Figure 18: Atmospheric concentrations of CO2, CH4 and N2O resulting from the six SRES scenarios and from the IS92a scenario computed with current methodology. [Based on Figures 3.12 and 4.14]

Measures to enhance carbon storage in terrestrial ecosystems could influence atmospheric CO₂ concentration, but the upper bound for reduction of CO₂ concentration by such means is 40 to 70 ppm. If all the carbon released by historic land-use changes could be restored to the terrestrial biosphere over the course of the century (e.g., by reforestation), CO₂ concentration would be reduced by 40 to 70 ppm. Thus, fossil fuel CO₂ emissions are virtually certain to remain the dominant control over trends in atmospheric CO₂ concentration during this century.

Model calculations of the abundances of the primary non-CO₂ greenhouse gases by the year 2100 vary considerably across the six illustrative SRES scenarios. In general A1B, A1T and B1 have the smallest increases, and A1FI and A2, the largest. The CH₄ changes from 1998 to 2100 range from -190 to +1970 ppb (-11 to +112%), and N₂O increases from +38 to +144 ppb (+12 to +46%) (see Figures 17b and c). The HFCs (134a, 143a, and 125) reach abundances of a few hundred to a thousand ppt from negligible levels today. The PFC CF4 is projected to increase to 200 to 400 ppt, and SF₆ is projected to increase to 35 to 65 ppt.

For the six illustrative SRES emissions scenarios, projected emissions of indirect greenhouse gases (NO_x, CO, VOC), together with changes in CH₄, are projected to change the global mean abundance of the tropospheric hydroxyl radical (OH), by -20% to +6% over the next century. Because of the importance of OH in tropospheric chemistry, comparable, but opposite sign, changes occur in the atmospheric lifetimes of the greenhouse gases CH₄ and HFCs. This impact depends in large part on the magnitude of and the balance between NO_x and CO emissions. Changes in tropospheric O₃ of –12 to +62% are calculated from 2000 until 2100. The largest increase predicted for the 21st century is for scenarios A1FI and A2 and would be more than twice as large as that experienced since the Pre-industrial Era. These O₃ increases are attributable to the concurrent and large increases in anthropogenic NO_x and CH₄ emissions.

The large growth in emissions of greenhouse gases and other pollutants as projected in some of the six illustrative SRES scenarios for the 21st century will degrade the global environment in ways beyond climate change. Changes projected in the SRES A2 and A1FI scenarios would degrade air quality over much of the globe by increasing background levels of tropospheric O₃. In northern mid-latitudes during summer, the zonal average of O₃ increases near the surface are about 30 ppb or more, raising background levels to about 80 ppb, threatening the attainment of current air quality standards over most metropolitan and even rural regions and compromising crop and forest productivity. This problem reaches across continental boundaries and couples emissions of NO_x on a hemispheric scale.

Figure 19: Simple model results: estimated historical anthropogenic radiative forcing up to the year 2000 followed by radiative forcing for the six illustrative SRES scenarios. The shading shows the envelope of forcing that encompasses the full set of thirty five SRES scenarios. The method of calculation closely follows that explained in the chapters. The values are based on the radiative forcing for a doubling of CO2 from seven AOGCMs. The IS92a, IS92c, and IS92e forcing is also shown following the same method of calculation. [Based on Figure 9.13a]

Except for sulphate and black carbon, models show an approximately linear dependence of the abundance of aerosols on emissions. The processes that determine the removal rate for black carbon differ substantially between the models, leading to major uncertainty in the future projections of black carbon. Emissions of natural aerosols such as sea salt, dust, and gas phase precursors of aerosols such as terpenes, sulphur dioxide (SO₂), and dimethyl sulphide oxidation may increase as a result of changes in climate and atmospheric chemistry.

The six illustrative SRES scenarios cover nearly the full range of forcing that results from the full set of SRES scenarios. Estimated total historical anthropogenic radiative forcing from 1765 to 1990 followed by forcing resulting from the six SRES scenarios are shown in Figure 19. The forcing from the full range of 35 SRES scenarios is shown on the figure as a shaded envelope, since the forcings resulting from individual scenarios cross with time. The direct forcing from biomass-burning aerosols is scaled with deforestation rates. The SRES scenarios include the possibility of either increases or decreases in anthropogenic aerosols (e.g., sulphate aerosols, biomass aerosols, and black and organic carbon aerosols), depending on the extent of fossil fuel use and policies to abate polluting emissions. The SRES scenarios do not include emissions estimates for non-sulphate aerosols. Two methods for projecting these emissions were considered in this report: the first scales the emissions of fossil fuel and biomass aerosols with CO while the second scales the emissions with SO₂ and deforestation. Only the second method was used for climate projections. For comparison, radiative forcing is also shown for the IS92a scenario. It is evident that the range for the new SRES scenarios is shifted higher compared to the IS92 scenarios. This is mainly due to the reduced future SO₂ emissions of the SRES scenarios compared to the IS92 scenarios, but also to the slightly larger cumulative carbon emissions featured in some SRES scenarios.

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