Ronald G Prinn

The atmosphere contains a large number of anthropogenic greenhouse gases besides carbon dioxide, which, because of their rising concentrations, have collectively contributed an amount of added radiative forcing comparable to that of CO2 since prein-dustrial times. Many of these non-CO2 gases (e.g., CH4, N2O, CF2Cl2, SF6) are emitted at the surface and contribute directly to this forcing. They are characterized by atmospheric lifetimes of decades to millennia (I define lifetime here as the amount of the gas in the global atmosphere divided by its global rate of removal). Other non-CO2 gases (e.g., isoprene, terpenes, NO, CO, SO2, (CH^S), most of which are also emitted at the surface, contribute indirectly to this forcing through production of either tro-pospheric ozone (which is a powerful greenhouse gas) or tropospheric aerosols (which can directly absorb sunlight or reflect it back to space, or indirectly change the reflection properties of clouds). This second group of climatically important non-CO2 gases is characterized by much shorter lifetimes (hours to months).

To aid the handling of the long-lived non-CO2 gases in the policy processes under the United Nations Framework Convention on Climate Change (UNFCCC), scientists have calculated so-called global warming potentials (GWPs). These dimensionless GWPs are intended to relate the time-integrated radiative forcing of climate by an emitted unit mass of a non-CO2 trace gas to the forcing caused by emission of a unit mass of CO2. The GWP concept has difficulties because the removal mechanisms for many gases (including CO2 itself) have complex interactions involving chemical and/or biological processes, and because the time period (e.g., decade, century) over which one integrates the instantaneous radiative forcing of a gas to compute its GWP is somewhat arbitrary.

A listing of the major non-CO2 greenhouse gases in the atmosphere along with their concentrations, temporal trends, emissions, lifetimes, and GWPs is given in Table 9.1, based on Prather et al. (2001). From this table it is evident that the radiative forcing by many non-CO2 gases is far greater per unit emitted mass than CO2, a characteristic that significantly offsets their much lower emissions relative to CO2. To illustrate this further,

Table 9.1. Chemically reactive greenhouse gases and their precursors: Abundances, trends, budgets, lifetimes, and GWPs (based on Prather et al. 2001)

Trend,

Annual

Abundance

a(ppt)

1990s

emission,

Lifetime

100-year

Chemical species

Formula

1998

1750

(ppt/yr) a

late 1990s

(years)

GWPb

Methane

CH4 (ppb)

1,745

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

>50,000

5,700

Perfluoroethane

C2F6

3.0

0

0.08

~2 Gg

10,000

11,900

Sulpher hexafluoride

SF6

4.2

0

0.24

~6 Gg

3,200

22,200

HFC-23

chf3

14

0

0.55

~7 Gg

260

12,000

HFC-134a

CF3C3H2F

7.5

0

2.0

~25 Gg

13.8

1,300

HFC-152a

CH3CHF2

0.5

0

0.1

~4 Gg

1.40

120

CFC-11

CFCCl3

268

0

-1.4

45

4,600

CFC-12

CF2C3l2

533

0

4.4

100

10,600

CFC-13

CF3Cl

4

0

0.1

640

14,000

CFC-113

CF23ClCFCl2

84

0

0.0

85

6,000

CFC-114

CF2ClCF2Cl

15

0

<0.5

300

9,800

CFC-115

CF3CF2Cl

7

0

0.4

1,700

7,200

Carbon tetrachloride

CC3l4 2

102

0

-1.0

35

1,800

Methyl chloroform

CH3CCl3

69

0

-14

4.8

140

HCFC-22

CHF2Cl

132

0

5

11.9

1,700

HCFC-l4lb

CH3C2 FCl2

10

0

2

9.3

700

HCFC-I42b

CH3CF2Cl

11

0

1

19

2,400

Halon-1211

CF2ClBr

3.8

0

0.2

11

1,300

Halon-1301

CF3Br

2.5

0

0.1

65

6,900

Halon-2402

CF3BrCF2Br

0.45

0

~0

{<20

Other chemically active gases directly or indirectly affecting radiative forcing Tropospheric ozone Tropospheric NOx Carbon monoxide Stratospheric water

a All abundances are tropospheric molar mixing ratios in ppt (10-12 ) and trends are in ppt y-1 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 refer to the 100-year horizon values.

