4.2 Trace Gases: Current Observations, Trends, and Budgets
4.2.1 Non-CO2 Kyoto Gases
4.2.1.1 Methane (CH4)
Methane’s globally averaged atmospheric surface abundance in 1998 was 1,745
ppb (see Figure 4.1), corresponding to a total burden of
about 4,850 Tg(CH4). The uncertainty in the burden is small (±5%)
because the spatial and temporal distributions of tropospheric and stratospheric
CH4 have been determined by extensive high-precision measurements
and the tropospheric variability is relatively small. For example, the Northern
Hemisphere CH4 abundances average about 5% higher than those in the
Southern Hemisphere. Seasonal variations, with a minimum in late summer, are
observed with peak-to-peak amplitudes of about 2% at mid-latitudes. The average
vertical gradient in the troposphere is negligible, but CH4 abundances
in the stratosphere decrease rapidly with altitude, e.g., to 1,400 ppb at 30
km altitude in the tropics and to 500 ppb at 30 km in high latitude northern
winter.
|
Figure 4.1: (a) Change in CH4 abundance (mole fraction,
in ppb = 10-9) determined from ice cores, firn, and whole air
samples plotted for the last 1,000 years. Data sets are as follows: Grip,
Blunier et al. (1995) and Chappellaz et al. (1997); Eurocore, Blunier
et al. (1993); D47, Chappellaz et al. (1997); Siple, Stauffer et al. (1985);
Global (inferred from Antarctic and Greenland ice cores, firn air, and
modern measurements), Etheridge et al. (1998) and Dlugokencky et al. (1998).
Radiative forcing, approximated by a linear scale since the pre-industrial
era, is plotted on the right axis.
(b) Globally averaged CH4 (monthly varying) and deseasonalised
CH4 (smooth line) abundance plotted for 1983 to 1999 (Dlugokencky
et al., 1998). (c) Instantaneous annual growth rate (ppb/yr) in global
atmospheric CH4 abundance from 1983 through 1999 calculated
as the derivative of the deseasonalised trend curve above (Dlugokencky
et al., 1998). Uncertainties (dotted lines) are ±1 standard deviation.
(d) Comparison of Greenland (GRIP) and Antarctic (D47 and Byrd) CH4
abundances for the past 11.5 kyr (Chappellaz et al., 1997). The shaded
area is the pole-to-pole difference where Antarctic data exist. (e) Atmospheric
CH4 abundances (black triangles) and temperature anomalies
with respect to mean recent temperature (grey diamonds) determined for
the past 420 kyr from an ice core drilled at Vostok Station in East Antarctica
(Petit et al., 1999).
|
The most important known sources of atmospheric methane are listed in Table
4.2. Although the major source terms of atmospheric CH4 have
probably been identified, many of the source strengths are still uncertain due
to the difficulty in assessing the global emission rates of the biospheric sources,
whose strengths are highly variable in space and time: e.g., local emissions
from most types of natural wetland can vary by a few orders of magnitude over
a few metres. Nevertheless, new approaches have led to improved estimates of
the global emissions rates from some source types. For instance, intensive studies
on emissions from rice agriculture have substantially improved these emissions
estimates (Ding and Wang, 1996; Wang and Shangguan, 1996). Further, integration
of emissions over a whole growth period (rather than looking at the emissions
on individual days with different ambient temperatures) has lowered the estimates
of CH4 emissions from rice agriculture from about 80 Tg/yr to about
40 Tg/yr (Neue and Sass, 1998; Sass et al., 1999). There have also been attempts
to deduce emission rates from observed spatial and temporal distributions of
atmospheric CH4 through inverse modelling (e.g., Hein et al., 1997;
Houweling et al., 1999). The emissions so derived depend on the precise knowledge
of the mean global loss rate and represent a relative attribution into aggregated
sources of similar properties. The results of some of these studies have been
included in Table 4.2. The global CH4 budget
can also be constrained by measurements of stable isotopes (
13C
and D)
and radiocarbon (14CH4) in atmospheric CH4
and in CH4 from the major sources (e.g., Stevens and Engelkemeir,
1988; Wahlen et al., 1989; Quay et al., 1991, 1999; Lassey et al., 1993; Lowe
et al., 1994). So far the measurements of isotopic composition of CH4
have served mainly to constrain the contribution from fossil fuel related sources.
The emissions from the various sources sum up to a global total of about 600
Tg/yr, of which about 60% are related to human activities such as agriculture,
fossil fuel use and waste disposal. This is consistent with the SRES estimate
of 347 Tg/yr for anthropogenic CH4 emissions in the year 2000.
