IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change Non- CO2 gases

Methane. Atmospheric CH4 concentrations have increased throughout most of the 20th century, but growth rates have been close to zero over the 1999–2005 period (Solomon et al., 2007; 2.1.1) due to relatively constant emissions during this period equaling atmospheric removal rates (Solomon et al., 2007; 2.1.1). Human emissions continue to dominate the total CH4 emissions budget (Solomon et al., 2007; 7.4.1). Agriculture and forestry developments are assessed in Chapters 8 and 9, respectively, in terms of their impact on the CH4 sink/source balance and mitigation strategies; waste handling is likewise assessed in Chapter 10.

The future increase in CH4 concentrations up to 2030 according to the SRES scenarios ranges from 8.1 GtCO2-eq to 10.3 GtCO2-eq (increase of 19–51% compared to 2000), and the increase under the Energy Modeling Forum (EMF)-21 baseline scenarios is quite similar (7.5 GtCO2-eq to 11.3 GtCO2-eq/yr). By 2100, the projected SRES increase in CH4 concentrations ranges from 5 GtCO2-eq to 18.7 GtCO2-eq (a change of –27% to +175% compared to 2000) and that of the EMF-21 ranges from 5.9 to 29.2 GtCO2-eq (a change of –2% to +390%).

Montreal gases. Emissions of ODS gases (also GHGs) controlled under the Montreal Protocol (CFCs, HCFCs) increased from a very low amount during the 1950–1960s to a substantial percentage – approximately 20% – of total GHG emissions by 1975. This percentage fluctuated slightly during the period between 1975 and 1989, but once the phase-out of CFCs was implemented, the ODS share in total GHG emissions fell rapidly, first to 8% (1995) and then to 4% (2000). Radiative forcing from these gases peaked in 2003 and is beginning to decline (Forster et al., 2007).

After 2000, ODS contributed 3–4% to total GHG emissions (Olivier et al., 2005, 2006). The ODS share is projected to decrease yet further due to the CFC phase-out in developing countries. Emissions of ODS are estimated at 0.5–1.15 Gt CO2-eq for the year 2015, dependent on the scenario chosen (IPCC, 2005); this would be about 1–2% of total GHG emissions for the year 2015, if emissions of all other GHGs are estimated at about 55 Gt CO2-eq (for the year 2015). The percentage of HCFC emissions in the total of CFC and HCFC emissions for the year 2015 is projected to be about 70%, independent of the scenario chosen.

Nitrous oxide. Atmospheric concentrations of N2O have been continuously increasing at an approximately constant growth rate since 1980 (IPCC, 2007a, SPM). Industrial sources, agriculture, forestry and waste developments are assessed in this report in terms of their impact on the N2O sink/source balance and mitigation strategies. The SRES emissions for 2030 range from 3 GtCO2-eq to 5.3 GtCO2-eq (a change of –13% to 55% compared to 2000). For comparison, the recent EMF-21 baseline range for 2030 is quite close to this (2.8 GtCO2-eq to 5.4 GtCO2-eq, an increase of –17% to 58% compared to 2000). By 2100, the range projected by the SRES scenarios is 2.6 GtCO2-eq to 8.1 GtCO2-eq (an increase of –23% to 140% compared to 2000), whereas the EMF-21 range is a little higher (3.2 GtCO2-eq to 11.5 GtCO2-eq, or an increase of –5% to 240% compared to 2000).

Fluorinated gases. Concentrations of many of these gases have increased by large factors (i.e., 1.3 and 4.3) between 1998 and 2005, and their radiative forcing is rapidly increasing (from low levels) by roughly 10% per year (Forster et al., 2007). Any projection of overall environmental impacts and emissions is complicated by the fact that several major applications retain the bulk of their fluorinated gases during their respective life cycles, resulting in the accumulation of significant stocks that need to be responsibly managed when these applications are eventually decommissioned. A comprehensive review of such assessments was published in an earlier IPCC Special Report (IPCC, 2005). This review reported growth in HFC emissions from about 0.4 GtCO2-eq in 2002 to 1.2 GtCO2-eq per year in 2015. Chapter 3 also describes in some detail the results of long-term GHG emissions scenarios. The range projected by SRES scenarios for 2030 is 1.0–1.6 GtCO2-eq (increase of 190–360% compared to 2000) and the EMF-21 baseline scenarios are quite close to this (1.2–1.7 GtCO2-eq per year, an increase of 115–240% compared to 2000). By 2100, the SRES range is 1.4–4 GtCO2-eq per year (an increase of 300% to more than 1000 % compared to 2000), whereas the new EMF-21 baseline scenarios are higher still (1.9–6.3 GtCO2-eq).

Air pollutants and other radiative substances. As noted above, some air pollutants, such as sulphur aerosol, have a significant effect on the climate system, although considerable uncertainties still surround the estimates of anthropogenic aerosol emissions. Data on non-sulphur aerosols are sparse and highly speculative, but in terms of global sulphur emissions, these appear to have declined from a range of 75 ± 10 MtS in 1990 to 55–62 MtS in 2000. Sulphur emissions from fossil fuel combustion lead to the formation of aerosols that affect regional climate and precipitation patterns and also reduce radiative forcing. There has been a slowing in the growth of sulphur emissions in recent decades, and more recent emission scenarios show lower emissions than earlier ones (Chapter 3, Section 3.2.2). Other air pollutants, such as NOx and black and organic carbon, are also important climatologically and adversely affect human health. The likely future development of these emissions is described in Section 3.2.2.