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Rainfall is low in most of the region, but it is highly variable seasonally and interannually. Average monthly rainfall ranges from 0 mm to 200 mm in January and from 0 mm to 500 mm in July (Oxford World Atlas, 1994). There was no discernible trend in annual precipitation during 1901-95 for the region as a whole (see Annex A, Figure A-9), nor in most parts of the region-except in the southWestern part of the Arabian peninsula, where there was a 200% increase (see Annex A, Figure A-1). This increase, however, is in relation to a very low base rainfall (<200 mm/yr). Precipitation tends to be very seasonal; in the Middle East countries, for example, precipitation occurs during winter, and the summer dry period lasts for 5-9 months (UNEP, 1997).
Winter precipitation is projected to increase slightly (<0.5 mm/day) throughout the region; summer precipitation is projected to remain the same in the northeastern part of the region and increase (0.5-1 mm/day) in the southWest (i.e., the southern part of the Arabian peninsula). These projected changes vary, however, from model to model and are unlikely to be significant.
Soil moisture is projected to decrease in most parts of the region because projected precipitation increases are small and evaporation will increase with rising temperatures (IPCC 1996, WG I, Figure 6.12).
The ratio of precipitation to potential evapotranspiration (P:PET) is a measure of moisture availability and an indicator of the type of vegetation that an area can support. The Middle East and Arid Asian region has a P:PET ratio in the range of 0.01-1.6; most of the area has a value of <0.45-typical of semi-arid and arid climates that support grasslands, shrublands, and some woodlands. Projected changes in P:PET for different model runs show no consistent trend (IPCC 1996, WG II, Chapter 14): Some models project a decrease (greater aridity) and some an increase. The lack of data specifically from the arid and semi-arid regions may contribute to this uncertainty.
This section discusses the region's major vulnerabilities-particularly those that apply to large parts of the region; many impacts are common to the region as a whole. Later in the section, some sector-specific impacts (e.g., those affecting agriculture or uplands), adaptations, and vulnerabilities for smaller parts of the region are highlighted.
The region is mostly semi-arid and arid, with significant areas of extreme aridity (i.e., deserts, as defined in IPCC 1996, WG II, Chapter 3). All areas experience wide fluctuations in rainfall, and their native plants and animals are adapted to coping with sequences of extreme climatic conditions. In many of these ecological systems, the initial climatic changes are unlikely to create conditions significantly outside the present range of variation (IPCC 1996, WG II, Chapter 3); thus, impacts from climate change may not be apparent for several decades.
Models that project changes in global vegetation in response to a doubled-CO2-driven climate suggest little change in desert (arid) communities. One study, which used four general circulation model (GCM) climate projections, concluded that deserts were the most stable of the 16 vegetation types considered (IPCC 1996, WG II, Chapter 3); it estimated that only 59-66% of all combined vegetation types for the world would remain in the same category, whereas 82-92% of existing deserts would retain this classification. The impact of climate change may be greater in the semi-arid areas of the region than in the arid areas. Agriculture, natural grasslands, livestock, and water resources in marginal areas are most likely to be vulnerable to climate change (Smith et al., 1996).
Although precipitation is projected by some models to increase slightly, this increase will have little impact because most of the region will remain arid. In rangelands or semi-arid lands, increased precipitation eventually may lead to improved soil conditions, including larger accumulations of organic matter, improved soil field capacity, enhanced moisture availability, and changed runoff patterns (IPCC 1996, WG II, Chapter 3). This process is slow (taking 400 years or more), however, so little advantage will accrue to these ecosystems during the next century. A more important process may be the demonstrated increase in the water-use efficiency of plants when high levels of CO2 are present in the atmosphere (Bazzaz et al., 1996). In atmospheres rich in CO2, plants can take up the CO2 necessary for photosynthesis with less water loss from their stomatal pores. This efficiency gain has been demonstrated to be most advantageous in plants growing in water-limited circumstances-and thus may lead to higher productivity throughout much of the region. The response of individual species and communities varies considerably, however. In high-CO2 conditions, some species conserve water by using less water to maintain previous rates of productivity (i.e., photosynthesis); other species increase their productivity and maintain relatively high rates of water use. There is little understanding about how whole communities of plants respond, and, unfortunately, few studies have been done within the region. Nevertheless, the net result of increased water-use efficiency should be increased plant productivity in arid and semi-arid ecosystems; this effect probably will outweigh any direct effects stemming from increased precipitation resulting from climate change.
The processes associated with changed climatic and atmospheric conditions are expected to lead to changes in the composition of many plant communities-such as the mix between grasses and shrubs or weeds and non-weeds (IPCC 1996, WG II, Chapters 2 and 3). There also is evidence that the forage quality (e.g., the ratio of nitrogen to carbon) and the protein content of some cereals may decline. Although it has not been possible to test the effects of high CO2 atmospheres on extensive areas of crops and pastures with large grazing animals (Diaz, 1995), it is possible that they may affect farm management systems significantly.
Even where wetter conditions prevail, however, any "greening" of the deserts and rangelands often might be counteracted by pressure from the expanding human population and associated desertification problems. Climatic conditions and social pressures vary greatly in a region that ranges from Mediterranean climate to extreme deserts, with economies as varied as those of the oil producing states, Israel, and the countries of the FSU. In the latter group of countries, the impacts of climate change are likely to be relatively small compared with the impacts of economic transitions on agricultural systems and the environment (see Section 7.3.4.1).
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