3.2 Current sensitivity/vulnerability

With higher temperatures, the water-holding capacity of the atmosphere and evaporation into the atmosphere increase, and this favours increased climate variability, with more intense precipitation and more droughts (Trenberth et al., 2003). The hydrological cycle accelerates (Huntington, 2006). While temperatures are expected to increase everywhere over land and during all seasons of the year, although by different increments, precipitation is expected to increase globally and in many river basins, but to decrease in many others. In addition, as shown in the Working Group I Fourth Assessment Report, Chapter 10, Section 10.3.2.3 (Meehl et al., 2007), precipitation may increase in one season and decrease in another. These climatic changes lead to changes in all components of the global freshwater system.

Climate-related trends of some components during the last decades have already been observed (see Table 3.1). For a number of components, for example groundwater, the lack of data makes it impossible to determine whether their state has changed in the recent past due to climate change. During recent decades, non-climatic drivers (Figure 3.1) have exerted strong pressure on freshwater systems. This has resulted in water pollution, damming of rivers, wetland drainage, reduction in streamflow, and lowering of the groundwater table (mainly due to irrigation). In comparison, climate-related changes have been small, although this is likely to be different in the future as the climate change signal becomes more evident.

Current vulnerabilities to climate are strongly correlated with climate variability, in particular precipitation variability. These vulnerabilities are largest in semi-arid and arid low-income countries, where precipitation and streamflow are concentrated over a few months, and where year-to-year variations are high (Lenton, 2004). In such regions a lack of deep groundwater wells or reservoirs (i.e., storage) leads to a high level of vulnerability to climate variability, and to the climate changes that are likely to further increase climate variability in future. In addition, river basins that are stressed due to non-climatic drivers are likely to be vulnerable to climate change. However, vulnerability to climate change exists everywhere, as water infrastructure (e.g., dikes and pipelines) has been designed for stationary climatic conditions, and water resources management has only just started to take into account the uncertainties related to climate change (see Section 3.6). In the following paragraphs, the current sensitivities of components of the global freshwater system are discussed, and example regions, whose vulnerabilities are likely to be exacerbated by climate change, are highlighted (Figure 3.2).

Figure 3.2. Examples of current vulnerabilities of freshwater resources and their management; in the background, a water stress map based on Alcamo et al. (2003a). See text for relation to climate change.

Surface waters and runoff generation

Changes in river flows as well as lake and wetland levels due to climate change depend on changes in the volume, timing and intensity of precipitation (Chiew, 2007), snowmelt and whether precipitation falls as snow or rain. Changes in temperature, radiation, atmospheric humidity, and wind speed affect potential evapotranspiration, and this can offset small increases in precipitation and exaggerate further the effect of decreased precipitation on surface waters. In addition, increased atmospheric CO₂ concentration directly alters plant physiology, thus affecting evapotranspiration. Many experimental (e.g., Triggs et al., 2004) and global modelling studies (e.g., Leipprand and Gerten, 2006; Betts et al., 2007) show reduced evapotranspiration, with only part of this reduction being offset by increased plant growth due to increased CO₂ concentrations. Gedney et al. (2006) attributed an observed 3% rise in global river discharges over the 20th century to CO₂-induced reductions in plant evapotranspiration (by 5%) which were offset by climate change (which by itself would have decreased discharges by 2%). However, this attribution is highly uncertain, among other reasons due to the high uncertainty of observed precipitation time series.

Different catchments respond differently to the same change in climate drivers, depending largely on catchment physiogeographical and hydrogeological characteristics and the amount of lake or groundwater storage in the catchment.

A number of lakes worldwide have decreased in size during the last decades, mainly due to human water use. For some, declining precipitation was also a significant cause; e.g., in the case of Lake Chad, where both decreased precipitation and increased human water use account for the observed decrease in lake area since the 1960s (Coe and Foley, 2001). For the many lakes, rivers and wetlands that have shrunk mainly due to human water use and drainage, with negative impacts on ecosystems, climate change is likely to exacerbate the situation if it results in reduced net water availability (precipitation minus evapotranspiration).

Groundwater

Groundwater systems generally respond more slowly to climate change than surface water systems. Groundwater levels correlate more strongly with precipitation than with temperature, but temperature becomes more important for shallow aquifers and in warm periods.