c Species with chemical feedbacks that change the duration of the atmospheric response; global mean atmospheric lifetime is given first followed by perturbation lifetime. Values are taken from the IPCC second assessment report (SAR) (Prather et al. 1995; Schimel et al. 1995) updated with data from Kurylo and Rodriguez (1999 and Prinn and Zander (1999) and new OH-scaling. 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 y-1, is driven by a large increase during 1998. For the period 1991 to 1999, the CO trend was -0.6 ppb y-1. CO is an indirect greenhouse gas.

208 | II. OVERVIEW OF THE CARBON CYCLE

Table 9.2. Current global emissions of non-CO2 greenhouse gases expressed as equivalent amounts of carbon (Ceq) in CO2 using GWPs with a 100-year time horizon

Chemical species Equivalent emissions (PgCeq y-1)

Nitrous oxide (N2O) 2.1

Kyoto flourine gases b 0.1

Carbon monoxide (CO) a,c 1.5

Nitrogen oxides (NO, NO2) d 0.15

Note: Emissions and GWPs from Table 9.1 unless otherwise noted.

aThese equivalent emissions do not include the CO2 produced from oxidation of these gases (0.45 PgC y-1 from CH4 and 1.2 PgC y-1 from CO).

Specifically CF4, C2F6, SF6, CHF3, CF3CH2F, and CH3CHF2 all regulated under the Kyoto Protocol. This list does not include the many ozone-depleting halocarbons already regulated under the Montreal Protocol.

cAssumes a GWP of 2, which is uncertain by at least a factor of 2 (Ramaswamy et al. 2001). CO generally produces O3 and depletes OH.

dAssumes a GWP of 5 for surface emissions, which is also highly uncertain. For aircraft emissions (which are a small fraction of the total), the GWP is about

450 (Ramaswamy et al. 2001). Assumes all emissions of NOx are NO with NO2

subsequently produced in the atmosphere. NOx usually produces O3 and OH.

Table 9.2 shows the current emissions of major non-CO2 gases converted into equivalent emissions of CO2 using their GWPs. These equivalent emissions do not include the CO2 produced from oxidation of CH4 (0.45 PgC y-1) and CO (1.2 PgC y-1), which is usually included in the CO2 carbon cycle. It is very clear that the total equivalent CO2 emissions in Table 9.2 are comparable to the actual total global CO2 emissions. Hence, these non-CO2 gases are very important in all climate change discussions, and methods to lower their emissions need to be carefully evaluated (Robertson, Chapter 29, this volume).

In this chapter the life cycles of key gases (or groups of gases) involved in climate are reviewed followed by a critical discussion of their GWPs and a summary of major unsolved problems. Aerosols are not addressed but have been extensively reviewed by the IPCC (Penner et al. 2001).

Methane

Methane is an important greenhouse substance because of its significant positive trend, its strong absorption of infrared radiation, and the location of its absorption bands at many wavelengths where CO2 and H2O do not absorb (so-called window regions).

The major sources of methane are biological (basically methanogens operating in oxygen-poor environments like natural wetlands, rice paddies, digestive systems of termites and cattle, animal waste treatment, sewage, landfills, etc.). Another (indirect) biological source is biomass burning. Many of these biological sources (rice, cattle, biomass burning, etc.) are governed largely by human activity. Also under dominant human influence is another significant CH4 source, namely escape of this gas during mining of coal and natural gas and leakages in natural gas distribution systems. The fossil sources can be differentiated from the biological sources because of the different carbon isotopic signatures of each, which show that roughly 20 percent of global total methane emissions come from fossil sources. The total methane source is estimated to be about 600 Tg y-1 with 60 percent due to human activities (Kurylo et al. 1999; Prather et al. 2001).

The major sink for methane is its reaction with the OH free radical:

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