| Overview | Energy Sources | Waste Management | Agricultural Sources | Industrial Processes | Methane Data Tables |
Estimated U.S. anthropogenic methane emissions totaled 30.9 million metric tons in 1996, virtually unchanged from 1995 levels but well below the 31.6 million metric tons emitted in 1990 (Table 16). Methane emissions are dwarfed by carbon dioxide emissions when measured as tons of native gas emitted (a ratio of 1 ton of methane for every 175 tons of carbon dioxide). The overall effect of methane on global climate is substantial, however, because the capacity of methane to trap heat in the atmosphere is estimated to be 21 times greater than that of carbon dioxide. Methane accounts for 10 percent of U.S. anthropogenic greenhouse gas emissions when weighted for global warming potential (see discussion in Chapter 1).
In comparison with estimates of carbon dioxide emissions, which are likely to be accurate
to within 3 to 5 percent, estimates of methane emissions carry significant uncertainty (see Appendix C). Most carbon
dioxide emissions are the result of fossil fuel combustion for energy production. For economic reasons, energy
consumption in the United States is carefully accounted for, and thus emissions estimates can be derived in a fairly
direct manner from well-documented data on fuel shipments. In contrast, methane emissions usually are accidental
or incidental to biological processes and are not metered in any systematic way.(37) Thus, emissions are difficult to
calculate, and estimates often require proxy measurements. For example, emissions from landfills are typically
estimated by using models developed from a combination of methane recovery data and laboratory experiments on
the methane yield of waste decomposition.
There are three principal sources of U.S. methane emissions: energy production and consumption, waste management,
and agriculture (Figure 7).
Emissions from energy sources represent just under 38 percent of all methane emissions, having increased by 430,000 metric tons
between 1995 and 1996, largely as the result of a rebound in natural gas vented at oil wells after a sharp 1-year drop
in 1995. Emissions from waste management, which account for about one-third of the total, have declined steadily
between 1990 and 1996 (Table 16) as the amount of waste reaching landfills has decreased and the volume of methane
recovered has increased. All but a very small portion of the remaining emissions can be attributed to agricultural
sources. More than 90 percent of emissions from agricultural sources result from animal husbandry, with about two-thirds of that share traced to enteric fermentation and one-third emitted from animal wastes. Between 1990 and 1994,
emissions from animal sources grew steadily as animal populations and animal sizes increased. More recently,
emissions from this source have begun to recede, with animal populations declining in response to market conditions.
Methane emissions from energy sources in 1996 were estimated at 11.6 million metric tons, up from 11.1 million metric
tons in 1995 yet nearly 500,000 metric tons below 1990 levels. The 1996 increase resulted froma large rise in gas vented
at oil wells during the year. This jump offsets a 1995 drop that appears to have been an isolated incident and thus does
not reflect a fundamental change in the long-term emissions trend. Emissions from energy sources are likely to remain
below 1990 levels due to a reduction in emissions from coal mines. Methane emissions from U.S. coal mines in 1996
were more than 15 percent below 1990 levels. Increased methane recovery and a decline in emissions from ventilation
systems in the Nation's gassiest mines can be credited for this trend.
