Climate Change 2001: The Scientific Basis

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(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

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 NO_x	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.

4.1.1 Sources of Greenhouse Gases