Climate Change 2001: Synthesis Report

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Water   3.22 Projected climate change would exacerbate water shortage and quality problems in many water-scarce areas of the world, but alleviate it in some other areas. Demand for water is generally increasing due to population growth and economic development, but is falling in some countries because of increased efficiency of use. Climate change is projected to reduce streamflow and groundwater recharge in many parts of the world but to increase it in some other areas (medium confidence). The amount of change varies among scenarios partly because of differences in projected rainfall (especially rainfall intensity) and partly because of differences in projected evaporation. Projected streamflow changes under two climate change scenarios are shown in Figure 3-5. Several hundred million to a few billion people are projected to suffer a supply reduction of 10% or more by the year 2050 for climate change projections corresponding to 1% per year increase in CO2 emissions (see Table 3-4). Freshwater quality generally would be degraded by higher water temperatures (high confidence), but this may be offset by increased flows in some regions. The effects of climate changes on water scarcity, water quality, and the frequency and intensity of floods and droughts will intensify challenges for water and flood management. Unmanaged and poorly managed water systems are the most vulnerable to adverse effects of climate change. WGI TAR Section 9.3.6 & WGII TAR Sections 4.3-4, 4.5.2, & 4.6   Table 3-4: Water resource effects of climate change if no climate policy interventions are made.*   2025 2050 2100 CO2 concentrationa 405-460 ppm 445-640 ppm 540-970 ppm Global mean temperature change from the year 1990b 0.4-1.1°C 0.8-2.6°C 1.4-5.8°C Global mean sea level rise from the year 1990b 3-14 cm 5-32 cm 9-88 cm Water Resource Effectsc Water supply [WGII TAR Sections 4.3.6 & 4.5.2] Peak river flow shifts from spring toward winter in basins where snowfall is an important source of water (high confidenced). Water supply decreased in many water-stressed countries, increased in some other water-stressed countries (high confidenced). Water supply effects amplified (high confidenced). Water quality [WGII TAR Section 4.3.10] Water quality degraded by higher temperatures. Water quality changes modified by changes in water flow volume. Increase in saltwater intrusion into coastal aquifers due to sea-level rise (medium confidenced). Water quality degraded by higher temperatures (high confidenced). Water quality changes modified by changes in water flow volume (high confidenced). Water quality effects amplified (high confidenced). Water demand [WGII TAR Section 4.4.3] Water demand for irrigation will respond to changes in climate; higher temperatures will tend to increase demand (high confidenced). Water demand effects amplified (high confidenced). Water demand effects amplified (high confidenced). Extreme events [WGI TAR SPM; WGII TAR SPM] Increased flood damage due to more intense precipitation events (high confidenced). Increased drought frequency (high confidenced). Further increase in flood damage (high confidenced). Further increase in drought events and their impacts. Flood damage several-fold higher than "no climate change scenarios." * Refer to footnotes a-d accompanying Table 3-1.

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