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of water resource stress, including the amount of water available per person and the ratio of volume of water withdrawn to volume of water potentially available.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).
	Total Population	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 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.
	Total Population	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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