Executive Summary
Overview
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This report, in accordance with Section 1605(a) of the Energy Policy Act
of 1992, provides estimates of U.S. emissions of greenhouse gases.1 Table
ES1 shows trends in emissions of the principal greenhouse gases, measured
in million metric tons of native gas. Throughout the remainder of the report,
emissions are given in carbon dioxide equivalents, which put the emissions
of each gas in comparable terms of their global warming potentials (GWPs)
relative to that of carbon dioxide.2 As shown in Table ES2, U.S. emissions
of greenhouse gases in 2005 totaled 7,147.2 million metric tons carbon
dioxide equivalent (MMTCO2e), 0.6 percent more than in 2004 (7,104.6 MMTCO2e).
The modest increase in total greenhouse gas emissions in 2005 is attributable
mainly to below-average growth in emissions of carbon dioxide (0.3 percent).
There were larger increases in emissions of nitrous oxide (1.9 percent)
and methane (0.9 percent), but collectively these two gases make up only
about 14 percent of total U.S. greenhouse gas emissions. Emissions of high-GWP
gases—hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride
(SF6) —increased by 7.2 percent, but their share of the total is only 2.2
percent. The U.S. economy grew by 3.2 percent in 2005. Consequently, U.S.
greenhouse gas intensity (greenhouse gas emissions per unit of real economic
output) was 2.5 percent lower in 2005 than in 2004. From 1990 to 2005,
U.S. greenhouse gas intensity declined by 25 percent, or by an average
of 1.9 percent per year.
U.S. greenhouse gas emissions in 2005 were 17 percent higher than the 1990
emissions level of 6,112.8 MMTCO2e—an average annual increase of 1.0 percent
over the period. Since 1990, U.S. emissions have increased more slowly
than the average annual growth in population (1.2 percent), primary energy
consumption (1.1 percent), electric power generation (1.9 percent), or
gross domestic product (3.0 percent). While the annual growth rate in carbon
dioxide emissions since 1990 (1.2 percent) has closely tracked annual growth
in population and energy consumption, the average annual rate of growth
in total greenhouse gas emissions has been lower (1.0 percent) because
of reductions in methane emissions and relatively slow annual growth in
nitrous oxide emissions (0.6 percent) since 1990.
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During 2005, approximately 83 percent of total U.S. greenhouse gas emissions
consisted of carbon dioxide from the combustion and nonfuel use of fossil
fuels (Figure ES1), such as coal, petroleum, and natural gas (after adjustments
for U.S. Territories and international bunker fuels). U.S. emissions trends
are driven largely by trends in fossil energy consumption. In recent years,
national energy consumption, like emissions, has grown relatively slowly,
with year-to-year deviations from trend growth caused by weather-related
phenomena, fluctuations in business cycles, changes in the fuel mix for
electric power generation, and developments in domestic and international
energy markets.
Other 2005 U.S. greenhouse gas emissions include carbon dioxide from non-combustion
sources (1.5 percent of total U.S. greenhouse gas emissions), methane (8.6
percent), nitrous oxide (5.1 percent), and other gases (2.2 percent). Methane
and nitrous oxide emissions are caused by the biological decomposition
of various waste streams and fertilizer; fugitive emissions from chemical
processes; fossil fuel production, transmission, and combustion; and many
smaller sources. The other gases include HFCs, used primarily as refrigerants;
PFCs, released as fugitive emissions from aluminum smelting and used in
semiconductor manufacture; and SF6, used as an insulator in utility-scale
electrical equipment.
