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fossil fuel
Page 91
11
Emission Rates and Concentrations
of Greenhouse Gases
Several atmospheric trace gases are relatively transparent to
incoming solar radiation but absorb, or trap, outbound infrared
radiation emitted from the earth's surface. Water vapor, one of
several naturally occurring greenhouse gases (including CO2, methane (CH4), and nitrous oxide (N2O)), also traps heat by mechanisms
associated with mechanical transfer and convection. The trapping of
heat by these gases is called the "greenhouse effect." This label
emphasizes the similarity of the phenomenon to the warming that
occurs in a greenhouse, although that warming is mostly associated
with blocking convective heat transfer. Without the naturally
occurring greenhouse effect, the earth's average temperature would
be about 33°C (59°F) colder than it is now, and the planet
would be much less suitable for human life. This report is not
concerned with the natural greenhouse effect, but rather with
additional global warming due to human-induced increases in the
atmospheric concentration of greenhouse gases. This enhancement of
the greenhouse effect is called "greenhouse warming." Table 11.1
presents key attributes of the principal greenhouse gases deriving,
in part or entirely, from human activities.
Carbon Dioxide
The increase from 280 ppmv (parts per million by volume) to 354
ppmv in the atmospheric concentration of CO2 since 1800 is well established from ice
core studies (Neftel et al., 1985; Friedli et al., 1986) and direct
atmospheric measurements (Keeling et al., 1989a). The error in the
ice core record is probably not more (and perhaps less) than
± 10 ppmv,1 and the
atmospheric record since 1957 is known to within ±1 ppmv.
The emission of CO2 into the
atmosphere via the burning of fossil fuels is also well
Page 92
TABLE 11.1 Key Greenhouse Gases Influenced by Human
Activity
CO2
CH4
CFC-11
CFC-12
N2O
Preindustrial atmospheric concentration
280 ppmv
0.8 ppmv
0
0
288 ppbv
Current atmospheric concentration (1990)a
354 ppmv
1.71 ppmv
280 pptv
484 pptv
310 ppbv
Current rate of annual atmospheric
accumulationb
1.8 ppmv (0.5%)
0.015 ppmv (0.9%)
9.5 pptv (4%)
17 pptv (4%)
0.8 ppbv (0.25%)
Atmospheric lifetime (years)c
(50–200)
10
65
130
150
NOTE: Ozone has not been included in the table
because of lack of precise data. Here ppmv = parts per million by
volume; ppbv = parts per billion by volume; and pptv = parts per
trillion by volume.
aThe 1990
concentrations have been estimated on the basis of an extrapolation
of measurements reported for earlier years, assuming that the
recent trends remained approximately constant.
bNet
annual emissions of CO2 from the
biosphere not affected by human activity, such as volcanic
emissions, are assumed to be small. Estimates of human-induced
emissions from the biosphere are controversial.
cFor each
gas in the table, except CO2, the
"lifetime" is defined as the ratio of the atmospheric concentration
to the total rate of removal. This time scale also characterizes
the rate of adjustment of the atmospheric concentrations if the
emissions rates are changed abruptly. CO2 is a special case because it is merely
circulated among various reservoirs (atmospheric, ocean, biota).
The "lifetime" of CO2 given in the
table is a rough indication of the time it would take for the
CO2 concentration to adjust to
changes in the emissions.
SOURCE: Adapted from Intergovernmental Panel on
Climate Change (1990). Reprinted by permission of Cambridge
University Press.
quantified over much of the last century (Marland, 1990). Other
anthropogenic input rates of CO2 are
not as well known, but it is unlikely that the total of such input
since 1700 has exceeded one-half of the fossil fuel inputs. The
1990 total of emissions resulting from other human activities, even
with the present rate of tropical deforestation, is about
one-quarter the rate from burning fossil fuels alone.