Floods and droughts

Disaster losses, mostly weather- and water-related, have grown much more rapidly than population or economic growth, suggesting a negative impact of climate change (Mills, 2005). However, there is no clear evidence for a climate-related trend in floods during the last decades (Table 3.1; Kundzewicz et al., 2005; Schiermeier, 2006). However, the observed increase in precipitation intensity (Table 3.1) and other observed climate changes, e.g., an increase in westerly weather patterns during winter over Europe, leading to very rainy low-pressure systems that often trigger floods (Kron and Bertz, 2007), indicate that climate might already have had an impact on floods. Globally, the number of great inland flood catastrophes during the last 10 years (between 1996 and 2005) is twice as large, per decade, as between 1950 and 1980, while economic losses have increased by a factor of five (Kron and Bertz, 2007). The dominant drivers of the upward trend in flood damage are socio-economic factors, such as increased population and wealth in vulnerable areas, and land-use change. Floods have been the most reported natural disaster events in Africa, Asia and Europe, and have affected more people across the globe (140 million/yr on average) than all other natural disasters (WDR, 2003, 2004). In Bangladesh, three extreme floods have occurred in the last two decades, and in 1998 about 70% of the country’s area was inundated (Mirza, 2003; Clarke and King, 2004). In some river basins, e.g., the Elbe river basin in Germany, increasing flood risk drives the strengthening of flood protection systems by structural means, with detrimental effects on riparian and aquatic ecosystems (Wechsung et al., 2005).

Droughts affect rain-fed agricultural production as well as water supply for domestic, industrial, and agricultural purposes. Some semi-arid and sub-humid regions of the globe, e.g., Australia (see Chapter 11, Section 11.2.1), western USA and southern Canada (see Chapter 14, Section 14.2.1), and the Sahel (Nicholson, 2005), have suffered from more intense and multi-annual droughts, highlighting the vulnerability of these regions to the increased drought occurrence that is expected in the future due to climate change.

Water quality

In lakes and reservoirs, climate change effects are mainly due to water temperature variations, which result directly from climate change or indirectly through an increase in thermal pollution as a result of higher demands for cooling water in the energy sector. This affects oxygen regimes, redox potentials,^[1] lake stratification, mixing rates, and biota development, as they all depend on temperature (see Chapter 4). Increasing water temperature affects the self-purification capacity of rivers by reducing the amount of oxygen that can be dissolved and used for biodegradation. A trend has been detected in water temperature in the Fraser River in British Columbia, Canada, for longer river sections reaching a temperature over 20°C, which is considered the threshold beyond which salmon habitats are degraded (Morrison et al., 2002). Furthermore, increases in intense rainfall result in more nutrients, pathogens, and toxins being washed into water bodies. Chang et al. (2001) reported increased nitrogen loads from rivers of up to 50% in the Chesapeake and Delaware Bay regions due to enhanced precipitation.

Numerous diseases linked to climate variations can be transmitted via water, either by drinking it or by consuming crops irrigated with polluted water (Chapter 8, Section 8.2.5). The presence of pathogens in water supplies has been related to extreme rainfall events (Yarze and Chase, 2000; Curriero et al., 2001; Fayer et al., 2002; Cox et al., 2003; Hunter, 2003). In aquifers, a possible relation between virus content and extreme rainfall has been identified (Hunter, 2003). In the USA, 20 to 40% of water-borne disease outbreaks can be related to extreme precipitation (Rose et al., 2000). Effects of dry periods on water quality have not been adequately studied (Takahashi et al., 2001), although lower water availability clearly reduces dilution.

At the global scale, health problems due to arsenic and fluoride in groundwater are more important than those due to other chemicals (United Nations, 2006). Affected regions include India, Bangladesh, China, North Africa, Mexico, and Argentina, with more than 100 million people suffering from arsenic poisoning and fluorosis (a disease of the teeth or bones caused by excessive consumption of fluoride) (United Nations, 2003; Clarke and King, 2004; see also Chapter 13, Section 13.2.3).

One-quarter of the global population lives in coastal regions; these are water-scarce (less than 10% of the global renewable water supply) (Small and Nicholls, 2003; Millennium Ecosystem Assessment, 2005b) and are undergoing rapid population growth. Saline intrusion due to excessive water withdrawals from aquifers is expected to be exacerbated by the effect of sea-level rise, leading to even higher salinisation and reduction of freshwater availability (Klein and Nicholls, 1999; Sherif and Singh, 1999; Essink, 2001; Peirson et al., 2001; Beach, 2002; Beuhler, 2003). Salinisation affects estuaries and rivers (Knighton et al., 1992; Mulrennan and Woodroffe, 1998; Burkett et al., 2002; see also Chapter 13). Groundwater salinisation caused by a reduction in groundwater recharge is also observed in inland aquifers, e.g., in Manitoba, Canada (Chen et al., 2004).