Carbon Dioxide
The preliminary estimate of U.S. carbon dioxide emissions from both energy
consumption and industrial processes in 2005 is 6,008.6 million metric
tons (MMT), which is 0.3 percent higher than in 2004 (5,988.7 MMT) and
accounts for 84 percent of total U.S. greenhouse gas emissions (see Table
ES3 for a breakdown of U.S. carbon dioxide emissions by source). U.S. carbon
dioxide emissions have grown by an average of 1.2 percent annually since
1990. Although short-term changes in carbon dioxide emissions can result
from temporary variations in weather, power generation fuel mixes, and
the economy, growth in carbon dioxide emissions in the longer term results
largely from population- and income-driven increases in energy use, as
well as consumer choices of energy-using equipment. The “carbon intensity”
of energy use (carbon dioxide emissions per unit of energy consumed) can
also influence the trend of growth in energy-related carbon dioxide emissions.
Figure ES2 shows recent trends in some common indexes used to measure the
carbon intensity of the U.S. economy. Carbon dioxide emissions per unit
of gross domestic product (GDP) have continued to fall relative to 1990;
by 2005, this measure was 23 percent lower than in 1990. Carbon dioxide
emissions per capita, however, were 1.0 percent above 1990 levels in 2005.
Population growth and other factors resulted in increased aggregate carbon
dioxide emissions per year from 1990 through 2005 (a total increase of
20 percent). Carbon dioxide emissions per unit of net electricity generation
in 2005 were 0.4 percent higher than in 2004.
Energy Consumption
The consumption of energy in the form of fossil fuel combustion is the
largest single contributor to anthropogenic greenhouse gas emissions in
the United States and the world. Of total 2005 U.S. carbon dioxide emissions
(adjusting for U.S. Territories and bunker fuels), 98 percent, or 5,903.2
MMT, resulted from the combustion of fossil fuels—0.3 percent more than
in 2004.
The Energy Information Administration (EIA) divides energy consumption
into four general end-use categories: residential, commercial, industrial,
and transportation. Emissions from electricity generators, which provide
electricity to the end-use sectors, are allocated in proportion to the
electricity consumed in, and losses allocated to, each sector. Figure ES3
illustrates trends in carbon dioxide emissions by energy consumption sector.
Average annual growth rates in carbon dioxide emissions by sector during
the 1990-2005 period were 2.0 percent for the commercial sector, 1.8 percent
for the residential sector, and 1.5 percent for the transportation sector.
For the industrial sector, carbon dioxide emissions have grown in some
years and fallen in others; industrial emissions in 2005 were slightly
below their 1990 level.
Carbon dioxide emissions from the transportation sector are the largest
source of energy-related carbon dioxide emissions. At 1,958.6 MMT, the
transportation sector accounted for 33 percent of total U.S. energy-related
carbon dioxide emissions in 2005. Transportation sector emissions increased
by 1.0 percent in 2005 relative to the 2004 level of 1,939.2 MMT. Almost
all (98 percent) of transportation sector carbon dioxide emissions result
from the consumption of petroleum products: motor gasoline, 1,170.5 MMT
(60 percent of total transportation sector emissions in 2005); middle distillates
(diesel fuel), 434.1 MMT (22 percent); jet fuel, 243.8 MMT (12 percent);
and residual oil (heavy fuel oil, largely for maritime use), 64.1 MMT (3.1
percent). The growth in transportation-related carbon dioxide emissions
in 2005 included increases in emissions from the use of jet fuel (6.5 MMT),
residual fuel oil (6.4 MMT), and diesel fuel (4.5 MMT).
Industrial sector carbon dioxide emissions, at 1,682.3 MMT, accounted for
28 percent of total U.S. energy-related carbon dioxide emissions in 2005.
The 2005 emissions level represents a 3.1-percent decrease from 2004 emissions
of 1,736.0 MMT. Although industrial production rose by 3.2 percent in 2005,
total industrial emissions fell, because three of the most energy-intensive
industries experienced downturns in 2005: primary metals (down 2.7 percent),
chemicals (down 6.9 percent), and petroleum (down 7.5 percent). In terms
of fuel shares, electricity consumption was responsible for 39 percent
of total industrial sector carbon dioxide emissions (662.8 MMT), natural
gas for 24 percent (399.7 MMT), petroleum for 26 percent (431.2 MMT), and
coal for 11 percent (184.5 MMT). Carbon dioxide emissions attributable
to industrial sector energy consumption in 2005 were 0.1 percent (1.3 MMT)
lower than in 1990.
