Working Group II: Impacts, Adaptation and Vulnerability

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3.4.4. Scenarios of Tropospheric Ozone 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, NOx, NH3, and VOCs for most of the United Nations Economic Commission for Europe (UN/ECE) region. 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.
World Regions
  Water Intensity (m3 cap-1 yr-1)a

  Total Water Withdrawals (km3)

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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