WGI AR5 Fig2-4

Figure 2.4 Globally averaged dry-air mole fractions at the Earth’s surface of the major halogen-containing well-mixed GHG. These are derived mainly using monthly mean measurements from the AGAGE and NOAA/ESRL/GMD networks. For clarity, only the most abundant chemicals are shown in different compound classes and results from different networks have been combined when both are available.

CFC atmospheric abundances are decreasing (Figure 2.4) because of the successful reduction in emissions resulting from the Montreal Protocol. By 2010, emissions from ODSs had been reduced by ~11 Pg CO₂-eq yr–1, which is five to six times the reduction target of the first commitment period (2008–2012) of the Kyoto Protocol (2 Pg CO₂-eq yr–1) (Velders et al., 2007). These avoided equivalent-CO₂ emissions account for the offsets to RF by stratospheric O3 depletion caused by ODSs and the use of HFCs as substitutes for them. Recent observations in Arctic and Antarctic firn air further confirm that emissions of CFCs are entirely anthropogenic (Martinerie et al., 2009; Montzka et al., 2011b). CFC-12 has the largest atmospheric abundance and GWP-weighted emissions (which are based on a 100-year time horizon) of the CFCs. Its tropospheric abundance peaked during 2000–2004. Since AR4, its global annual mean mole fraction declined by 13.8 ppt to 528.5 ppt in 2011. CFC-11 continued the decrease that started in the mid-1990s, by 12.9 ppt since 2005. In 2011, CFC-11 was 237.7 ppt. CFC-113 decreased by 4.3 ppt since 2005 to 74.3 ppt in 2011. A discrepancy exists between top-down and bottom-up methods for calculating CFC-11 emissions (Montzka et al., 2011b). Emissions calculated using top-down methods come into agreement with bottom-up estimates when a lifetime of 64 years is used for CFC-11 in place of the accepted value of 45 years; this longer lifetime (64 years) is at the upper end of the range estimated by Douglass et al. (2008) with models that more accurately simulate stratospheric circulation. Future emissions of CFCs will largely come from ‘banks’ (i.e., material residing in existing equipment or stores) rather than current production.

The mean decrease in globally, annually averaged carbon tetrachloride (CCl₄) based on NOAA and AGAGE measurements since 2005 was 7.4 ppt, with an atmospheric abundance of 85.8 ppt in 2011 (Table 2.1). The observed rate of decrease and inter-hemispheric difference of CCl₄ suggest that emissions determined from the observations are on average greater and less variable than bottom-up emission estimates, although large uncertainties in the CCl₄ lifetime result in large uncertainties in the top-down estimates of emissions (Xiao et al., 2010; Montzka et al., 2011b). CH3CCl3 has declined exponentially for about a decade, decreasing by 12.0 ppt since 2005 to 6.3 ppt in 2011. HCFCs are classified as ‘transitional substitutes’ by the Montreal Protocol. Their global production and use will ultimately be phased out, but their global production is not currently capped and, based on changes in observed spatial gradients, there has likely been a shift in emissions within the NH from regions north of about 30°N to regions south of 30°N (Montzka et al., 2009). Global levels of the three most abundant HCFCs in the atmosphere continue to increase. HCFC-22 increased by 44.5 ppt since 2005 to 213.3 ppt in 2011. Developed country emissions of HCFC-22 are decreasing, and the trend in total global emissions is driven by large increases from south and Southeast Asia (Saikawa et al., 2012). HCFC-141b increased by 3.7 ppt since 2005 to 21.4 ppt in 2011, and for HCFC-142b, the increase was 5.73 ppt to 21.1 ppt in 2011. The rates of increase in these three HCFCs increased since 2004, but the change in HCFC-141b growth rate was smaller and less persistent than for the other two, which approximately doubled from 2004 to 2007 (Montzka et al., 2009).

In summary, for ODS, whose production and consumption are controlled by the Montreal Protocol, it is certain that the global mean abundances of major CFCs are decreasing and HCFCs are increasing. Atmospheric burdens of CFC-11, CFC-12, CFC-113, CCl₄, CH3CCl3 and some halons have decreased since 2005. HCFCs, which are transitional substitutes for CFCs, continue to increase, but the spatial distribution of their emissions is changing.