At 1,253.8 MMT, residential carbon dioxide emissions represented 21 percent
of U.S. energy-related carbon dioxide emissions in 2005. The 2005 residential
emissions were 3.3 percent higher than the 2004 level of 1,213.9 MMT. The
residential sector’s pro-rated share of electric power sector carbon dioxide
emissions, 885.7 MMT, accounted for more than two-thirds of all emissions
in the residential sector.3 Natural gas accounted for 21 percent (261.7
MMT) and petroleum (mainly distillate fuel oil) represented 8.4 percent
(105.3 MMT). Since 1990, when residential sector carbon dioxide emission
totaled 953.7 MMT, the growth in residential carbon dioxide emissions has
averaged 1.8 percent per year.
Commercial sector carbon dioxide emissions accounted for about 18 percent
of total energy-related carbon dioxide emissions in 2005, at 1,050.6 MMT,
of which 78 percent (821.1 MMT) was the sector’s pro-rated share of electricity-related
emissions. Natural gas contributed 16 percent (166.3 MMT) and petroleum
5.3 percent (55.4 MMT). Commercial sector carbon dioxide emissions increased
by 1.6 percent from the 2004 level of 1,034.1 MMT. Since 1990, carbon dioxide
emissions in the commercial sector have increased on average by 2.0 percent
per year, the largest growth of any end-use sector. Commercial sector carbon
dioxide emissions have risen by 269.9 MMT since 1990, accounting for 28
percent of the total increase in U.S. energy-related carbon dioxide emissions.
Carbon dioxide emissions from the U.S. electric power sector increased
by 2.8 percent (65.6 MMT), from 2,309.4 MMT in 2004 to 2,375.0 MMT in 2005.
Carbon dioxide emissions from the electric power sector have grown by 32
percent since 1990, while total carbon dioxide emissions from all energy-related
sources have grown by 19 percent. Carbon dioxide emissions from the electric
power sector represented 40 percent of total U.S. energy-related carbon
dioxide emissions in 2005; however, as noted above, in calculating emissions
from the end-use sectors EIA distributes electric power sector emissions
to the four sectors in proportion to their respective shares of total electricity
purchases. Therefore, electric power emissions are already included in
the sectoral totals. By fuel, emissions from natural-gas-fired generation
increased by 7.7 percent, emissions from coal-fired generation increased
by 2.1 percent, and emissions from petroleum-fired generation increased
by 2.3 percent in 2005 from their 2004 levels (see box on page xiv for
allocation of all greenhouse gases to EIA’s end-use sectors).
Nonfuel Uses of Energy Inputs
Nonfuel uses of fossil fuels, principally petroleum, both emit carbon dioxide
and sequester carbon over their life cycles. In 2005, nonfuel uses of fossil
fuels resulted in emissions of 106.4 MMT carbon dioxide, a 4.7-percent decrease
from the 2004 level of 111.7 MMT. Emissions from nonfuel uses of energy
fuels are included in the unadjusted energy consumption subtotals in Table
ES3.
On the sequestration side of the ledger, nonfuel uses of fossil fuels also
resulted in carbon sequestration equal to 300.9 MMTCO2e in 2005, a 3.3-percent
decrease from the 2004 level of 311.1 MMTCO2e. The major fossil fuel products
that emit and sequester carbon include liquefied petroleum gas (LPG) and
feedstocks for plastics and other petrochemicals. Asphalt and road oils
are a major source of sequestration, but they do not emit carbon dioxide.
