3.4.4.1. Reference Conditions

Tropospheric ozone forms part of the natural shield that protects living organisms from harmful UV-B rays. In the lowest portion of the atmosphere, however, excess accumulations of ozone can be toxic for a wide range of plant species (Fuhrer, 1996; Semenov et al., 1998, 1999).Ozone is produced by a chain of chemical and photochemical reactions involving, in particular, NO, NO₂, and VOCs (Finlayson-Pitts and Pitts, 1986; Derwent et al., 1991; Alexandrov et al., 1992; Simpson, 1992, 1995a; Peters et al., 1995). These chemical precursors of ozone can be human-derived (e.g., energy production, transport) or natural (e.g., biogenic emissions, forest fires). Surface ozone concentrations are highly variable in space and time (Table 3-2); the highest values typically are over industrial regions and large cities.

Global background concentrations of ground-level ozone (annual means) are about 20-25 ppb (Semenov et al., 1999). Background concentrations have increased in Europe during the 20th century from 10-15 to 30 ppb (Grennfelt, 1996). In the northern hemisphere as a whole, trends in concentrations since 1970 show large regional differences: increases in Europe and Japan, decreases in Canada, and only small changes in the United States (Lelieveld and Thompson, 1998). In an effort to reverse the upward trends still recorded in many regions, a comprehensive protocol to abate acidification, eutrophication, and ground-level ozone was signed in 1999, setting emissions ceilings for sulfur, NO_x, NH₃, and VOCs for most of the United Nations Economic Commission for Europe (UN/ECE) region.

3.4.4.2. Development and Application of Tropospheric Ozone Scenarios

Results from the first intercomparison of model-based estimates of global tropospheric ozone concentration assuming the new SRES emissions scenarios (see Section 3.8.1) are reported in TAR WGI Chapter 4. Estimates of mean ground-level O₃ concentrations during July over the industrialized continents of the northern hemisphere under the SRES A2 and A1FI scenarios are presented in Table 3-2. These scenarios produce concentrations at the high end of the SRES range, with values in excess of 70 ppb for 2100 emissions (TAR WGI Chapter 4). Local smog events could enhance these background levels substantially, posing severe problems in achieving the accepted clean-air standard of <80 ppb in most populated areas.

Regional projections of ozone concentration also are made routinely, assuming various emissions reduction scenarios (e.g., SEPA, 1993; Simpson, 1995b; Simpson et al., 1995). These projections sometimes are expressed in impact terms—for example, using AOT40 (the integrated excess of O₃ concentration above a threshold of 40 ppb during the vegetative period), based on studies of decline in tree growth and crop yield (Fuhrer, 1996; Semenov et al., 1999).

There are few examples of impact studies that have evaluated the joint effects of ozone and climate change. Some experiments have reported on plant response to ozone and CO₂ concentration (Barnes et al., 1995; Ojanperä et al., 1998), and several model-based studies have been conducted (Sirotenko et al., 1995; Martin, 1997; Semenov et al., 1997, 1998, 1999).

Table 3-3: Estimates of global and regional water intensity and water withdrawals in 1995 and scenarios for 2025.
Aggregate World Regions		Water Intensity (m3 cap-1 yr-1)a 2025					Total Water Withdrawals (km3) 2025
	1995b	BAUb,c	TECb,d	VALb,e	CDSf	1995b	BAUb,c	TECb,d	VALb,e	CDSf
Africa	5678	2804	2859	2974	2858	167	226	228	204	240
Asia	3884	2791	2846	3014	2778	1913	2285	2050	1499	2709
Central America	6643	4429	4507	4895	4734	126	171	140	112	145
CISg	17049	16777	17124	17801	14777	274	304	226	186	480
Europe	4051	3908	3922	4119	3765	375	359	256	201	415
North America	17625	14186	14186	15533	14821	533	515	323	245	668
Oceania	64632	46455	46455	51260	42914	27	27	28	20	32
South America	30084	21146	21576	23374	21176	157	208	162	128	211
World	7305	5167	5258	5563	5150	3572	4095	3413	2595	4899
a Calculated by using estimates of water availability from UN Comprehensive Assessment of the Freshwater Resources of the World (Shiklomanov, 1998) and population from footnoted source. b World Commission on Water for the 21st Century (Alcamo et al., 2000). c Business-as-usual scenario (domestic water intensity increases, then stabilizes with increasing incomes, some increase in water-use efficiency). d Technology, Economics, and Private Sector scenario (relative to BAU: similar population and income level; domestic water-use intensity one-third lower; higher water-use efficiency in industrialized countries). e Values and Lifestyles scenario (relative to BAU: lower population and higher income; domestic water-use intensity two-thirds lower; much higher water-use efficiency in all countries). f Conventional Development scenario (Raskin et al., 1997—population slightly higher than in BAU scenario; per capita water use falls in developed world and rises in developing world). g Commonwealth of Independent States.

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