Climate Change 2001: Working Group I: The Scientific Basis

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4.6 Overall Impact of Global Atmospheric Chemistry Change

The projected growth in emissions of greenhouse gases and other pollutants in the IPCC SRES scenarios for the 21st century is expected to increase the atmospheric burden of non-CO₂ greenhouse gases substantially and contribute a sizable fraction to the overall increase in radiative forcing of the climate. These changes in atmospheric composition may, however, degrade the global environment in ways beyond climate change.

The impact of metropolitan pollution, specifically O₃ and CO, on the background air of the Atlantic and Pacific Oceans has been highlighted by many studies over the past decade. These have ranged from observations of anthropogenic pollution reaching across the Northern Hemisphere (e.g., Parrish et al., 1993; Jaffe et al., 1999) to analyses of rapidly increasing emissions of pollutants (NO_x, CO, VOC) in, for example, East Asia (Kato and Akimoto 1992; Elliott et al., 1997). CTM studies have tried to quantify some of these projections for the near term: Berntsen et al. (1999) predict notable increases in CO and O₃ coming into the north-west USA from a doubling of current Asian emissions; Jacob et al. (1999) calculate that monthly mean O₃ abundances over the USA will increase by 1 to 6 ppb from a tripling of these emissions between 1985 and 2010; and Collins et al. (2000) project a 3 ppb increase from 1990 to 2015 in monthly mean O₃ over north-west Europe due to rising North American emissions. The impact of metropolitan pollution will expand over the coming decades as urban areas grow and use of resources intensifies.

What is new in this IPCC assessment is the extension of these projections to the year 2100, whereupon the cumulative impact of all Northern Hemisphere emissions, not just those immediately upwind, may for some scenarios double O₃ abundances over the northern mid-latitudes. Surface O₃ abundances during July over the industrialised continents of the Northern Hemisphere are about 40 ppb with 2000 emissions; and under SRES scenarios A2 and A1FI they would reach 45 to 50 ppb with 2030 emissions, 60 ppb with 2060 emissions, and >70 ppb with 2100 emissions. Since regional ozone episodes start with these background levels and build upon them with local smog production, it may be impossible under these circumstances to achieve a clean-air standard of <80 ppb over most populated regions. This problem reaches across continental boundaries and couples emissions of NO_x on a hemispheric scale. In the 21st century a global perspective will be needed to meet regional air quality objectives. The impact of this threatened degradation of air quality upon societal behaviour and policy decisions will possibly change the balance of future emissions impacting climate change (e.g., more fuel burn (CO₂) to achieve lower NO_x as in aviation; Penner et al., 1999).

Under some emission scenarios, the large increases in tropo-spheric O₃ combined with the decreases in OH may alter the oxidation rate and the degradation paths for hydrocarbons and other hazardous substances. The damage caused by higher O₃ levels to both crops and natural systems needs to be assessed, and societal responses to this threat would likely change the emissions scenarios evaluated here (e.g., the current SRES scenarios anticipate the societal demand to control urban aerosols and acid rain by substantially cutting sulphur emissions).

Coupling between atmospheric chemistry, the biosphere, and the climate are not at the stage that these feedbacks can be included in this assessment. There are indications, however, that the evolution of natural emissions and physical climate projected over the next century will change the baseline atmospheric chemistry and lead to altered biosphere-atmosphere exchanges and continued atmospheric change independent of anthropogenic emissions.