Working Group II: Impacts, Adaptation and Vulnerability

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4.5.2. Impacts of Climate Change on Water Resources: A Global Perspective

There are several indicators of water resource stress, including the amount of water available per person and the ratio of volume of water withdrawn to volume of water potentially available. When withdrawals are greater than 20% of total renewable resources, water stress often is a limiting factor on development (Falkenmark and Lindh, 1976); withdrawals of 40% or more represents high stress. Similarly, water stress may be a problem if a country or region has less than 1,700 m3 yr-1 of water per capita (Falkenmark and Lindh, 1976). Simple numerical indices, however, give only partial indications of water resources pressures in a country or region because the consequences of “water stress” depend on how the water is managed.

Table 4-6: Number of people living in water-stressed countries that are adversely affected by climate change, under a “business-as-usual” emissions scenario (IS92a) (Arnell, 2000).

Population in
Water-Stressed Countries

Number of People (millions) in Water-Stressed
Countries with Increase in Water Scarcity
  (millions) (millions) HadCM2 HadCM3 ECHAM4 CGCM1 CSIRO CCSR GFDL NCAR
2025 8055 5022 338–623 545 488 494 746 784 403 428
2050 9505 5915 2209–3195 1454 662 814 1291 1439
a Water-stressed countries use more than 20% of their available resources.

At the global scale, assessments of water stress usually are made by country because that is the unit at which water-use data generally are available. In 1990, approximately one-third of the world’s population lived in countries using more than 20% of their water resources, and by 2025 about 60% of a larger total would be living in such stressed countries, in the absence of climate change (WMO, 1997), largely because population growth. Arnell (1999b, 2000) estimates the effect of a number of climate change scenarios on national water resource availability and compares this with estimated future demands for water (increasing following the CDS outlined in Section 4.4). Table 4-6 shows the numbers of people living in countries using more than 20% of their water resources in 2025 and 2050 and in which the amount of resources decreases by more than 10% as a result of climate change. There is considerable variability between scenarios, essentially reflecting how resources change in populous countries, but by the 2020s the table indicates that about 0.5 billion people could see increased water resources stress as a result of climate change. Significant geographic variations are hidden in Table 4-6. Under most of the scenarios considered, climate change increases stresses in many countries in southern and western Africa and the Middle East, whereas it ameliorates stresses in parts of Asia. Alcamo et al. (1997) found broadly similar results.

Figure 4-4: National water resources per capita (m3 yr-1), in 1990 and 2050 under several climate change scenarios, for some countries (Arnell, 2000). Blue diamonds represent 1990; long pink bars 2050 with no climate change; and short black bars 2050 under different climate change scenarios.

Figure 4-4 shows water resources per capita in 1990 and 2050 for a set of countries,as listed in Table 14-3 of the WGII contribution to the SAR, showing resources per capita in 2050 without climate change (long line) and under eight climate change scenarios (short lines) (Arnell, 2000). There are some differences with the earlier table because of the use of updated data sets, but similar conclusions can be drawn. Climate change tends to have a small effect relative to population growth, and the range of magnitudes of effect between scenarios also is little changed; the effects are still uncertain. For most of the example countries, climate change may result in either an increase or a decrease, although for some the climate change signal is more consistent (reductions in South Africa, Cyprus, and Turkey, for example, and increases in China). Note that these figures represent national averages, and different parts of each country may be differently affected.

Table 4-7: Effect of stabilization of CO2 concentrations on numbers of people living in water-stressed countries adversely affected by climate change (Arnell et al., 2001). Climate change under each emissions scenario is simulated with HadCM2 general circulation model; other climate models could give different indications of the effect of stabilization.
Population in
Water Stressed Countries
Number of People (millions) in
Water-Stressed Countries with
Increase in Water Scarcity
  (millions) (millions) IS92a S750 S550
2025 8055 5022 338–623 242 175
2050 9505 5915 2209–3195 2108 1705

Table 4-7 gives an indication of the potential effect of stabilizing GHG concentrations on the total number of people living in water-stressed countries adversely affected by climate change (Arnell et al., 2001). The results are conditional on the climate model used and the stabilization scenario, but this study—using just the HadCM2 climate model—suggests that by the 2050s the “weaker” stabilization target has little effect on the total number of impacted people, and although the “stronger” target reduces the impact of climate change, it does not eliminate it. The changes by the 2020s are very much affected by climatic variability between the various GCM runs.

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