Agriculture Reference
In-Depth Information
Table 8.1
Estimates of the global methane budget
(
Tg CH
4
year
−
1
)
from different
sources and sinks
Reference
Fung
et al
.
(1991)
Hein
et al
.
(1997)
Olivier
et al
.
(1999)
Lelieveld
et al
.
(1998)
Mosier
et al
.
(1998b)
Cao
et al
.
(1998)
Houweling
et al
.
(1999)
Prather
et al
.
(2001)
Base year
1980s
1990
1992
1994
——
1998
—
Natural sources
Wetlands
225
b
115
237
92
145
Termites
20
—
20
20
Oceans
10
—
15
15
Hydrates
5
—
10
—
Anthropogenic
Energy
75
97
109
110
89
Landfills
40
35
36
40
73
Ruminants
80
90
a
93
a
115
89
93
a
a
Waste treatment —
25
14
—
b
Rice agriculture
100
88
60
25-54
53
—
Biomass burning
55
40
23
40
34
40
Other
—
—
—
15
20
Total source
500
587
600
598
Sinks
Tropospheric OH
10
—
30
44
30
30
Stratosphere
450
489
510
506
Soils
—
46
40
40
Total sink
460
535
580
576
a
Waste treatment included under ruminants.
b
Rice included under wetlands.
Source
: adapted from Prather
et al
. (2001).
Estimates of CH
4
emissions from ricefields have improved greatly in the
past decade and the contribution of ricefields to the global CH
4
budget
is far smaller than originally thought (Table 8.2). However, there is still
considerable uncertainty. Recent estimates compiled by the IPCC range from 25
to 60 Tg CH
4
year
−
1
out of a total global emission of about 600 Tg CH
4
year
−
1
(Table 8.1), but credible estimates of less than 10 Tg CH
4
year
−
1
are also made
(Table 8.2). These compare with 100-111 Tg CH
4
year
−
1
from fossil fuels,
80-115 Tg CH
4
year
−
1
from ruminants, and 35-75 Tg CH
4
year
−
1
from landfills.
Rice therefore ranks about fourth among anthropogenic sources of methane.
8.1.2 PROCESSES GOVERNING METHANE EMISSIONS FROM RICE
Reviewers of this topic include Schutz
et al
. (1989a,b), Conrad (1993), Neue
and Roger (1994) and Segers (1998). The rate of emission depends on the linked
rates of CH
4
production, transport and oxidation, which are sensitive to a host
of soil, plant, climate and management variables. Production occurs through
