Water is indispensable for all forms of life. It is needed in almost all human activities. Access to safe freshwater is now regarded as a universal human right (United Nations Committee on Economic, Social and Cultural Rights, 2003), and the Millennium Development Goals include the extended access to safe drinking water and sanitation (UNDP, 2006). Sustainable management of freshwater resources has gained importance at regional (e.g., European Union, 2000) and global scales (United Nations, 2002, 2006; World Water Council, 2006), and 'Integrated Water Resources Management' has become the corresponding scientific paradigm.
Figure 3.1 shows schematically how human activities affect freshwater resources (both quantity and quality) and their management. Anthropogenic climate change is only one of many pressures on freshwater systems. Climate and freshwater systems are interconnected in complex ways. Any change in one
of these systems induces a change in the other. For example, the draining of large wetlands may cause changes in moisture recycling and a decrease of precipitation in particular months, when local boundary conditions dominate over the large-scale circulation (Kanae et al., 2001). Conversely, climate change affects freshwater quantity and quality with respect to both mean states and variability (e.g., water availability as well as floods and droughts). Water use is impacted by climate change, and also, more importantly, by changes in population, lifestyle, economy, and technology; in particular by food demand, which drives irrigated agriculture, globally the largest water-use sector. Significant changes in water use or the hydrological cycle (affecting water supply and floods) require adaptation in the management of water resources.
In the Working Group II Third Assessment Report (TAR; IPCC, 2001), the state of knowledge of climate change impacts on hydrology and water resources was presented in the light of literature up to the year 2000 (Arnell et al., 2001). These findings are summarised as follows.
• There are apparent trends in streamflow volume, both increases and decreases, in many regions.
• The effect of climate change on streamflow and groundwater recharge varies regionally and between scenarios, largely following projected changes in precipitation.
• Peak streamflow is likely to move from spring to winter in many areas due to early snowmelt, with lower flows in summer and autumn.
• Glacier retreat is likely to continue, and many small glaciers may disappear.
• Generally, water quality is likely to be degraded by higher water temperatures.
• Flood magnitude and frequency are likely to increase in most regions, and volumes of low flows are likely to decrease in many regions.
• Globally, demand for water is increasing as a result of population growth and economic development, but is falling in some countries, due to greater water-use efficiency.
• The impact of climate change on water resources also depends on system characteristics, changing pressures on the system, how the management of the system evolves, and what adaptations to climate change are implemented.
• Unmanaged systems are likely to be most vulnerable to climate change.
• Climate change challenges existing water resource management practices by causing trends not previously experienced and adding new uncertainty.
• Adaptive capacity is distributed very unevenly across the world.
These findings have been confirmed by the current assessment. Some of them are further developed, and new findings have been added. This chapter gives an overview of the future impacts of climate change on freshwater resources and their management, mainly based on research published after the Third Assessment Report. Socio-economic aspects, adaptation issues, implications for sustainable development, as well as uncertainties and research priorities, are also covered. The focus is on terrestrial water in liquid form, due to its importance for freshwater management. Various aspects of climate change impacts on water resources and related vulnerabilities are presented (Section 3.4) as well as the impacts on water-use sectors (Section 3.5). Please refer to Chapter 1 for further information on observed trends, to Chapter 15 (Sections 15.3 and 15.4.1) for freshwater in cold regions and to Chapter 10 of the Working Group I Fourth Assessment Report (Meehl et al., 2007) - Section 10.3.3 for the cryosphere, and Section 10.3.2.3 for impacts on precipitation, evapotranspiration and soil moisture. While the impacts of increased water temperatures on aquatic ecosystems are discussed in this volume in Chapter 4 (Section 4.4.8), findings with respect to the effect of changed flow conditions on aquatic ecosystems are presented here in Section 3.5. The health effects of changes in water quality and quantity are covered in Chapter 8, while regional vulnerabilities related to freshwater are discussed in Chapters 9-16.
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