It is estimated that, of the amount of carbon sequestered in the form of
plastic, about 11.1 MMTCO2e was emitted as carbon dioxide from the burning
of the plastic components of municipal solid waste to produce electricity
in 2005. The 2004 estimate of 19.4 MMTCO2e is used in this report as an
estimate for total 2005 emissions of carbon dioxide from the burning of
wastes. The U.S. Environmental Protection Agency (EPA) estimates total
emissions from waste burning, and its 2005 value was not available at the
time this report was published.
Adjustments to Energy Consumption
Total U.S. carbon dioxide emissions and the estimates of energy consumption
on which they are based correspond to EIA’s coverage of energy consumption,
including the 50 States and the District of Columbia. Under the United
Nations Framework Convention on Climate Change (UNFCCC), however, the United
States is also responsible for counting emissions emanating from its Territories,
and their emissions are added to the U.S. total. Conversely, because the
IPCC definition of energy consumption excludes international bunker fuels
from the statistics of all countries, emissions from international bunker
fuels are subtracted from the U.S. total. Military bunker fuels are also
subtracted, because they are also excluded by the IPCC from national emissions
totals. On net, these adjustments resulted in the subtraction of 42.1 MMT
from total U.S. carbon dioxide emissions related to energy consumption
(5,945.3 MMT), resulting in an adjusted total of 5,903.2 MMT for energy-related
carbon dioxide emissions in 2005 (Table ES3).
Energy-related carbon dioxide emissions for the U.S. Territories are added
as an adjustment, in keeping with IPCC guidelines for national emissions
inventories. The Territories included are Puerto Rico, the U.S. Virgin
Islands, American Samoa, Guam, the U.S. Pacific Islands, and Wake Island.
Most of these emissions are from petroleum products; however, Puerto Rico
and the Virgin Islands consume coal in addition to petroleum products.
Total energy-related carbon dioxide emissions from the U.S. Territories
in 2005 are estimated at 58.6 MMT (Table ES3).
In 2005, approximately 100.7 MMT carbon dioxide was emitted in total from
international bunker fuels, including 90.6 MMT attributable to civilian
consumption of bunker fuels and 10.1 MMT from military use.4 In Table ES3,
total emissions from international bunker fuels are included as a negative
adjustment to U.S. energy-related carbon dioxide emissions. Just over two-thirds
of the carbon dioxide emissions associated with international bunker fuels
comes from the combustion of jet fuels; residual and distillate fuels account
for the other one-third, with most of that coming from residual fuel.
Other Carbon Dioxide Emissions
In addition to carbon dioxide emissions from fossil fuel combustion and
use, a total of 105.4 MMT was emitted from other sources in 2005 (Table
ES3). Cement manufacture (45.9 MMT) and industrial sources (28.1 MMT) accounted
for nearly three-fourths of the total carbon dioxide emissions from other
sources. Energy sector components in the other emissions category included
the stripping of carbon dioxide from natural gas (17.3 MMT) and natural
gas flaring (5.9 MMT). An additional 8.3 MMT carbon dioxide is estimated
to have been released from the burning of wastes other than municipal solid
waste in the electric power sector.
Methane
U.S. anthropogenic methane emissions totaled 611.9 MMTCO2e5 (26.6 million
metric tons of methane) in 2005, representing 8.6 percent of total U.S.
greenhouse gas emissions. Methane emissions in 2005 were 0.9 percent (5.3
MMTCO2e) higher than the 2004 level of 606.5 MMTCO2e (Table ES4). The increase
is attributable primarily to increases in methane emissions from landfills
(6.9 MMTCO2e) and, to a lesser extent, emissions associated with animal
husbandry. Those increases were offset to some degree by a decrease in
methane emissions from coal mines (1.8 MMTCO2e) and a small decrease in
emissions from the cultivation of crops. Despite the 0.9-percent increase
in 2005, methane emissions still were 89.8 MMTCO2e (13 percent) below their
1990 level of 701.7 MMTCO2e.
