Substantial, pre-industrial abundances for CH₄ and N₂O are found in the tiny bubbles of ancient air trapped in ice cores. Both gases have large, natural emission rates, which have varied over past climatic changes but have sustained a stable atmospheric abundance for the centuries prior to the Industrial Revolution (see Figures 4.1 and 4.2). Emissions of CH₄ and N₂O due to human activities are also substantial and have caused large relative increases in their respective burdens over the last century. The atmospheric burdens of CH₄ and N₂O over the next century will likely be driven by changes in both anthropogenic and natural sources. A second class of greenhouse gases – the synthetic HFCs, PFCs, SF₆, CFCs, and halons – did not exist in the atmosphere before the 20th century (Butler et al., 1999). CF₄, a PFC, is detected in ice cores and appears to have an extremely small natural source (Harnisch and Eisenhauer, 1998). The current burdens of these latter gases are derived from atmospheric observations and represent accumulations of past anthropogenic releases; their future burdens depend almost solely on industrial production and release to the atmosphere. Stratospheric H₂O could increase, driven by in situ sources, such as the oxidation of CH₄ and exhaust from aviation, or by a changing climate.

Tropospheric O₃ is both generated and destroyed by photochemistry within the atmosphere. Its in situ sources are expected to have grown with the increasing industrial emissions of its precursors: CH₄, NO_x, CO and VOC. In addition, there is substantial transport of ozone from the stratosphere to the troposphere (see also Section 4.2.4). The effects of stratospheric O₃ depletion over the past three decades and the projections of its recovery, following cessation of emissions of the Montreal Protocol gases, was recently assessed (WMO, 1999).

The current global emissions, mean abundances, and trends of the gases mentioned above are summarised in Table 4.1a. Table 4.1b lists additional synthetic greenhouse gases without established atmospheric abundances. For the Montreal Protocol gases, political regulation has led to a phase-out of emissions that has slowed their atmospheric increases, or turned them into decreases, such as for CFC-11. For other greenhouse gases, the anthropogenic emissions are projected to increase or remain high in the absence of climate-policy regulations. Projections of future emissions for this assessment, i.e., the IPCC Special Report on Emission Scenarios (SRES) (Nakic´enovic´ et al., 2000) anticipate future development of industries and agriculture that represent major sources of greenhouse gases in the absence of climate-policy regulations. The first draft of this chapter and many of the climate studies in this report used the greenhouse gas concentrations derived from the SRES preliminary marker scenarios (i.e., the SRES database as of January 1999 and labelled ‘p’ here). The scenario IS92a has been carried along in many tables to provide a reference of the changes since the SAR. The projections of greenhouse gases and aerosols for the six new SRES marker/illustrative scenarios are discussed here and tabulated in Appendix II.

An important policy issue is the complete impact of different industrial or agricultural sectors on climate. This requires aggregation of the SRES scenarios by sector (e.g., transportation) or sub-sector (e.g., aviation; Penner et al., 1999), including not only emissions but also changes in land use or natural ecosystems. Due to chemical coupling, correlated emissions can have synergistic effects; for instance NO_x and CO from transportation produce regional O₃ increases. Thus a given sector may act through several channels on the future trends of greenhouse gases. In this chapter we will evaluate the data available on this subject in the current literature and in the SRES scenarios.

