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OCR for page 79
9
Historical Trends in Atmospheric Methane
Concentration and the
Temperature Sensitivity of Methane Outgassing
from Boreal and Polar Regions
ROBERT C. HARRISS
Langley Research Center
National Aeronautics and Space Administration
Recent studies have documented two trends in atmospheric
methane (CH4) concentrations. First, a modern trend of increas-
ing global atmospheric CH4 has been documented in the trapped gas
in polar ice cores (Craig and Chou, 1982; Khali! and Rasmussen,
1987) and with regular monitoring of ambient CH4 at remote lo-
cations around the world (Steele et al., 1987; Blake and Rowland,
1988~. These data indicate that CH4 has increased from a concen-
tration of approximately 650 parts per billion by volume (ppbv) 200
years ago to 1,690 ppbv in 1988 (Figure ~1~. This recent increase
in atmospheric CH4 over the past several hundred years correlates
with the growth of the human population and industrial society and
is hypothesized to be a result of increased CH4 emissions related
primarily to the expansion of rice agriculture, domestication of rumi-
nant animals, landfi~ling of organic wastes, and the mining and use
of fossil fuels (Ehhalt, 1985; Pearman and Eraser, 1988~.
A second trend from low concentrations of CH4 (350 ppbv) at
times of glacial magnum (approximately 20,000 years B.P.), in-
creasing to 650 ppbv during interglacial times (Figure 9-2) has been
observed in ice core samples from Antarctica and Greenland (Stauf-
fer et al., 1988~. This variability in prehistoric CH4 is hypothesized
to be due to the expansion of arctic and boreal peatiands following
glacial retreat (Harries et al., 1985~. These high-latitude ecosystems
are commonly wetlands with oxygen-deficient, organic-rich soils that
79
OCR for page 80
l
80
1 ,600
>
Q
Q
Q
A_
z
~ 1,200
z
z
800
ROBERT a. HARRISS
.
#~'
+~~ - art;
1600 1700 1800 1900
YEAR
FIGURE 9-1 Atmospheric methane (CH4) variations (in ppbv) over the past
few centuries. Each point represents measurements or (in the earlier centuries)
estimates with error bars. (Adapted from Khalil and Rasmussen, 1987.)
800
> 700
Q
~ 600
c
o
._
c 500
a)
o
In, 400
-
300
200
0 Dye 3
· Byrd Station
:~l
Ti*
. . , . , . , .
1 00 80
60 40 20 0
Age (103 years)
FIGURE 9-2 Atmospheric methane (CH4) variations in glacial and interglacial
times, as determined from ice corings. Abscissa is thousands of years before
present. (Reprinted, by permission, from Stouffer et al., 1988. Copyright (~)1988
by Macmillan Magazines Ltd.)
OCR for page 81
ATMOSPHERIC METHANE
81
emit CH4 as a byproduct of microbial decomposition processes. It has
been estimated that these boreal and arctic peatIands may currently
produce about 60 percent of the global CH4 flux to the atmosphere
from natural wetlands (Matthews and F ung, 1987~.
Methane is also present in large quantities as "frozen" CH4
cIathrates in subsurface sediments of the polar regions, and as
trapped gas in permafrost (Bell, 1982; Revelle, 1983~. If substan-
tial warming of the polar oceans and landscape were to occur, these
"frozen" sources of CH4 would be released to migrate through the soil
and overlying sediments into the atmosphere. However, at present
this issue is highly speculative.
There is little doubt that increasing concentrations of CH4 and
other trace gases can have a profound influence on the earth's at-
mospheric chemistry and on climate (Thompson and Cicerone, 1986;
Ramanathan, 1988~. The question of what influence climate change
has on emissions of trace gases from the global biosphere is less
certain. If emissions of CH4 are enhanced by global warming, a
positive feedback can result, with the increasing concentrations of
atmospheric CH4 further enhancing the tendency for a greenhouse
warming. Such positive feedback could contribute to abrupt climate
changes, resulting in considerable ecological and societal disruption.
At present much less is known about negative feedbacks on climate
warming such as increased evaporation, which would lead to more
clouds and a higher albedo, with subsequent cooling of the earth's
surface. Increased atmospheric water vapor could also lead to higher
concentrations of atmospheric hydroxy} (OH), which destroys CH4,
consequently mitigating the increase in source emissions due to cli-
mate warming.