The current emissions from CH4 hydrate deposits appear small, about
10 Tg/yr. However, these deposits are enormous, about 107 TgC (Suess
et al., 1999), and there is an indication of a catastrophic release of a gaseous
carbon compound about 55 million years ago, which has been attributed to a large-scale
perturbation of CH4 hydrate deposits (Dickens, 1999; Norris and Röhl,
1999). Recent research points to regional releases of CH4 from clathrates
in ocean sediments during the last 60,000 years (Kennett et al., 2000), but
much of this CH4 is likely to be oxidised by bacteria before reaching
the atmosphere (Dickens, 2001). This evidence adds to the concern that the expected
global warming may lead to an increase in these emissions and thus to another
positive feedback in the climate system. So far, the size of that feedback has
not been quantified. On the other hand, the historic record of atmospheric CH4
derived from ice cores (Petit et al., 1999), which spans several large temperature
swings plus glaciations, constrains the possible past releases from methane
hydrates to the atmosphere. Indeed, Brook et al. (2000) find little evidence
for rapid, massive CH4 excursions that might be associated with large-scale
decomposition of methane hydrates in sediments during the past 50,000 years.
| Table 4.2: Estimates of the global methane budget
(in Tg(CH4)/yr) from different sources compared with the values
adopted for this report (TAR). |
 |
| Reference: |
Fung et al. |
Hein et al. |
Lelieveld et al. |
Houweling et al. |
Mosier et al. |
Olivier et al. |
Cao et al. |
SAR |
TARa |
| |
(1991) |
(1997) |
(1998) |
(1999) |
(1998a) |
(1999) |
(1998) |
|
|
| Base year: |
1980s |
- |
1992 |
- |
1994 |
1990 |
- |
1980s |
1998 |
|
| Natural sources |
|
|
|
|
|
|
|
|
|
| Wetlands |
115
|
237
|
225c
|
145
|
|
|
92
|
|
|
| Termites |
20
|
-
|
20
|
20
|
|
|
|
|
|
| Ocean |
10
|
-
|
15
|
15
|
|
|
|
|
|
| Hydrates |
5
|
-
|
10
|
-
|
|
|
|
|
|
| Anthropogenic sources |
|
|
|
|
|
|
|
|
|
| Energy |
75
|
97
|
110
|
89
|
|
109
|
|
|
|
| Landfills |
40
|
35
|
40
|
73
|
|
36
|
|
|
|
| Ruminants |
80
|
90b
|
115
|
93
|
80
|
93b
|
|
|
|
| Waste treatment |
-
|
b
|
25
|
-
|
14
|
b
|
|
|
|
| Rice agriculture |
100
|
88
|
c
|
-
|
25-54
|
60
|
53
|
|
|
| Biomass burning |
55
|
40
|
40
|
40
|
34
|
23
|
|
|
|
| Other |
-
|
-
|
-
|
20
|
15
|
|
|
|
|
| Total source |
500
|
587
|
600
|
|
|
|
|
597
|
598
|
|
| Imbalance (trend) |
|
|
|
|
|
|
|
+37
|
+22
|
|
| Sinks |
|
|
|
|
|
|
|
|
|
| Soils |
10
|
-
|
30
|
30
|
44
|
|
|
30
|
30
|
| Tropospheric OH |
450
|
489
|
510
|
|
|
|
|
490
|
506
|
| Stratospheric loss |
-
|
46
|
40
|
|
|
|
|
40
|
40
|
| Total sink |
460
|
535
|
580
|
|
|
|
|
560
|
576
|
|
The mean global loss rate of atmospheric CH4 is dominated by its
reaction with OH in the troposphere.
OH + CH4 
CH3 + H2O
This loss term can be quantified with relatively good accuracy based on the
mean global OH concentration derived from the methyl chloroform (CH3CCL3) budget
described in Section 4.2.6 on OH. In that way we obtain
a mean global loss rate of 507 Tg(CH4)/yr for the current tropospheric
removal of CH4 by OH. In addition there are other minor removal processes
for atmospheric CH4. Reaction with Cl atoms in the marine boundary
layer probably constitutes less than 2% of the total sink (Singh et al., 1996).
A recent process model study (Ridgwell et al., 1999) suggested a soil sink of
38 Tg/yr, and this can be compared to SAR estimates of 30 Tg/yr. Minor amounts
of CH4 are also destroyed in the stratosphere by reactions with OH,
Cl, and O(1D), resulting in a combined loss rate of 40 Tg/yr. Summing
these, our best estimate of the current global loss rate of atmospheric CH4
totals 576 Tg/yr (see Table 4.2), which agrees reasonably
with the total sources derived from process models. The atmospheric lifetime
of CH4 derived from this loss rate and the global burden is 8.4 years.
Attributing individual lifetimes to the different components of CH4
loss results in 9.6 years for loss due to tropospheric OH, 120 years for stratospheric
loss, and 160 years for the soil sink (i.e., 1/8.4 yr = 1/9.6 yr + 1/120 yr
+ 1/160 yr).
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