Methane emissions come from four source categories, three major and one
minor. The major sources are energy, agriculture, and waste management,
and the minor source is industrial processes. The three major sources accounted
for 42, 30, and 28 percent, respectively, of total 2005 U.S. emissions
of methane. Trends in the major sources of anthropogenic methane emissions
since 1990 are illustrated in Figure ES4.
Energy Sources
Total U.S. energy consumption fell by 0.3 percent from 2004 to 2005; and
methane emissions from energy sources (coal mining, natural gas systems,
petroleum systems, stationary combustion, and mobile source combustion)
in 2005, at 254.9 MMTCO2e, were 1.3 percent below the 2004 level of 258.3
MMTCO2e. Methane emissions from energy sources have fallen by 7.3 percent
since 1990.
Agricultural Sources
Methane emissions from agricultural sources, at 183.0 MMTCO2e, represented
30 percent of total U.S. anthropogenic methane emissions in 2005. Emissions
increased by 1.0 percent (1.9 MMTCO2e) in 2005 relative to 2004. Increases
greater than 1 percent in emissions from enteric fermentation in domesticated
animals and from animal waste were offset to some extent by decreases in
emissions from rice cultivation and burning of crop residues. Of the total
2005 methane emissions from agricultural sources, 93 percent (170.9 MMTCO2e)
resulted from livestock management, of which 68 percent (115.6 MMTCO2e)
was from enteric fermentation and the remainder (55.3 MMTCO2e) was from
anaerobic decomposition of livestock wastes. Methane emissions from rice
cultivation (10.9 MMTCO2e) and crop residue burning (1.2 MMTCO2e) together
represented about 6.6 percent of total methane emissions from agricultural
sources in 2005, which have increased by 5.5 percent since 1990.
Waste Management
Methane emissions from waste management, which at 171.5 MMTCO2e accounted
for 28 percent of U.S. anthropogenic methane emissions in 2005, were 4.3
percent above the 2004 level of 164.5 MMTCO2e. Landfills—the largest single
source of U.S. anthropogenic methane emissions—represented 91 percent (155.7
MMTCO2e) of total U.S. methane emissions from waste management in 2005.
The remainder of 2005 methane emissions from waste management (15.8 MMTCO2e)
was associated with domestic wastewater treatment. Methane emissions from
waste management have fallen by 32 percent (79.1 MMTCO2e) from their 1990
level of 250.6 MMTCO2e, due largely to increased methane recovery at landfills,
which increased to 122.5 MMTCO2e in 2005 from 21.7 MMTCO2e in 1990. Even
at these higher methane recovery levels, however, waste management emissions,
after bottoming out in 2002, increased in both 2004 and 2005.
Industrial Processes
Methane emissions from industrial processes totaled 2.5 MMTCO2e in 2005,
including 1.5 MMTCO2e from chemical manufacturing and 1.0 MMTCO2e from
iron and steel production. The 2005 total was 7.7 percent below the 2004
total of 2.7 MMTCO2e. Since 1990, growth in methane emissions from chemical
production has been more than offset by reductions in emissions from iron
and steel production, leaving the 2005 total for industrial process emissions
8.4 percent below the total for 1990.
Nitrous Oxide
Estimated U.S. anthropogenic emissions of nitrous oxide in 2005 totaled
366.6 MMTCO2e, or 1.2 MMT nitrous oxide. Nitrous oxide emissions represented
5.1 percent of total U.S. greenhouse gas emissions in 2005 and were 1.9
percent (6.7 MMTCO2e) above the 2004 level of 359.9 MMTCO2e. Most of the
increase from 2004 can be attributed to increased emissions from agricultural
sources, which rose by 7.0 MMTCO2e in 2005. Emissions from waste management
contributed about 1 percent to the total growth from 2004 to 2005, whereas
emissions from industrial sources were 5.3 percent lower in 2005 than in
2004 (Table ES5 and Figure ES5).