Very little quantitative information is currently available on the
long-term, integrated response of sources or sinks of atmospheric CH4
to climate change. The data that are available on the temperature
sensitivity of CH4 sources from organic soils and sediments clearly
indicate that CH4 emission rates increase with increasing surface
temperature (Baker-Blocker et al., 1977; Crill et al., 1988~. Fig-
ure ~3 illustrates the response of CH4 emissions to seasonal warming
of boreal peatiand soib in northern Minnesota. From these data it
is reasonable to hypothesize that the initial response to warming of
both natural and anthropogenic organic soils and sediments, which
are the dominant sources of atmospheric CH4, will be an increasing
flux of CH4 to the atmosphere. A large fraction of the worId's old
carbon is in boreal and arctic regions, so these ecosystems are of
OCR for page 82
82
1 000
-
C~
~100
-
X
I
ROBERT C. HARRISS
10
1
OPEN BOG .` .. S /
·
me,.
· ~
5 10 15
SOILTEMPERATURE (°C)
FIGURE 9-3 Effect of soil temperature on methane (CH4) emissions to the
atmosphere from an open bog in the Marcell Experimental Forest, Minnesota.
particular importance to understanding the response of CH4 sources
to climate change. Thus, if the greenhouse warming preclicted by
models (e.g., see Ramanathan, 1988) is realized, it is likely that the
rate of increase in atmospheric CH4 will increase further over the next
few decades clue to enhanced flux from boreal and arctic wetlands,
rice paddy soils, landfills, and other soil and sediment sources.
The mid-term (10- to 100-year) response of CH4 sources to a
greenhouse warming of the earth's surface is impossible to predict at
present. Major scientific issues related to negative feedback mecha-
nisms on the CH4 increase (e.g., increases in atmospheric OH and
drying of wetland soils in major source regions) must be resolved.
Recent advances in environmental measurement technologies and
techniques make possible, for the first time, regional- to global-scale
quantification of biosphere-atmosphere interactions. The uncertain-
ties in how CH4 sources and sinks will respond to future climate
change or to socioeconomic developments that influence CH4 sources
can be reduced by vigorous research programs in global tropospheric
chemistry (NRC, 1984) and earth system science (NRC, 1985~. Spe-
cific research to better understand the importance of increasing at-
mospheric CH4 as a cause and/or consequence of climate change
must include the following components:
OCR for page 83
ATMOSPHERIC METHANE
83
1. The quantification of both natural and anthropogenic CH4
sources must be improved. Such improvements will require integrated
ground, aircraft, and satellite measurements, which will provide ac-
curate estimates of CH4 flux to the atmosphere at regional scales.
Isotope studies using newly developed accelerator mass spectromet-
ric techniques (Lowe et al., 1988), combined with more detailed
temporal resolution from ice core sampling, could resolve the issue
of what contribution fossil sources of CH4 make to the variability in
atmospheric CH4 over long and short time scales.
2. The boreal and arctic regions store much of the earth's
soil carbon in wetlands, which are important sources of atmospheric
CH4. It will be especially important to quantify the temperature sen-
sitivity of the physical and biological processes responsible for CH4
outgassing from these ecosystems, since they are in regions where
climate models predict an enhanced greenhouse warming effect.
3. The impact of increasing concentrations of tropospheric car-
bon monoxide, ozone, and CH4 on global OH distributions can be
resolved with the availability of quantitative source data and the de-
velopment of advanced three-dimensional photochem~cal models for
prediction of OH.
4. The flux of CH4 from the troposphere to the stratosphere can
be quantified with a comprehensive program of aircraft and satellite
measurements. Methane decomposes in the stratosphere to products
such as water vapor that can alter chemical and radiative transfer
processes.
In summary, during the past decade it has become clear that
current changes in the earth's atmospheric composition are global in
scale. Studies on atmospheric CH4 will provide critical information
on how the earth's biosphere and atmosphere will respond to the
global warming that is forecast by climate models to occur in the
next few decades. Such research will also provide the necessary
scientific data to make sound regulatory decisions if policymakers
decide to arrest or reverse the growth of sources of CH4 produced by
human activities.
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Bell, P.R. 1982. Methane hydrate and the carbon dioxide question. In Carbon
Dioxide Review 1982, W.C. Clark (ed.~. Oxford University Press, New
York, pp. 401-406.
OCR for page 84
84
ROBERT a. HARRISS
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Representative terms from entire chapter:
greenhouse warming