As the demand for natural gas has increased during this decade, the volume of fugitive methane emissions from gas
processing plants and gate stations has also increased. Similarly, as additional pipeline has been added to the
distribution system, the opportunity for leaks has increased.(38) As a result, there have been slow, steady increases in
methane emissions from the oil and natural gas system, accelerated by growth in the volume of associated gas vented
from oil wells, to a level of 6.73 million metric tons in 1994 (Table 17). In 1995, a sharp decrease in vented gas, from 1
million metric tons to just 680,000 metric tons, led to a concomitant drop in overall emissions from the oil and gas
system to 6.32 million metric tons.(39)
The drop proved temporary as venting levels rebounded to more
than 1.14 million metric tons in 1996, raising overall emissions to 6.8 million metric tons and renewing the upward
trend in emissions from this source.(40)
In 1996, U.S. methane emissions from coal mining declined slightly from 1995 levels, dropping by 50,000 metric tons
to 3.93 million metric tons (Table 18). Emissions levels continue to be significantly below the 1990 estimate of 4.63
million metric tons. Increased methane recovery and reduced emissions from the ventilation systems of the Nation's
gassiest mines are responsible for the reduced emissions levels.(41) A cursory examination of national coal production
data for 1996 would suggest that this decline is remarkable. In 1996, a record 1.06 billion short tons of coal were
produced, driven by a nearly 6-percent increase in coal consumption for electricity generation. Further, growth in
production from underground mines matched that of surface mines at nearly 3 percent. Underground coal had lost
market share to surface coal during the 1990s as a result of the sulfur restrictions in the Clean Air Act Amendments
of 1990. In 1996, however, increased productivity in underground mines and a drop in prices for sulfur allowances to
as low as $65.00 per ton once again made medium- and high-sulfur coal from Eastern underground mines competitive.
Despite record coal production in 1996, emissions from coal mines decreased slightly from 1995 levels. The decline can
be attributed to a reduction of more than 100,000 metric tons in emissions from the ventilation systems of the Nation's
gassiest mines--a result ofproduction consolidation, aggressive pre-mining degasification, and methane recovery for
energy. More than two-thirds of the reduction in emissions from ventilation systems occurred at the mines of two
companies: Jim Walters Resources, operating in the Warrior Basin in Alabama, and the CONSOL Coal Group mines
in Buchanan, Virginia.
Preliminary estimates of 1996 emissions from residential wood combustion indicate a small decrease of about 1,000
metric tons from 1995 levels. Yet, emissions from residential wood combustion remained 13,000 metric tons higher than
in 1990, accounting for more than two-thirds of the increase in overall methane emissions from stationary combustion
between 1990 and 1996. Coal and fuel oil consumption increased at electric utilities
during 1996 as power markets grew more competitive and natural gas prices rose. In the residential sector, natural gas
consumption grew rapidly. These increases offset the small decline in emissions from residential wood consumption,
leading to an increase of 2,000 metric tons in overall methane emissions from stationary combustion.
Estimates of residential wood combustion are highly uncertain.(42) The universe of wood consumers is large and
heterogenous, and wood for residential consumption usually comes from sources outside the documented economy.
The Energy Information Administration (EIA) relies on its Residential Energy Consumption Survey (RECS) to estimate
residential wood consumption. The survey includes only primary residences and thus systematically underestimates
consumption by an estimated 5 percent.(43) More importantly, the last RECS was completed in 1993. Residential wood
consumption since that time is estimated by scaling the 1993 number to heating degree days. Therefore, the estimate
is driven by weather patterns, which are, of course, unpredictable from year to year.
Methane emissions from mobile combustion in 1996 were 249,000 metric tons, virtually unchanged from 1995
emissions, which were 7,000 metric tons above the 1994 level but more than 24,000 metric tons below the 1990 level
(Table 20). Methane emissions from mobile sources declined slowly but steadily from 1980 through 1994, primarily
because of a 58-percent decrease in emissions from passenger cars. Catalytic converters, used on U.S. automobiles
to control emissions, havegrown more efficient in reducing methane emissions over time. Thus, as the U.S. fleet is
replaced, the remaining automobiles have lower emissions profiles than their earlier counterparts. The last large leap
in catalytic converter technology occurred around 1988, and the annual reductions attributable to fleet turnover are
now diminishing. Since 1994, decreases in emissions from passenger cars have been more than offset by increases in
emissions from the rapidly growing fleet of light-duty trucks and by a 4.5-percent increase in vehicle miles traveled.
In 1996, vehicle miles traveled in the United States increased by about 2 percent, offsetting any marginal reductions
in methane emissions attributable to fleet turnover.
In 1990, approximately 940,000 metric tons of methane were recovered for energy use, and an additional 300,000 metric
tons were recovered and flared. By 1996, these numbers had grown to an estimated 1.68 million metric tons and 650,000
metric tons, respectively, preventing more than 2.3 million metric tons of potential methane emissions. Future rates
of methane recovery are subject to a complex and often conflicting mix of regulatory, tax, and energy market influences
and thus are uncertain.