Water quality problems and their effects are different in type and magnitude in developed and developing countries, particularly those stemming from microbial and pathogen content (Lipp et al., 2001; Jiménez, 2003). In developed countries, flood-related water-borne diseases are usually contained by well-maintained water and sanitation services (McMichael et al., 2003) but this does not apply in developing countries (Wisner and Adams, 2002). Regretfully, with the exception of cholera and salmonella, studies of the relationship between climate change and micro-organism content in water and wastewater do not focus on pathogens of interest in developing countries, such as specific protozoa or parasitic worms (Yarze and Chase, 2000; Rose et al., 2000; Fayer et al., 2002; Cox et al., 2003; Scott et al., 2004). One-third of urban water supplies in Africa, Latin America and the Caribbean, and more than half in Asia, are operating intermittently during periods of drought (WHO/UNICEF, 2000). This adversely affects water quality in the supply system.

Erosion and sediment transport

Rainfall amounts and intensities are the most important factors controlling climate change impacts on water erosion (Nearing et al., 2005), and they affect many geomorphologic processes, including slope stability, channel change, and sediment transport (Rumsby and Macklin, 1994; Rosso et al., 2006). There is no evidence for a climate-related trend in erosion and sediment transport in the past, as data are poor and climate is not the only driver of erosion and sediment transport. Examples of vulnerable areas can be found in north-eastern Brazil, where the sedimentation of reservoirs is significantly decreasing water storage and thus water supply (De Araujo et al., 2006); increased erosion due to increased precipitation intensities would exacerbate this problem. Human settlements on steep hill slopes, in particular informal settlements in metropolitan areas of developing countries (United Nations, 2006), are vulnerable to increased water erosion and landslides.

Water use, availability and stress

Human water use is dominated by irrigation, which accounts for almost 70% of global water withdrawals and for more than 90% of global consumptive water use, i.e., the water volume that is not available for reuse downstream (Shiklomanov and Rodda, 2003). In most countries of the world, except in a few industrialised nations, water use has increased over the last decades due to demographic and economic growth, changes in lifestyle, and expanded water supply systems. Water use, in particular irrigation water use, generally increases with temperature and decreases with precipitation. There is no evidence for a climate-related trend in water use in the past. This is due to the fact that water use is mainly driven by non-climatic factors and to the poor quality of water-use data in general and time series in particular.

Water availability from surface sources or shallow groundwater wells depends on the seasonality and interannual variability of streamflow, and safe water supply is determined by seasonal low flows. In snow-dominated basins, higher temperatures lead to reduced streamflow and thus decreased water supply in summer (Barnett et al., 2005), for example in South American river basins along the Andes, where glaciers are shrinking (Coudrain et al., 2005). In semi-arid areas, climate change may extend the dry season of no or very low flows, which particularly affects water users unable to rely on reservoirs or deep groundwater wells (Giertz et al., 2006)

Currently, human beings and natural ecosystems in many river basins suffer from a lack of water. In global-scale assessments, basins with water stress are defined either as having a per capita water availability below 1,000 m³/yr (based on long-term average runoff) or as having a ratio of withdrawals to long-term average annual runoff above 0.4. These basins are located in Africa, the Mediterranean region, the Near East, South Asia, Northern China, Australia, the USA, Mexico, north-eastern Brazil, and the western coast of South America (Figure 3.2). Estimates of the population living in such severely stressed basins range from 1.4 billion to 2.1 billion (Vörösmarty et al., 2000; Alcamo et al., 2003a, b; Oki et al., 2003a; Arnell, 2004b). In water-scarce areas, people and ecosystems are particularly vulnerable to decreasing and more variable precipitation due to climate change. For example, in the Huanghe River basin in China (Yang et al., 2004), the combination of increasing irrigation water consumption facilitated by reservoirs, and decreasing precipitation associated with global El Niño-Southern Oscillation (ENSO) events over the past half century, has resulted in water scarcity (Wang et al., 2006). The irrigation-dominated Murray-Darling Basin in Australia suffers from decreased water inflows to wetlands and high salinity due to irrigation water use, which affects aquatic ecosystems (Goss, 2003; see also Chapter 11, Section 11.7).

Current adaptation

At the Fourth World Water Forum held in Mexico City in 2006, many of the involved groups requested the inclusion of climate change in Integrated Water Resources Management (World Water Council, 2006). In some countries (e.g., Caribbean, Canada, Australia, Netherlands, UK, USA and Germany), adaptation procedures and risk management practices for the water sector have already been developed that take into account climate change impacts on freshwater systems (compare with Section 3.6).