Agriculture
Agricultural sources, at 279.9 MMTCO2e, accounted for 76 percent of total
U.S. nitrous oxide emissions in 2005. Agricultural emissions in 2005 were
2.6 percent above the 2004 total of 272.9 MMTCO2e, primarily as the result
of an increase of 2.9 percent (6.2 MMTCO2e) in emissions from the nitrogen
fertilization of agricultural soils. Emissions from nitrogen fertilization,
at 218.1 MMTCO2e, accounted for 60 percent of nitrous oxide emissions from
agriculture in 2005. Emissions from the solid waste of domesticated animals,
at 61.2 MMTCO2e, made up 22 percent of agricultural nitrous oxide emissions
in 2005, and burning of crop residues produced another 0.6 MMTCO2e. Total
U.S. emissions of nitrous oxide from agriculture sources have increased
by 12.3 percent since 1990.
Energy Use
U.S. nitrous oxide emissions associated with fossil fuel combustion in
2005 were 67.3 MMTCO2e, or 18 percent of total nitrous oxide emissions.
Most of the energy-related emissions of nitrous oxide in 2005 (78 percent
or 52.6 MMTCO2e) were from mobile sources, principally, motor vehicles
equipped with catalytic converters. The remainder (22 percent or 14.7 MMTCO2e)
was from stationary source combustion of fossil fuels. Nitrous oxide emissions
from energy sources have increased by 32 percent since 1990.
Industrial Processes and Waste Management
Industrial processes and waste management facilities were responsible for
3.6 percent and 1.6 percent, respectively, of total U.S. nitrous oxide
emissions in 2005. Industrial process emissions in 2005 (13.2 MMTCO2e) were
5.3 percent below the 2004 level of 14.0 MMTCO2e. Industrial process emissions
have fallen by 54 percent since 1990 due to decreases in nitrous oxide
emissions from the manufacture of adipic acid.
Nitrous oxide emissions from waste management facilities in 2005 (6.2 MMTCO2e)
increased by 1.1 percent from the 2004 level of 6.1 MMTCO2e. Most of the
2005 emissions (5.8 MMTCO2e) were from human sewage in wastewater and the
remainder from waste combustion. Nitrous oxide emissions from waste management
facilities have increased by 26 percent since 1990.
Other Gases: Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride
HFCs, PFCs, and SF6 are three classes of high-GWP gases that accounted
for 2.2 percent of total U.S. greenhouse gas emissions in 2005. At 160.2
MMTCO2e, their emissions were 7.2 percent higher than in 2004 (149.5 MMTCO2e).
The increase in emissions of the high-GWP gases from 2004 to 2005 resulted
largely from an 8.9-percent increase in HFC emissions, which more than
offset decreases in emissions of PFCs (5.2 percent lower than in 2004)
and SF6 (2.1 percent lower) (Table ES6).
At 131.8 MMTCO2e, emissions of HFCs made up the majority of U.S. emissions
of high-GWP greenhouse gases, followed by SF6 at 15.7 MMTCO2e and PFCs
at 6.7 MMTCO2e. Another group of high-GWP gases, consisting of other HFCs,
other PFCs, and perfluoropolyethers (PFPEs), includes HFC-152a, HFC-227ea,
HFC-245fa, HFC-4310mee, and a variety of PFCs and PFPEs. They are grouped
together in this report to protect confidential data. In 2005, their combined
emissions totaled 6.1 MMTCO2e. Emissions of the gases in this group in
2005 were 13 percent higher than in 2004 and an order of magnitude higher
than in 1990, when they totaled less than 0.4 MMTCO2e. Since 1990, HFC
emissions from U.S. sources have increased by 265 percent, PFC emissions
have fallen by 67 percent, and SF6 emissions have fallen by 49 percent.