Table 4.1(a): Chemically reactive greenhouse gases and their precursors: abundances, trends, budgets, lifetimes, and GWPs.
Chemical species	Formula	Abundance a ppt		Trend ppt/yr a	Annual emission	Lifetime	100-yr GWP b
		1998	1750	1990s	late 90s	(yr)
Methane	CH4 (ppb)	1745	700	7.0	600 Tg	8.4/12 c	23
Nitrous oxide	N2O (ppb)	314	270	0.8	16.4 TgN	120/114 c	296
Perfluoromethane	CF4	80	40	1.0	~15 Gg	>50000	5700
Perfluoroethane	C2F6	3.0	0	0.08	~2 Gg	10000	11900
Sulphur hexafluoride	SF6	4.2	0	0.24	~6 Gg	3200	22200
HFC-23	CHF3	14	0	0.55	~7 Gg	260	12000
HFC-134a	CF3CH2F	7.5	0	2.0	~25 Gg	13.8	1300
HFC-152a	CH3CHF2	0.5	0	0.1	~4 Gg	1.40	120
Important greenhouse halocarbons under Montreal Protocol and its Amendments
CFC-11	CFCl3	268	0	-1.4		45	4600
CFC-12	CF2Cl2	533	0	4.4		100	10600
CFC-13	CF3Cl	4	0	0.1		640	14000
CFC-113	CF2ClCFCl2	84	0	0.0		85	6000
CFC-114	CF2ClCF2Cl	15	0	<0.5		300	9800
CFC-115	CF3CF2Cl	7	0	0.4		1700	7200
Carbon tetrachloride	CCl4	102	0	-1.0		35	1800
Methyl chloroform	CH3CCl3	69	0	-14		4.8	140
HCFC-22	CHF2Cl	132	0	5		11.9	1700
HCFC-141b	CH3CFCl2	10	0	2		9.3	700
HCFC-142b	CH3CF2Cl	11	0	1		19	2400
Halon-1211	CF2ClBr	3.8	0	0.2		11	1300
Halon-1301	CF3Br	2.5	0	0.1		65	6900
Halon-2402	CF2BrCF2Br	0.45	0	~ 0		<20
Other chemically active gases dirctly or indirectly affecting radiative forcing
Tropospheric ozone	O3 (DU)	34	25	?	see text	0.01-0.05	-
Tropospheric NOx	NO + NO2	5-999	?	?	~52 TgN	<0.01-0.03	-
Carbon monoxide	CO (ppb)d	80	?	6	~2800 Tg	0.08 - 0.25	d
Stratospheric water	H2O (ppm)	3-6	3-5	?	see text	1-6	-
a All abundances are tropospheric molar mixing ratios in ppt (10 -12 )and trends are in ppt/yr unless superseded by units on line (ppb = 10 -9 , ppm = 10 -6 ). Where possible, the 1998 values are global, annual averages and the trends are calculated for 1996 to 1998. b GWPs are from Chapter 6 of this report and refer to the 100-year horizon values. c Species with chemical feedbacks that change the duration of the atmospheric response; global mean atmospheric lifetime (LT) is given first followed by perturbation lifetime (PT). Values are taken from the SAR (Prather et al., 1995; Schimel et al., 1996) updated with WMO98 (Kurylo and Rodriguez, 1999; Prinn and Zander, 1999) and new OH-scaling, see text. Uncertainties in lifetimes have not changed substantially since the SAR. d CO trend is very sensitive to the time period chosen. The value listed for 1996 to 1998, +6 ppb/yr, is driven by a large increase during 1998. For the period 1991 to 1999, the CO trend was -0.6 ppb/yr. CO is an indirect greenhouse gas: for comparison with CH4 see this chapter; for GWP, see Chapter 6.

Table 4.1(b): Additional synthetic greenhouse gases.
Chemical species	Formula	Lifetime	GWP b
		(yr)
Perfluoropropane	C3F8	2600	8600
Perfluorobutane	C4F10	2600	8600
Perfluorocyclobutane	C4F8	3200	10000
Perfluoropentane	C5F12	4100	8900
Perfluorohexane	C6F14	3200	9000
Trifluoromethyl- sulphur pentafluoride	SF5CF3	1000	17500
Nitrogen trifluoride	NF3	>500	10800
Trifluoroiodomethane	CF3I	<0.005	1
HFC-32	CH2F2	5.0	550
HFC-41	CH3F	2.6	97
HFC-125	CHF2CF3	29	3400
HFC-134	CHF2CHF2	9.6	1100
HFC-143	CH2FCHF2	3.4	330
HFC-143a	CH3CF3	52	4300
HFC-152	CH2FCH2F	0.5	43
HFC-161	CH3CH2F	0.3	12
HFC-227ea	CF3CHFCF3	33	3500
HFC-236cb	CF3CF2CH2F	13.2	1300
HFC-236ea	CF3CHFCHF2	10.0	1200
HFC-236fa	CF3CH2CF3	220	9400
HFC-245ca	CH2FCF2CHF2	5.9	640
HFC-245ea	CHF2CHFCHF2	4.0
HFC-245eb	CF3CHFCH2F	4.2
HFC-245fa	CHF2CH2CF3	7.2	950
HFC-263fb	CF3CH2CH3	1.6
HFC-338pcc	CHF2CF2CF2CF2H	11.4
HFC-356mcf	CF3CF2CH2CH2F	1.2
HFC-356mff	CF3CH2CH2CF3	7.9
HFC-365mfc	CF3CH2CF2CH3	9.9	890
HFC-43-10mee	CF3CHFCHFCF2CF3	15	1500
HFC-458mfcf	CF3CH2CF2CH2CF3	22
HFC-55-10mcff	CF3CF2CH2CH2CF2CF3	7.7
HFE-125	CF3OCHF2	150	14900
HFE-134	CF2HOCF2H	26	2400
HFE-143a	CF3OCH3	4.4	750
HFE-152a	CH3OCHF2	1.5
HFE-245fa2	CHF2OCH2CF3	4.6	570
HFE-356mff2	CF3CH2OCH2CF3	0.4

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