V IRGILIO M AGO

The vulnerability of the hydrological cycle to changes in climate has been widely acknowledged (Vorosmarty et al., 2005; Gosling and Arnell, 2016). At the global scale, climate change is expected to reduce the volume of both renewable surface and groundwater resources (Kundzewicz et al., 2008, Jiménez Cisneros et al., 2014). Fung et al. (2011) showed that beyond 2°C of temperature rise, elevated water stress is projected, as climate becomes the major limiting factor in water availability. Jiménez Cisneros et al. (2014) determined that the projected impacts of climate change on freshwater resources increase considerably with higher greenhouse gas concentrations and temperature rises. The different elements of the hydrological cycle are discussed in this section.

2.2.1.1 Precipitation

Global trends in precipitation are not as readily apparent as patterns of temperature change, partly due to regional variations masking global signals (Rowell, 2012). Precipitation changes are more spatially and temporally variable than temperature (Kundzewicz and Doll, 2009). However, Zhang et al. (2007) compared model results and observations for the 20th Century and concluded that climate change is already driving changes in precipitation. In areas such as

southern Africa and Australia, both model projections and observational data show increases in precipitation. By contrast, northern Africa and Southeast Asia show

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decreases in precipitation. Large reductions in the amount of winter precipitation falling as snow in mountainous and high-latitude regions are projected as global temperatures increase (Barnett et al., 2005).

Alterations in the distribution of precipitation between high and low frequency events will also prove extremely important (Allen and Ingram, 2002). Overall, the global hydrological cycle is projected to intensify (Fung et al., 2011; Arnell and Gosling, 2013). Precipitation is projected to be more concentrated in heavy rainfall events, while a reduction in moderate precipitation events are likely to be

observed. Hegerl et al. (2004) compared two different models to show that

increases in precipitation on the wettest day are greater than the increases in the mean precipitation change. Higher intensity rainfall may increase erosion and the occurrence of natural disasters, such as landslides and floods (Nearing et al., 2004).

2.2.1.2 Glaciers

As global temperatures increase, glacial ice loss will continue. Glaciers are extremely sensitive to changes in climate and changes are already being observed. Reductions in glacier area have been observed in all areas in recent years (Vaughan et al., 2013; Gardner et al., 2013), along with the disappearance of glaciers in some regions. Knoll and Kerschner (2009) found losses from glaciers in Italy’s South Tyrol had accelerated since 1983, but that the exact changes varied greatly amongst the individual glaciers. Huss and Fischer (2016) found that small glaciers in the Swiss Alps are particularly sensitive to changes in climate. Their results projected that over half of small glaciers in Switzerland will disappear in the next 25 years. Continued loss of glacial ice is projected to lead to a shift in seasonal flow in many glacial catchments (Jiménez Cisneros et al., 2014). Peak discharges are projected to occur in spring, whereas reductions in summer discharges are likely (Sorg et al., 2012).

2.2.1.3 Runoff, River Flows and Water Stress

Projected changes in precipitation will lead to changes to runoff, river flows and water scarcity across the world. A comparison of 12 global climate models (GCMs) by Milly et al. (2005) showed that there are regional variations in runoff projections. While eastern Africa and Eurasia are likely to experience increases in runoff, of 10-40%, areas such as mid-latitude North America, the Middle East and southern Africa could see decreases in runoff of up to 30%. This shows that future changes

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in precipitation and runoff will be highly spatially variable and that changes in some regions may not be projected well by current models. Schewe et al. (2014) found a similar regional pattern of projected changes in runoff and river flow. However, they also noted the large spread of projections between different climate and hydrological models in some areas of the world such as northern Africa.

As the number of intense rainfall events increases, the likelihood of flooding also increases (Githui et al., 2009). Betts et al. (2018) found that, with both 1.5°C and 2°C of warming, flooding events across the world increase in length in all models. By contrast, the reduction in moderate precipitation events could lead to increased water stress in countries with dry seasons. Gosling and Arnell (2016) found that more people are likely to experience higher water stress as a result of climate change than a reduction in water stress. Paltsev et al. (2016) found that the largest relative changes in water stress occur in Africa. They found that globally, at least 1 billion additional people are projected to experience at least moderately stressed water conditions worldwide by the end of the century.

There is already evidence of earlier spring snowmelt occurring in alpine regions (Laternser and Schneebeli, 2003). The projected precipitation shift from snowfall to rain may severely alter the winter flood regimes of mountain catchments, reducing the chance of snowmelt floods but increasing the possibility of very high river winter flows, or even flash floods. Berghuijs et al. (2014) found that shifts from snow to rainfall could lead to reductions in streamflow across catchments in the United States.

2.2.1.4 Groundwater

Potential impacts on groundwater recharge have not been investigated to the same extent as impacts on surface water resources (Kundzewicz and Doll, 2009). Groundwater is often more protected from seasonal variations and pollution than surface waters, making it an important resource in less developed countries. Although groundwater is already a vital resource for many countries, its

importance is likely to increase in the future, as surface water quantity and quality alters. Modelling results suggest that some areas of the world, including parts of China and the USA, are projected to experience increases in groundwater by 2050, whereas other areas, such as the Mediterranean and southwestern Africa, are projected to see decreases (Kundzewicz and Doll, 2009). Despite uncertainty in the magnitude of groundwater changes, model results have clearly shown that

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sizeable alterations to available groundwater resources will be observed. Portmann et al. (2013) also investigated the impacts of climate change on renewable global groundwater resources, using five GCMs in the hydrological model ‘WaterGAP’. Despite some variation between models, the results suggested that South America and the Mediterranean are likely to experience decreases in groundwater recharge, whereas western regions of North America could see increases in groundwater.

2.2.1.5 Water Quality

Rising temperatures will affect the rate of chemical and biological processes within aquatic systems (Jiménez Cisneros et al., 2014). Furthermore, lakes and slow- flowing freshwater bodies may experience algal blooms as a result of stagnant water; which will be of particular concern for areas that are projected to experience a decrease in precipitation (Whitehead et al., 2009). Algal blooms can block light and reduce dissolved oxygen concentrations, negatively impacting aquatic life. Increased volumes of suspended solids in the water column, occurring as a result of projected higher runoff volumes, would reduce the quality of the river water (Grayson et al., 1997). Fine sediment may smother the substrate, depriving benthic organisms of light and oxygen. Projected changes to water quality will impact drinking water (Jiménez Cisneros et al., 2014).

2.2.1.6 Soil Erosion and Sediment

Higher rainfall and increased runoff are likely to result in higher soil erosion. Even in areas of the world which are not projected to experience increases in average rainfall, soil erosion may increase as a result of more intense rainfall events. Extreme events have been projected to account for around half of the total soil erosion in semi-arid regions of Australia, Africa and Spain (Yang et al., 2003, Bussi et al., 2013). In addition, Knutson et al. (2010) found that projected

increases in cyclones in the tropics could result in more frequent landslides and greater soil erosion. Greater soil erosion will lead to higher sediment loads in river systems (Whitehead et al., 2009). However, the projections of changes to soil erosion occurring as a result of climate change are still very uncertain (Jiménez Cisneros et al., 2014).

Therefore, global climate change is likely to have a range of impacts on both the quality and quantity of water, which will affect the whole hydrological cycle.

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