Page 93
Calculating the effective atmospheric lifetime of the actual
emissions is complex because CO2
molecules are constantly exchanged between the atmosphere, the
oceans, and the biosphere. Although a typical molecule stays in the
atmosphere about 4 years, atmospheric CO2 concentrations vary seasonally and
between the northern and southern hemispheres. However, because of
the exchange with other "sinks," the "lifetime" associated with an
individual CO2 anomaly, or surge in
atmospheric concentration, is 50 to 200 years. In addition, the
total natural flow of CO2 into and
out of the atmosphere is about 30 times greater than that caused by
human activities. This natural carbon cycle, however, was
approximately constant over the 10,000 years prior to the
Industrial Revolution, even though there were changes in the
biosphere—deserts emerged and forests migrated, for example.
What matters for greenhouse warming is the increased concentration
of CO2 (and other greenhouse gases)
in the atmosphere rather than the total annual flow into and out of
the atmosphere.
The increase in atmospheric CO2
concentration since 1860, which is approximately the beginning of
the fossil fuel era, has been slightly more than 60 percent of the
concentration that would be in the atmosphere if all the fossil
fuel emissions had remained in the atmosphere. The results of most
models of the ocean-atmosphere carbon system are consistent with
this approximate 60-40 split of the total fossil fuel emissions.
However, the ocean-atmosphere models cannot reproduce the observed
(from direct atmospheric measurements and the derived
concentrations from ice cores) CO2
increase in this century if one adds to the fossil fuel CO2 emissions the estimated net flux of
CO2 that results from land cover
change. On the other hand, to match the ice core CO2 record for the period 1700 to 1900, it
is necessary to invoke a terrestrial source of CO2 in addition to the fossil fuel
emissions. Improved understanding of various aspects of relevant
phenomena is reported from time to time (e.g., Keeling et al.,
1989a,b; Kirchman et al., 1991). At present, however, there are no
data indicating that much of the missing carbon is in the deep
ocean (to which the transfer of CO2
would be expected to be very slow), nor are there as yet any
observations to suggest that it might have become incorporated into
the soil and/or biomass of the continents.
Although there are substantial uncertainties in our
understanding of the components of the carbon cycle, the
atmospheric concentration of CO2 has
been steadily increasing because of human activities2 in a manner that can be described by
using a simple empirical relationship; e.g.,
increment in atmospheric CO2 at
time t = 60% of accumulated fossil fuel
emissions.
However, there is no real assurance that the nature of the
unknown sink is consistent with such an extrapolation, which
essentially assumes the terrestrial
Page 94
flux from land cover change. This uncertainty is especially
troublesome in discussions of the atmospheric condition at times
when CO2 (or the equivalent in other
greenhouse gases) may have become twice or more the present
value.
In short, there is a clear record of recent increases in
atmospheric CO2, a reasonably good
estimate of the anthropogenic emissions, and a range of CO2 uptake by the oceans with which the
scientific community seems comfortable. However, the panel
concludes that there are no data or compelling hypotheses that
identify sinks and transfer mechanisms that could redress the
observed very large imbalance in this century between the emissions
and the known sinks.
Methane
The atmospheric concentration of CH4 in 1990, about 1.71 ppmv, is more than
double the preindustrial value of about 0.8 ppmv (Craig and Chou,
1982; Blake and Rowland, 1988). Concentrations currently are
increasing at about 0.9 percent per year (Blake and Rowland, 1988).
Two main pathways for CH4 generation
have been identified: (1) reduction of CO2 with hydrogen, fatty acids, or alcohols
as hydrogen donors and (2) transmethylation of acetic acid or
methyl alcohol by CH4-producing
bacteria. The principal phenomena involved include biological
processes in natural wetlands and rice paddies, burning of plant
material in tropical and subtropical regions, consumption of
biomass by termites, anaerobic decay of organic waste in landfills,
and ventilation of coal mines. Additional sources include enteric
fermentation by ruminant animals, leakage from natural gas
pipelines, and venting from oil and gas wells. The amount of
CH4 currently released from these
various sources, however, is not known with precision. The
Intergovernmental Panel on Climate Change (IPCC) estimated 1990
total CH4 releases to be between 400
and 600 teragrams (Tg) per year (Intergovernmental Panel on Climate
Change, 1990).
