One of the major impacts of global warming is likely to be on hydrology and water resources, which in turn will have a significant impact across many sectors of the economy, society, and environment (Figures 11.2 and 11.3). Characteristics of many ecosystems are heavily influenced by water availability. Water is fundamental for human life and many activities, in-
cluding agriculture, industry, and power generation. On the global scale, climate change is likely to worsen water resource stress in some regions but perhaps ameliorate stress in others. At the regional scale there are mixed signals.
A major impact of climate change over the African continent is a shift in the temporal and spatial distribution of precipitation. This will result in a shift of runoff or hydrological resources in both time and space.
Future climatic changes in the Nile River basin would be significant and possibly severe. For example, with 4°C warming and a 20 percent decrease in precipitation, Nile River flow decreases 98 percent. This represents a significant reduction in water supply (Gleick, 1993,1998). Based on the results of the river flow responses, climate variables alone can cause a 50 percent change in runoff in the Gambia River catchment. In general, a 1 percent change in rainfall will result in 3 percent change in runoff (Jallow et al., 1999). For the Zambezi River basin, simulated runoff under climate change is projected to decrease by about 40 percent or more (Cambula, 1999).
Several simulation studies suggest that some areas of the Asian continent are expected to experience an increase in water availability, while other areas will have reduced water resources available.
The Himalayas have nearly 1,500 glaciers that provide snow and glacial-melt waters to keep the major rivers perennial throughout the year. Glacial melt is expected to increase under changed climate conditions, which would lead to increased summer flows in some river systems for a few decades, followed by a reduction in flow as the glaciers disappear (IPCC, 1998).
Large-scale shrinkage of the permafrost region in boreal Asia is also likely. Due to global warming, permafrost thawing will start over vast territories (IPCC, 1998). The perennially frozen rocks will completely degrade within the present southern regions. In the northern regions of boreal Asia, the mean annual temperature of permafrost and hence the depth of seasonal thawing (active layer thickness) will increase (Izrael, Anokhin, and Eliseev, 1997).
The average annual runoff in the river basins of the Tigris, Euphrates, Indus, and Brahmaputra Rivers would decline by 22,25,27, and 14 percent, respectively, by the year 2050 (Izrael, Anokhin, and Eliseev, 1997). Runoff in the Yangtze and Huang He Rivers has the potential to increase up to 37 and 26 percent. Increases in annual runoff are also projected in the Siberian large rivers: Yenise by 15 percent, Lena by 27 percent, Ob by 12 percent, and Amur by 14 percent.
Surface runoff is projected to decrease drastically in arid and semiarid central Asia under climate change scenarios and would significantly affect the volume of available water for withdrawal for irrigation and other purposes (Gruza et al., 1997).
In temperate Asia (Mongolia, northern China, and Japan), an increase in surface runoff seems likely, but a decline is possible in southern China. The hydrological characteristics of Japanese rivers and lakes are sensitive to climate change.
The perennial rivers originating in the high Himalayas receive water from snow and glaciers. Because the melting season of snow coincides with the summer monsoon season, any intensification of the monsoons is likely to contribute to flood disasters in the Himalayan catchments. Such impacts will be observed more in the western Himalayas compared to the eastern Himalayas, due to the higher contribution of snow-melt runoff in the west (Singh, 1998).
Global warming will adversely affect water resources in Australia. Although increases in stream flow are possible in northern Australia, decreases in stream flow seem likely in other parts of the country due to a decrease in rainfall. Estimated changes in stream flow in the Murray-Darling Basin range from 0 to -20 percent by 2030, and +5 to -45 percent by 2070 (Commonwealth Scientific and Industrial Research Organisation [CSIRO], 2001). Estimates also show large decreases in both the maximum and minimum monthly runoff. This implies large increases in drought frequency (Arnell, 2000). Another study (Kothavala, 1999) has also concluded that there will be longer and more severe droughts under doubled CO2 conditions than in the control simulation.
Application of the CSIRO (1996) scenarios also suggests a possible combination of small or larger decreases in mean annual rainfall, higher temperatures and evaporation, and a higher frequency of floods and droughts in northern Victorian rivers (Schreider et al., 1996). A study of the Macquarie River basin in New South Wales indicated inflow reductions of 10 to 30 percent for doubled CO2 and reduced stream flows if irrigation demand remains constant or increases (Hassall and Associates et al., 1998). There is also concern about the adverse effects of increased drought frequency on water quality through possible increases in toxic algal blooms (MurrayDarling Basin Commission [MDBC], 1999).
Calculations at the continental scale (Arnell, 2000) indicate that under most climate change scenarios northern Europe would see an increase in annual average stream flow, but southern Europe would experience a reduction in stream flow. In much of midlatitude Europe annual runoff would de crease or increase by around 10 percent by the 2050s, but the change may be significantly larger further north and south.
In Mediterranean regions, climate change is likely to considerably exaggerate the range in flows between winter and summer. In maritime western Europe the range is also likely to increase but to a lesser extent. In more continental and upland areas, where snowfall makes up a large portion of winter precipitation, a rise in temperature would mean that more precipitation falls as rain, and, therefore, winter runoff increases and spring snow melt decreases (Arnell, 2000).
In some Latin American areas the availability of freshwater will be substantially changed by global warning, especially in areas where it is possible that the combined effect of less rainfall and more evaporation could take place and lead to less runoff (Marengo, 1995).
Hydrological scenarios for Central America show that a significant limitation of the potential water resources will occur due to an increase in evapotranspiration and changes in precipitation. Studies on vulnerability of hydrologic regions in Mexico and all Central American countries to future changes in climate suggest that potential changes in temperature and precipitation may have a dramatic impact on the pattern and magnitude of runoff, on soil moisture, and on evaporation (Arnell, 2000). In the Uruguay River basin a decrease in runoff during low-flow periods of the year is anticipated. Argentina could foresee a reduction in water availability from the snow melt in the high Andes and in central western regions (Marengo, 1995).
In North America global warming may lead to substantial changes in mean annual stream flows, the seasonal distribution of flows, and the probability of extreme high or low flow conditions. Runoff changes will depend on changes in temperatures and other climatic variables, and warmer temperatures may cause runoff to decline even where precipitation increases (Nash and Gleick, 1993; Matalas, 1998).
The Greenland ice sheet already suffers melting in summer over much of its margin. There is a trend toward an increase in the area and duration of this melt (Abdalati and Steffen, 1997). This is likely to continue. Airborne altimetric monitoring has shown that over the period 1993 to 1998, the Greenland ice sheet was slowly thickening at higher elevations, while at lower elevations, thinning of around 1 m/year was underway (Krabill et al., 1999). If warming continues, the Greenland ice sheet will eventually disappear, but this will take many centuries.
Over the Antarctic ice sheet, where only a few limited areas show summer melting, the likely response is toward a slight thickening as precipitation rates increase (Vaughan et al., 1999).
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