2.3.3 Other Kyoto Protocol Gases
At the time of the TAR, N2O had the fourth largest RF among the LLGHGs behind CO2, CH4 and CFC-12. The TAR quoted an atmospheric N2O abundance of 314 ppb in 1998, an increase of 44 ppb from its pre-industrial level of around 270 ± 7 ppb, which gave an RF of +0.15 ± 0.02 W m–2. This RF is affected by atmospheric CH4 levels due to overlapping absorptions. As N2O is also the major source of ozone-depleting nitric oxide (NO) and nitrogen dioxide (NO2) in the stratosphere, it is routinely reviewed in the ozone assessments; the most recent assessment (Montzka et al., 2003) recommended an atmospheric lifetime of 114 years for N2O. The TAR pointed out large uncertainties in the major soil, agricultural, combustion and oceanic sources of N2O. Given these emission uncertainties, its observed rate of increase of 0.2 to 0.3% yr–1 was not inconsistent with its better-quantified major sinks (principally stratospheric destruction). The primary driver for the industrial era increase of N2O was concluded to be enhanced microbial production in expanding and fertilized agricultural lands.
Ice core data for N2O have been reported extending back 2,000 years and more before present (MacFarling Meure et al., 2006; Section 6.6). These data, as for CO2 and CH4, show relatively little changes in mixing ratios over the first 1,800 years of this record, and then exhibit a relatively rapid rise (see FAQ 2.1, Figure 1). Since 1998, atmospheric N2O levels have steadily risen to 319 ± 0.12 ppb in 2005, and levels have been increasing approximately linearly (at around 0.26% yr–1) for the past few decades (Figure 2.5). A change in the N2O mixing ratio from 270 ppb in 1750 to 319 ppb in 2005 results in an RF of +0.16 ± 0.02 W m–2, calculated using the simplified expression given in Ramaswamy et al. (2001). The RF has increased by 11% since the time of the TAR (Table 2.1). As CFC-12 levels slowly decline (see Section 2.3.4), N2O should, with its current trend, take over third place in the LLGHG RF ranking.
Figure 2.5. Hemispheric monthly mean N2O mole fractions (ppb) (crosses for the NH and triangles for the SH). Observations (in situ) of N2O from the Atmospheric Lifetime Experiment (ALE) and GAGE (through the mid-1990s) and AGAGE (since the mid-1990s) networks (Prinn et al., 2000, 2005b) are shown with monthly standard deviations. Data from NOAA/GMD are shown without these standard deviations (Thompson et al., 2004). The general decrease in the variability of the measurements over time is due mainly to improved instrumental precision. The real signal emerges only in the last decade.
Since the TAR, understanding of regional N2O fluxes has improved. The results of various studies that quantified the global N2O emissions from coastal upwelling areas, continental shelves, estuaries and rivers suggest that these coastal areas contribute 0.3 to 6.6 TgN yr–1 of N2O or 7 to 61% of the total oceanic emissions (Bange et al., 1996; Nevison et al., 2004b; Kroeze et al., 2005; see also Section 7.4). Using inverse methods and AGAGE Ireland measurements, Manning et al. (2003) estimated EU N2O emissions of 0.9 ± 0.1 TgN yr–1 that agree well with the United Nations Framework Convention on Climate Change (UNFCCC) N2O inventory (0.8 ± 0.1 TgN yr–1). Melillo et al. (2001) provided evidence from Brazilian land use sequences that the conversion of tropical forest to pasture leads to an initial increase but a later decline in emissions of N2O relative to the original forest. They also deduced that Brazilian forest soils alone contribute about 10% of total global N2O production. Estimates of N2O sources and sinks using observations and inverse methods had earlier implied that a large fraction of global N2O emissions in 1978 to 1988 were tropical: specifically 20 to 29% from 0° to 30°S and 32 to 39% from 0° to 30°N compared to 11 to 15% from 30°S to 90°S and 22 to 34% from 30°N to 90°N (Prinn et al., 1990). These estimates were uncertain due to their significant sensitivity to assumed troposphere-stratosphere exchange rates that strongly influence inter-hemispheric gradients. Hirsch et al. (2006) used inverse modelling to estimate significantly lower emissions from 30°S to 90°S (0 to 4%) and higher emissions from 0° to 30°N (50 to 64%) than Prinn et al. (1990) during 1998 to 2001, with 26 to 36% from the oceans. The stratosphere is also proposed to play an important role in the seasonal cycles of N2O (Nevison et al., 2004a). For example, its well-defined seasonal cycle in the SH has been interpreted as resulting from the net effect of seasonal oceanic outgassing of microbially produced N2O, stratospheric intrusion of low-N2O air and other processes (Nevison et al., 2005). Nevison et al. also estimated a Southern Ocean (30°S–90°S) N2O source of 0.9 TgN yr–1, or about 5% of the global total. The complex seasonal cycle in the NH is more difficult to reconcile with seasonal variations in the northern latitude soil sources and stratospheric intrusions (Prinn et al., 2000; T. Liao et al., 2004). The destruction of N2O in the stratosphere causes enrichment of its heavier isotopomers and isotopologues, providing a potential method to differentiate stratospheric and surface flux influences on tropospheric N2O (Morgan et al., 2004).
