Working Group I: The Scientific Basis


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4.1.1 Sources of Greenhouse Gases

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, NOx, 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 NOx 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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