Future trends in MSW generation are more predictable. According to the EPA's Office of Solid Waste, MSW generation
in the United States is expected to increase by slightly more than 4 percent between 1996 and 2000.(45) While per capita
generation is expected to remain unchanged, a growing population will increase overall generation. The increase can
be expected to bring the volume of waste generated in 2000 to 340.6 million short tons. In contrast to waste generation,
which is trending upward, the share of waste generatedthat will reach landfills is expected to decline from 62 percent
in 1996 to 55 percent in 2000. With waste combustion expected to remain stable as a share of waste generation at
approximately 15 percent, predicted declines in landfilling are attributable to increased curbside recycling.
Methane generation from wastewater is the result of anaerobic decomposition of organic matter in the water. Thus,
emissions are driven by the share of organic matter in the wastewater stream and the conditions under which it
decomposes. Wastewater may be treated aerobically or anaerobically. If it is untreated, the organic matter may
decompose under a combination of conditions. Waste with a high organic content, such as pulp and paper waste or
foodstuff, will deplete available oxygen rapidly, with significant portions degrading anaerobically. Because of
difficulties in characterizing wastewater management practices and flaring or energy recovery practices, estimates of
emissions are scaled to U.S. population data. Driven by a slowly growing U.S. population, methane emissions from
domestic and commercial wastewater treatment rose by less than 1 percent between 1995 and 1996, increasing
emissions from this source to 0.16 million metric tons. Industrial wastewater treatment estimates are equally uncertain,
but because of their potential magnitude, they have been excluded pending the availability of more reliable data.
(46)
Agriculture represents a significant source of methane, with emissions of nearly 9 million metric tons in 1996, or 28
percent of all U.S. anthropogenic methane emissions. Approximately 94 percent of all methane emissions from
agricultural sources can be traced to animal husbandry, with emissions from enteric fermentation accounting for 5.46
million metric tons and emissions from the solid waste of livestock representing another 2.76 million metric tons.
Enteric fermentation occurs when carbohydrates are broken down in the digestive track of herbivores, such as cattle,
sheep, and goats. As microorganisms in the forestomach (rumen) of these animals assist in the digestion of the large
quantities of cellulose found in the plant material, they produce methane, nearly all (90 percent) of which is released
as part of normal respiration and eructation. The remainder is released as flatus. As the solid waste of livestock
decomposes under anaerobic conditions, methane is also released. Other minor sources of agricultural methane--rice
cultivation and crop residue burning--together accounted for 540,000 metric tons of methane emissions in 1996.
About one-half of the increase in emissions from dairy cattle waste between 1990 and 1994 was attributable to a shift
in the method used for handling the solid waste of dairy cattle in six States: Arizona, Florida, Nevada, North Carolina,
North Dakota, and Texas. Waste management techniques in these States shifted toward liquid systems, especially
anaerobic lagoons. Solid waste managed in anaerobic lagoons realizes a much larger share of its maximum potential
methane production than waste managed in any other fashion. The remaining portion of the increase can be traced to
growing cattle populations and a rise in the average size and productivity of dairy cattle between 1990 and 1994. With
dairy cattle populations and average sizes stabilizing in 1995 and receding toward 1990 levels in 1996, almost all the
remaining increase in emissions from dairy cattle waste compared to 1990 is the result of the shift toward higher
emitting management practices.
Methane emissions from the waste of market swine also escalated between 1990 and 1994 and stabilized in 1995 in step
with swine populations. In 1996, swine populations dropped to their lowest levels since 1990, reducing emissions to
just over 4 percent above 1990 levels.
The October 1996 edition of Emissions of Greenhouse Gases in the United States 1995 erroneously stated that market swine populations grew by 13 percent in 1995, thus overestimating emissions for that year by about 130,000 metric tons. Swine populations were virtually unchanged in 1995 from 1994 levels, as reflected in the revised emissions estimate for that year.
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