Global measurements suggest that CH4 concentrations are greatest at latitudes
poleward of 30°N. The approximately 0.14 ppmv greater
concentration of CH4 in the northern
hemisphere over the southern hemisphere is not surprising given
that most CH4 is produced over land
(Wuebbles and Edmonds, 1991).
The major sink for CH4 is
reaction with the hydroxyl radical (OH) in the atmosphere. In
addition, soils may be a sink for CH4, but this has not yet been determined
quantitatively. CH4 has a relatively
short lifetime in the atmosphere of 10 ± 2 years (Prinn et
al., 1987). Recent work on the reaction of OH with CH4 (Vaghjiani and Ravishankara, 1991)
indicates a longer CH4 lifetime (by
about 25 percent) and a smaller inferred flux (about 100 Tg CH4 per year) than was previously estimated.
Reduction in the amount
Page 95
of global atmospheric OH may provide an explanation for 20 to 50
percent of the increase in atmospheric CH4 concentration, but this is highly
uncertain given the unavailability of reliable direct measurements
of tropospheric OH concentrations (Wuebbles and Edmonds, 1991).
Methane emissions from wetlands are sensitive to temperature and
soil moisture (Crutzen, 1989). Future climatic changes could thus
significantly affect the fluxes of CH4 from both natural wetlands and rice
paddies. A highly uncertain but potentially large source of CH4 is the methane hydrates stored in
permafrost sediments, CH4 trapped in
permafrost, and decomposable organic matter frozen in the
permafrost (Cicerone and Oremland, 1988). Future climate warming
could release these sources of CH4,
thereby further adding to the warming. However, the lowering of the
water table in tundra as a result of a warmer, drier climate could
decrease CH4 fluxes into the
atmosphere owing to increased microbial oxidation. This might
provide a negative feedback for atmospheric CH4 (Whalen and Reeburgh, 1990). The net
effect of climatic changes on atmospheric concentrations of CH4 is highly uncertain.
In short, the atmospheric concentration of CH4 has more than doubled over preindustrial
levels. In addition to many natural sources, there are a number of
anthropogenic sources of atmospheric CH4. The extent of relevant human activities
has increased over the last 100 years, and an inferred decrease in
atmospheric OH during this period also could contribute to the
CH4 increase. However, the
quantitative importance of each of the factors contributing to the
observed increase in CH4 is not
known at present.
Halocarbons
With the exception of methyl chloride, most halocarbons come
entirely from human activities. Atmospheric concentrations of the
major anthropogenic halocarbons in 1990 are 280 parts per trillion
by volume (pptv) for CFC-11; 484 pptv for CFC-12; 60 pptv for
CFC-113; and 146 pptv for carbon tetrachloride (CCl4). Their annual rates of increase over
the past few years are 4 percent for CFC-11; 4 percent for CFC-12;
10 percent for CFC-113; and 1.5 percent for CCl4 (Fraser and Derek, 1989;
Intergovernmental Panel on Climate Change, 1990; Wuebbles and
Edmonds, 1991). Concentrations of the fully halogenated
chlorofluorocarbons (CFCs) are slightly greater in the northern
hemisphere than in the southern hemisphere. This fact is consistent
with the geographic distribution of releases (>90 percent from
industrialized nations), a 45°N to 45°S mixing time of
about 1 year, and the very long atmospheric lifetimes of these
CFCs.
Halocarbons are used as aerosol propellants (CFCs 11, 12, and
114), foam blowing agents (CFCs 11 and 12), solvents (CFC 113,
methyl chloroform (CH3CCl3), and CCl4), and fire retardants (halons 1211 and
1301).
Page 96
Methyl chloride is primarily released from the oceans and during
biomass burning.
