2.4.6.2 Scenarios of atmospheric composition

Projections of atmospheric composition account for the concurrent effects of air pollution and climate change, which can be important for human health, agriculture and ecosystems. Scenarios of CO₂ concentration ([CO₂]) are needed in some CCIAV studies, as elevated [CO₂] can affect the acidity of the oceans (IPCC, 2007; Chapter 6, Section 6.3.2) and both the growth and water use of many terrestrial plants (Chapter 4, Section 4.4.1; Chapter 5, Section 5.4.1), with possible feedbacks on regional hydrology (Gedney et al., 2006). CO₂ is well mixed in the atmosphere, so concentrations at a single observing site will usually suffice to represent global conditions. Observed [CO₂] in 2005 was about 379 ppm (Forster et al., 2007) and was projected in the TAR using the Bern-CC model to rise by 2100 to reference, low, and high estimates for the SRES marker scenarios of B1: 540 [486 to 681], A1T: 575 [506 to 735], B2: 611[544 to 769], A1B: 703 [617 to 918], A2: 836 [735 to 1080], and A1FI: 958 [824 to 1248] ppm (Appendix II in IPCC, 2001a). Values similar to these reference levels are commonly adopted in SRES-based impact studies; for example, Arnell et al. (2004) employed levels assumed in HadCM3 AOGCM climate simulations, and Schröter et al. (2005b) used levels generated by the IMAGE-2 integrated assessment model. However, recent simulations with coupled carbon cycle models indicate an enhanced rise in [CO₂] for a given emissions scenario, due to feedbacks from changing climate on the carbon cycle, suggesting that the TAR reference estimates are conservative (Meehl et al., 2007).

Elevated levels of ground-level ozone (O₃) are toxic to many plants (see Chapter 5, Box 5.2) and are strongly implicated in a range of respiratory diseases (Chapter 8, Section 8.2.6). Increased atmospheric concentrations of sulphur dioxide are detrimental to plants, and wet and dry deposition of atmospheric sulphur and nitrogen can lead to soil and surface water acidification, while nitrogen deposition can also serve as a plant fertiliser (Carter et al., 2001; see also Chapter 4, Section 4.4.1; Chapter 5, Section 5.4.3.1). Projections with global atmospheric chemistry models for the high-emissions SRES A2 scenario indicate that global mean tropospheric O₃ concentrations could increase by 20 to 25% between 2015 and 2050, and by 40 to 60% by 2100, primarily as a result of emissions of NO_x, CH₄, CO₂, and compounds from fossil fuel combustion (Meehl et al., 2007). Stricter air pollution standards, already being implemented in many regions, would reduce, and could even reverse, this projected increase (Meehl et al., 2007). Similarly, the range of recent scenarios of global sulphur and NO_x emissions that account for new abatement policies has shifted downwards compared with the SRES emissions scenarios (Smith et al., 2005; Nakićenović et al., 2007).

For the purposes of CCIAV assessment, global projections of pollution are only indicative of local conditions. Levels are highly variable in space and time, with the highest values typically occurring over industrial regions and large cities. Although projections are produced routinely for some regions in order to support air pollution policy using high-resolution atmospheric transport models (e.g., Syri et al., 2004), few models have been run assuming an altered climate, and simulations commonly assume emissions scenarios developed for air pollution policy rather than climate policy (see Alcamo et al., 2002; Nakićenović et al., 2007). Exceptions include regionally explicit global scenarios of nitrogen deposition on a 0.5° latitude * 0.5° longitude grid for studying biodiversity loss in the Millennium Ecosystem Assessment (Alcamo et al., 2005) and simulations based on SRES emissions for sulphur and nitrogen over Europe (Mayerhofer et al., 2002) and Finland (Syri et al., 2004), and for surface ozone in Finland (Laurila et al., 2004).