations exposed to river flooding globally could be avoided at 1.5°C compared to 2°C of warming.

Under scenarios SSP1–5, Arnell and Lloyd-Hughes (2014) <sup>[[#fn:r408|408]]</sup> found that the number of people exposed to increased flooding in 2050 under warming of about 1.5°C could be reduced by 26–34 million compared to the number exposed to increased flooding associated with 2°C of warming. Variation between socio-economic levels, however, is projected to be larger than variation between the two levels of global warming. Kinoshita et al. (2018) <sup>[[#fn:r409|409]]</sup> found that a serious increase in potential flood fatality (5.7%) is projected without any adaptation if global warming increases from 1.5°C to 2°C, whereas the projected increase in potential economic loss (0.9%) is relatively small. Nevertheless, their study indicates that socio-economic changes make a larger contribution to the potentially increased consequences of future floods, and about half of the increase in potential economic losses could be mitigated by autonomous adaptation.

There is limited information about the global and regional projected risks posed by droughts at 1.5°C and 2°C of global warming. However, hazards by droughts at 1.5°C could be reduced compared to the hazards at 2°C in some regions, in particular in the Mediterranean region and southern Africa (Section 3.3.4). Under constant socio-economic conditions, the population exposed to drought at 2°C of warming is projected to be larger than at 1.5°C ( ''low to medium confidence'' ) (Smirnov et al., 2016; Sun et al., 2017; Arnell et al., 2018; Liu et al., 2018) <sup>[[#fn:r410|410]]</sup> . Under the same scenario, the global mean monthly number of people expected to be exposed to extreme drought at 1.5°C in 2021–2040 is projected to be 114.3 million, compared to 190.4 million at 2°C in 2041–2060 (Smirnov et al., 2016) <sup>[[#fn:r411|411]]</sup> . Under the SSP2 population scenario, Arnell et al. (2018) <sup>[[#fn:r412|412]]</sup> projected that 39% (range 36–51%) of impacts on populations exposed to drought could be globally avoided at 1.5°C compared to 2°C warming.

Liu et al. (2018) <sup>[[#fn:r413|413]]</sup> studied the changes in population exposure to severe droughts in 27 regions around the globe for 1.5°C and 2°C of warming using the SSP1 population scenario compared to the baseline period of 1986–2005 based on the Palmer Drought Severity Index (PDSI). They concluded that the drought exposure of urban populations in most regions would be decreased at 1.5°C (350.2 ± 158.8 million people) compared to 2°C (410.7 ± 213.5 million people). Liu et al. (2018) <sup>[[#fn:r414|414]]</sup> also suggested that more urban populations would be exposed to severe droughts at 1.5ºC in central Europe, southern Europe, the Mediterranean, West Africa, East and West Asia, and Southeast Asia, and that number of affected people would increase further in these regions at 2°C. However, it should be noted that the PDSI is known to have limitations (IPCC SREX, Seneviratne et al., 2012) <sup>[[#fn:r415|415]]</sup> , and drought projections strongly depend on considered indices (Section 3.3.4); thus only ''medium confidence'' is assigned to these projections. In the Haihe River basin in China, a study has suggested that the proportion of the population exposed to droughts is projected to be reduced by 30.4% at 1.5°C but increased by 74.8% at 2°C relative to the baseline value of 339.65 million people in the 1986–2005 period, when assessing changes in droughts using the Standardized Precipitation-Evaporation Index, using a Penman–Monteith estimate of potential evaporation (Sun et al., 2017) <sup>[[#fn:r416|416]]</sup> .

Alfieri et al. (2018) <sup>[[#fn:r417|417]]</sup> estimated damage from flooding in Europe for the baseline period (1976–2005) at 5 billion euro of losses annually, with projections of relative changes in flood impacts that will rise with warming levels, from 116% at 1.5°C to 137% at 2°C.

Kinoshita et al. (2018) <sup>[[#fn:r418|418]]</sup> studied the increase of potential economic loss under SSP3 and projected that the smaller loss at 1.5°C compared to 2°C (0.9%) is marginal, regardless of whether the vulnerability is fixed at the current level or not. By analysing the differences in results with and without flood protection standards, Winsemius et al. (2016) <sup>[[#fn:r419|419]]</sup> showed that adaptation measures have the potential to greatly reduce present-day and future flood damage. They concluded that increases in flood-induced economic impacts (% gross domestic product, GDP) in African countries are mainly driven by climate change and that Africa’s growing assets would become increasingly exposed to floods. Hence, there is an increasing need for long-term and sustainable investments in adaptation in Africa.

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<span id="groundwater"></span>
==== 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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<span id="water-quality"></span>
==== 3.4.2.4 Water quality ====

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Working Group II of AR5 concluded that most observed changes to water quality from climate change are from isolated studies, mostly of rivers or lakes in high-income countries, using a small number of variables (Jiménez Cisneros et al., 2014) <sup>[[#fn:r427|427]]</sup> . AR5 assessed that climate change is projected to reduce raw water quality, posing risks to drinking water quality with conventional treatment ( ''medium to high confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r428|428]]</sup> .

