Observations during the 20th century have demonstrated clearly the multifaceted nature of anthropogenic environmental changes. Therefore, it is reasonable to expect that changes in climate anticipated for the future will occur in combination with other changes in the environment. Some of these changes will occur independently of climate change (e.g., groundwater depletion, acidification); others are a cause of climate change (e.g., changing atmospheric CO₂ concentration); and still others are a direct consequence of climate change (e.g., sea-level rise). All of these could have a role in modifying the impacts of future climate change. Hence, realistic scenarios of nonclimatic environmental factors are required to facilitate analysis of these combined effects and quantify them in impact assessments.

This section introduces environmental changes that are of importance at scales from subcontinental to global and describes how scenarios commonly are constructed to represent them. Requirements for environmental scenarios are highly application- and region-specific. For example, scenarios of CO₂ concentration may be important in considering future vegetation productivity under a changing climate but are unlikely to be required for assessment of human health impacts. Most of the scenarios treated here relate to atmospheric composition: CO₂, SO₂, sulfur and nitrogen deposition, tropospheric O₃, and surface UV-B radiation. Scenarios of water resources and marine pollution also are examined. Changes in the terrestrial environment are addressed in Section 3.3, and changes in sea level are addressed in Section 3.6.

3.4.2.1. Reference Conditions

Aside from its dominant role as a greenhouse gas, atmospheric CO₂ also has an important direct effect on many organisms, stimulating photosynthetic productivity and affecting water-use efficiency in many terrestrial plants. In 1999, the concentration of CO₂ in the surface layer of the atmosphere (denoted as [CO₂]) was about 367 ppm (see Table 3-2), compared with a concentration of approximately 280 ppm in preindustrial times (see TAR WGI Chapter 3). CO₂ is well mixed in the atmosphere, and, although concentrations vary somewhat by region and season (related to seasonal uptake by vegetation), projections of global mean annual concentrations usually suffice for most impact applications. Reference levels of [CO₂] between 300 and 360 ppm have been widely adopted in CO₂-enrichment experiments (Cure and Acock, 1986; Poorter, 1993; see Table 3-2) and in model-based impact studies. [CO₂] has increased rapidly during the 20th century, and plant growth response could be significant for responsive plants, although the evidence for this from long-term observations of plants is unclear because of the confounding effects of other factors such as nitrogen deposition and soil fertility changes (Kirschbaum et al., 1996).

3.4.2.2. Development and Application of [CO₂] Scenarios

Projections of [CO₂] are obtained in two stages: first, the rate of emissions from different sources is evaluated; second, concentrations are evaluated from projected emissions and sequestration of carbon. Because CO₂ is a major greenhouse gas, CO₂ emissions have been projected in successive IPCC scenarios (Scenarios A-D—Shine et al., 1990; IS92 scenarios—Leggett et al., 1992; SRES scenarios—Nakicenovic et al., 2000). To obtain scenarios of future [CO₂] from those of emissions, global models of the carbon cycle are required (e.g., Schimel et al., 1995). Some estimates of [CO₂] for the SRES emissions scenarios are given in Table 3-2.

In recent years, there has been growing interest in emissions scenarios that lead to [CO₂] stabilization (see Section 3.8.4). Typically, levels of [CO₂] stabilized between 350 and 1000 ppm have been examined; these levels usually are achieved during the 22nd or 23rd century, except under the most stringent emissions targets (Schimel et al., 1997a). Work to develop storylines for a set of stabilization scenarios is reported in Chapter 2 of WGIII. Whatever scenarios emerge, it is likely to be some time before a set of derivative CO₂-stabilization impact and adaptation assessments are completed, although a few exploratory studies already have been conducted (UK-DETR, 1999).

Experimental CO₂-enrichment studies conventionally compare responses of an organism for a control concentration representing current [CO₂] with responses for a fixed concentration assumed for the future. In early studies this was most commonly a doubling (Cure and Acock, 1986), to coincide with equilibrium climate model experiments (see Section 3.5). However, more recent transient treatment of future changes, along with the many uncertainties surrounding estimates of future [CO₂] and future climate, present an infinite number of plausible combinations of future conditions. For example, Table 3-2 illustrates the range of [CO₂] projected for 2050 and 2100 under the SRES emissions scenarios, using simple models. To cover these possibilities, although doubled [CO₂] experiments are still common, alternative concentrations also are investigated (Olesen, 1999)—often in combination with a range of climatic conditions, by using devices such as temperature gradient tunnels (Wheeler et al., 1996).

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