There is no significant tropospheric removal mechanism for the
fully halogenated halocarbons such as CFCs 11, 12, 113, 114, and
115 or for halon-1301. They are primarily removed by
photodissociation in the middle to upper stratosphere. The fully
halogenated halocarbons therefore have long atmospheric lifetimes,
ranging from about 60 years (CFC-11) to about 400 years
(CFC-115).
Emissions of many halocarbons are governed by the Montreal
Protocol on Substances That Deplete the Ozone Layer. The Protocol
specifies that the production and consumption of CFCs 11, 12, 113,
114, and 115 be limited to 50 percent of their 1986 levels by 1998
and of halons to their 1986 levels after 1994. The London Protocol,
adopted in June 1990, requires a total phaseout of CFCs, halons,
and CCl4 by 2000 in industrialized
countries and by 2010 in developing countries. Emissions of these
gases will continue after their production has ceased, however,
because of their presence in refrigerants, foams, fire retardants,
and so on.
Nitrous Oxide
The 1990 average atmospheric concentration of nitrous oxide
(N2O) is about 310 ppbv and is
increasing at a rate of about 0.2 to 0.3 percent per year (Khalil
and Rasmussen, 1988). Ice core data suggest that preindustrial
atmospheric concentrations of N2O
were about 288 ppbv (Khalil and Rasmussen, 1988). The atmospheric
concentration of N2O is about 1 ppbv
higher in the northern hemisphere than in the southern hemisphere
(Rasmussen and Khalil, 1986).
Denitrification in aerobic soils is thought to be a dominant
source of atmospheric N2O (Keller et
al., 1986). There is, however, considerable uncertainty in the
source budget for N2O and in the
causes for its increasing concentration in the atmosphere. The two
possible sources that most likely account for the observed increase
are (1) nitrification and denitrification of nitrogen from
industrially produced fertilizers and (2) high-temperature
combustion, such as from coal-burning power plants (Wuebbles and
Edmonds, 1991). Some results are contradictory, however, as to
whether combustion is the major source or totally unimportant (Hao
et al., 1987; Muzio and Kramlich, 1988). The oceans also are a
significant, although probably not dominant, source of N2O (McElroy and Wofsy, 1986). An accurate
determination of the global annual ocean flux is difficult because
of uncertainties associated with quantifying the gas exchange
coefficient and because the partial pressure of N2O in the surface waters varies
considerably, ranging from being supersaturated by up to 40 percent
in upwelling regions to being undersaturated by a few percent in
areas around Antarctica and in upwelling
Page 97
currents (Cline et al., 1987). It is still unclear whether
N2O is primarily produced from
nitrification in near-surface waters or denitrification in
oxygen-deficient deep waters. Quantification of global N2O emissions from soils is difficult
because of the heterogeneity of terrestrial ecosystems and the
variability in environmental conditions. N2O emissions from the use of nitrate and
ammonium fertilizers are difficult to quantify because the N2O fluxes depend on numerous factors,
including type of fertilizer, soil type, soil temperature, weather,
and farming practices. Conversion of nitrogen to N2O ranges from 0.01 to 2 percent (Conrad
et al., 1983). Leaching of nitrogen fertilizers into groundwater
may result in additional fluxes of N2O into the atmosphere.
The major atmospheric process removing N2O is photochemical decomposition in the
stratosphere. N2O is not chemically
active in the troposphere. N2O has a
lifetime in the atmosphere of about 150 years.
In short, we know that the atmospheric concentration of N2O is now about 8 percent higher than in
the preindustrial period. N2O has a
relatively long atmospheric lifetime. It is difficult to account
for the source of the observed increase in atmospheric N2O, but it is thought to be anthropogenic.
The observed rate of growth indicates a 30 percent imbalance
between sources and sinks (Hao et al., 1987).
Notes
1. Some believe the error for concentrations derived from ice
core data may be as large as ±20 ppmv.
2. That human activities are causing the increasing atmospheric
concentration of CO2 is known from
carbon isotope studies.
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