Human-made PFCs, HFCs and SF6 are very effective absorbers of infrared radiation, so that even small amounts of these gases contribute significantly to the RF of the climate system. The observations and global cycles of the major HFCs, PFCs and SF6 were reviewed in Velders et al. (2005), and this section only provides a brief review and an update for these species. Table 2.1 shows the present mixing ratio and recent trends in the halocarbons and their RFs. Absorption spectra of most halocarbons reviewed here and in the following section are characterised by strongly overlapping spectral lines that are not resolved at tropospheric pressures and temperatures, and there is some uncertainty in cross section measurements. Apart from the uncertainties stemming from the cross sections themselves, differences in the radiative flux calculations can arise from the spectral resolution used, tropopause heights, vertical, spatial and seasonal distributions of the gases, cloud cover and how stratospheric temperature adjustments are performed. IPCC/TEAP (2005) concluded that the discrepancy in the RF calculation for different halocarbons, associated with uncertainties in the radiative transfer calculation and the cross sections, can reach 40%. Studies reviewed in IPCC/TEAP (2005) for the more abundant HFCs show that an agreement better than 12% can be reached for these when the calculation conditions are better constrained (see Section 2.10.2).
The HFCs of industrial importance have lifetimes in the range 1.4 to 270 years. The HFCs with the largest observed mole fractions in 1998 (as reported in the TAR) were, in descending order, HFC-23, HFC-134a and HFC-152a. In 2005, the observed mixing ratios of the major HFCs in the atmosphere were 35 ppt for HFC-134a, 17.5 ppt for HFC-23 (2003 value), 3.7 ppt for HFC-125 and 3.9 ppt for HFC-152a (Table 2.1). Within the uncertainties in calibration and emissions estimates, the observed mixing ratios of the HFCs in the atmosphere can be explained by the anthropogenic emissions. Measurements are available from GMD (Thompson et al., 2004) and AGAGE (Prinn et al., 2000; O’Doherty et al., 2004; Prinn et al., 2005b) networks as well as from University of East Anglia (UEA) studies in Tasmania (updated from Oram et al., 1998; Oram, 1999). These data, summarised in Figure 2.6, show a continuation of positive HFC trends and increasing latitudinal gradients (larger trends in the NH) due to their predominantly NH sources. The air conditioning refrigerant HFC-134a is increasing at a rapid rate in response to growing emissions arising from its role as a replacement for some CFC refrigerants. With a lifetime of about 14 years, its current trends are determined primarily by its emissions and secondarily by its atmospheric destruction. Emissions of HFC-134a estimated from atmospheric measurements are in approximate agreement with industry estimates (Huang and Prinn, 2002; O’Doherty et al., 2004). IPCC/TEAP (2005) reported that global HFC-134a emissions started rapidly increasing in the early 1990s, and that in Europe, sharp increases in emissions are noted for HFC-134a from 1995 to 1998 and for HFC-152a from 1996 to 2000, with some levelling off through 2003. The concentration of the foam blower HFC-152a, with a lifetime of only about 1.5 years, is rising approximately exponentially, with the effects of increasing emissions only partly offset by its rapid atmospheric destruction. Hydrofluorocarbon-23 has a very long atmospheric lifetime (approximately 270 years) and is mainly produced as a by-product of HCFC-22 production. Its concentrations are rising approximately linearly, driven by these emissions, with its destruction being only a minor factor in its budget. There are also smaller but rising concentrations of HFC-125 and HFC-143a, which are both refrigerants.
The PFCs, mainly CF4 (PFC-14) and C2F6 (PFC-116), and SF6 have very large radiative efficiencies and lifetimes in the range 1,000 to 50,000 years (see Section 2.10, Table 2.14), and make an essentially permanent contribution to RF. The SF6 and C2F6 concentrations and RFs have increased by over 20% since the TAR (Table 2.1 and Figure 2.6), but CF4 concentrations have not been updated since 1997. Both anthropogenic and natural sources of CF4 are important to explain its observed atmospheric abundance. These PFCs are produced as by-products of traditional aluminium production, among other activities. The CF4 concentrations have been increasing linearly since about 1960 and CF4 has a natural source that accounts for about one-half of its current atmospheric content (Harnisch et al., 1996). Sulphur hexafluoride (SF6) is produced for use as an electrical insulating fluid in power distribution equipment and also deliberately released as an essentially inert tracer to study atmospheric and oceanic transport processes. Its concentration was 4.2 ppt in 1998 (TAR) and has continued to increase linearly over the past decade, implying that emissions are approximately constant. Its very long lifetime ensures that its emissions accumulate essentially unabated in the atmosphere.