Since AR5, studies have detected climate change impacts on several indices of water quality in lakes, watersheds and regions (e.g., Patiño et al., 2014; Aguilera et al., 2015; Watts et al., 2015; Marszelewski and Pius, 2016; Capo et al., 2017) <sup>[[#fn:r429|429]]</sup> . The number of studies utilising RCP scenarios at the regional or watershed scale have gradually increased since AR5 (e.g., Boehlert et al., 2015; Teshager et al., 2016; Marcinkowski et al., 2017) <sup>[[#fn:r430|430]]</sup> . Few studies, have explored projected impacts on water quality under 1.5°C versus 2°C of warming, however, the differences are unclear ( ''low confidence'' ) (Bonte and Zwolsman, 2010 <sup>[[#fn:r431|431]]</sup> ; Hosseini et al., 2017) <sup>[[#fn:r432|432]]</sup> . The daily probability of exceeding the chloride standard for drinking water taken from Lake IJsselmeer (Andijk, the Netherlands) is projected to increase by a factor of about five at 2°C relative to the present-day warming level of 1°C since 1990 (Bonte and Zwolsman, 2010) <sup>[[#fn:r433|433]]</sup> . Mean monthly dissolved oxygen concentrations and nutrient concentrations in the upper Qu’Appelle River (Canada) in 2050–2055 are projected to decrease less at about 1.5°C of warming (RCP2.6) compared to concentrations at about 2°C (RCP4.5) (Hosseini et al., 2017) <sup>[[#fn:r434|434]]</sup> . In three river basins in Southeast Asia (Sekong, Sesan and Srepok), about 2°C of warming (corresponding to a 1.05°C increase in the 2030s relative to the baseline period 1981–2008, RCP8.5), impacts posed by land-use change on water quality are projected to be greater than at 1.5°C (corresponding to a 0.89°C increase in the 2030s relative to the baseline period 1981–2008, RCP4.5) (Trang et al., 2017) <sup>[[#fn:r435|435]]</sup> . Under the same warming scenarios, Trang et al. (2017) <sup>[[#fn:r436|436]]</sup> projected changes in the annual nitrogen (N) and phosphorus (P) yields in the 2030s, as well as with combinations of two land-use change scenarios: (i) conversion of forest to grassland, and (ii) conversion of forest to agricultural land. The projected changes in N (P) yield are +7.3% (+5.1%) under a 1.5°C scenario and –6.6% (–3.6%) under 2°C, whereas changes under the combination of land-use scenarios are (i) +5.2% (+12.6%) at 1.5°C and +8.8% (+11.7%) at 2°C, and (ii) +7.5% (+14.9%) at 1.5°C and +3.7% (+8.8%) at 2°C (Trang et al., 2017) <sup>[[#fn:r437|437]]</sup> .

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<span id="soil-erosion-and-sediment-load"></span>
==== 3.4.2.5 Soil erosion and sediment load ====

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Working Group II of AR5 concluded that there is little or no observational evidence that soil erosion and sediment load have been altered significantly by climate change ( ''low to medium confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r438|438]]</sup> . As the number of studies on climate change impacts on soil erosion has increased where rainfall is an important driver (Lu et al., 2013) <sup>[[#fn:r439|439]]</sup> , studies have increasingly considered other factors, such as rainfall intensity (e.g., Shi and Wang, 2015; Li and Fang, 2016) <sup>[[#fn:r440|440]]</sup> , snow melt, and change in vegetation cover resulting from temperature rise (Potemkina and Potemkin, 2015) <sup>[[#fn:r441|441]]</sup> , as well as crop management practices (Mullan et al., 2012) <sup>[[#fn:r442|442]]</sup> . WGII AR5 concluded that increases in heavy rainfall and temperature are projected to change soil erosion and sediment yield, although the extent of these changes is highly uncertain and depends on rainfall seasonality, land cover, and soil management practices (Jiménez Cisneros et al., 2014) <sup>[[#fn:r443|443]]</sup> .

While the number of published studies of climate change impacts on soil erosion have increased globally since 2000 (Li and Fang, 2016) <sup>[[#fn:r444|444]]</sup> , few articles have addressed impacts at 1.5°C and 2°C of global warming. The existing studies have found few differences in projected risks posed on sediment load under 1.5°C and 2°C ( ''low confidence'' ) (Cousino et al., 2015; Shrestha et al., 2016) <sup>[[#fn:r445|445]]</sup> . The differences between average annual sediment load under 1.5°C and 2°C of warming are not clear, owing to complex interactions among climate change, land cover/surface and soil management (Cousino et al., 2015; Shrestha et al., 2016) <sup>[[#fn:r446|446]]</sup> . Averages of annual sediment loads are projected to be similar under 1.5°C and 2°C of warming, in particular in the Great Lakes region in the USA and in the Lower Mekong region in Southeast Asia (Cross-Chapter Box 6 in this chapter, Cousino et al., 2015; Shrestha et al., 2016) <sup>[[#fn:r447|447]]</sup> .

<span id="terrestrial-and-wetland-ecosystems"></span>