Emissions of the high-GWP gases specified in the Kyoto Protocol are very
small (at most a few thousand metric tons). On the other hand, some of
the gases (including PFCs and SF6) have atmospheric lifetimes measured
in the thousands of years, and consequently they are potent greenhouse gases
with GWPs thousands of times higher than that of carbon dioxide per unit
of molecular weight. Some of the commercially produced HFCs (134a, 152a,
4310mee, and 227ea), which are used as replacements for chlorofluorocarbons
(CFCs) and hydrochlorofluorocarbons (HCFCs), have shorter atmospheric lifetimes,
ranging from 1 to 33 years.
Land-Use Change and Forestry
Forest lands in the United States are net absorbers of carbon dioxide from
the atmosphere, primarily as a result of the reversal of the extensive
deforestation that occurred in the United States during the late 19th and
early 20th centuries. Since then, millions of acres of formerly cultivated
land have been abandoned and have returned to forest, with the regrowth
of forests sequestering carbon on a large scale. The process is steadily
diminishing, however, because the rate at which forests absorb carbon slows
as the trees mature, and because the rate of reforestation has slowed.
The EPA estimates annual U.S. carbon sequestration from land-use change
and forestry in 2004 at 780.1 MMTCO2e,6 representing an offset of 11 percent
of total 2004 U.S. greenhouse gas emissions (7,104.6 MMTCO2e). In 1990,
carbon sequestration attributable to land use and forestry was 910.4 MMTCO2e,
or 15 percent of total 1990 U.S. greenhouse gas emissions (6,112.8 MMTCO2e).7 The EPA’s 2004 estimates for carbon sequestration from land-use change
and forestry include 637.2 MMTCO2e from forested land, 88.0 MMTCO2e from
urban trees, 9.3 MMTCO2e from landfilled yard trimmings and food scraps,
and 45.6 MMTCO2e from all other sources, including net emissions of 7.3
MMTCO2e from grassland soil stocks (Table ES7).
Uncertainty in Emissions Estimates
The emissions numbers presented in this report are, for the most part,
estimates based on estimated activity data and estimated emission factors.
As such, they have an element of uncertainty, given that the activity data
and emission factors on which the emission estimates are based also have
a range of possible values. The activity data and emission factors can
themselves be characterized by systematic biases and/or random errors.
In 2000, EIA employed a Monte Carlo analysis to estimate the range of uncertainty,
at a 95-percent confidence level, around estimated emissions of carbon
dioxide, methane, and nitrous oxide (HFCs, PFCs, and SF6 were not part
of the analysis).8
The Monte Carlo simulations revealed that uncertainty varies by type of
gas. There is less uncertainty around the simulated mean for carbon dioxide
(-1.4 percent to +1.3 percent) than for methane (-15.6 percent to 16.0
percent) or nitrous oxide (-53.5 percent to +54.2 percent). The simulations
also showed that the uncertainty around the simulated mean of total greenhouse
gas emissions (excluding HFCs, PFCs, and SF6) is -4.4 percent to +4.6 percent.
The certainty of emissions data varies by category and by source. For example,
methane emissions from existing underground coal mines are relatively certain.
In general, however, the estimates for carbon dioxide emissions are more
certain than the estimates for other gases. It is likely that the estimate
of total U.S. carbon dioxide emissions is accurate to within 5 percent.
For methane emissions, most of the estimates are much more uncertain, with
a level of uncertainty that may exceed 30 percent. Estimates of methane
emissions may also understate actual emissions as a result of the
exclusion of sources that are unknown or difficult to quantify. For example,
EIA does not include sources such as abandoned coal mines and industrial
wastewater. Nitrous oxide emissions estimates are much less certain than
those for carbon dioxide or methane emissions, in part because nitrous
oxide emissions have been studied far less than emissions of the other
greenhouse gases and in part because the largest apparent sources of nitrous
oxide emissions are area sources that result from biological activity,
which makes for emissions that are highly variable and hard to measure
or characterize. The uncertainty for nitrous oxide emissions may exceed
100 percent.
Executive Summary Notes & Sources
Executive Summary Tables
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