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Emissions of Greenhouse Gases Report
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Carbon Dioxide Emissions
Total Emissions | Energy-Related Emissions | Weather Effects on Energy-Related Carbon Dioxide Emissions, 2006 and 2007 |
Residential Sector | Commercial Sector | Industrial Sector | Transportation Sector | Electric Power Sector |
Nonfuel Uses of Energy Inputs | Adjustments to Energy Consumption | Other Sources |
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Total Emissions |
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Total U.S. carbon dioxide emissions in 2007 increased by 75.9 million metric
tons (1.3 percent) compared with 2006 emissions (see Figure 5 on right), to 6,022 million
metric tons (MMT). The increase offset a 1.4-percent drop in 2006 (to 5,946
MMT), raising the total back close to the 2005 level (6,032 MMT).
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The important factors that contributed to the increase in carbon dioxide
emissions in 2007 included: unfavorable weather, with both heating and
cooling degree-days above 2006 levels (see discussion on "Weather Effects on energy-Related Carbon Dioxide Emissions, 2006 and 2007"); and the combination
of a 2.5-percent increase in electricity demand and a 14.2-percent decline
in hydropower generation that resulted in a 2.9-percent increase in emissions
from the electric power sector. |
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Energy-related carbon dioxide emissions account for 98 percent of U.S.
carbon dioxide emissions (see Table 5 below). The vast majority of carbon dioxide
emissions come from fossil fuel combustion, with smaller amounts from the
nonfuel use of energy inputs, and the total adjusted for emissions from
U.S. Territories and international bunker fuels. Other sources include
emissions from industrial processes, such as cement and limestone production. |
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figure data
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Energy-Related Emissions |
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Energy-related carbon dioxide emissions account for more than 80 percent
of U.S. greenhouse gas emissions. EIA breaks energy use into four end-use
sectors (see Table 6 below), and emissions from the electric power sector are attributed
to the end-use sectors.
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Growth in energy-related carbon dioxide emissions has resulted largely
from increases associated with electric power generation and transportation
fuel use. All other energy-related carbon dioxide emissions (from direct
fuel use in the residential, commercial, and industrial sectors) have been
either flat or declining in recent years (see Figure 6 on right). |
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Reasons for the growth in electric power and transportation sector emissions
include: increased demand for electricity for computers and electronics
in homes and offices; strong growth in demand for commercial lighting and
cooling; substitution of new electricity-intensive technologies, such as
electric arc furnaces for steelmaking, in the industrial sector; and increased
demand for transportation services as a result of relatively low fuel prices
and robust economic growth in the 1990s and early 2000s.6 |
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Other U.S. energy-related carbon dioxide emissions have remained flat or
declined, for reasons that include increased efficiencies in heating technologies,
as well as declining activity in older smokestack industries and the
growing importance of less energy-intensive industries, such as computers
and electronics. |
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Weather Effects on energy-Related Carbon Dioxide Emissions, 2006 and 2007 |
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Annual variations in CO2 emissions in the residential and commercial sectors
over the 2005-2007 period illustrate the impacts that changes in weather
can have on emissions in those sectors. Total energy-related CO2 emissions
for all the end-use sectors combined (residential, commercial, industrial,
and transportation) were down in 2006 by about 69 million metric tons (MMT),
but in 2007 they were up by about 84 MMT, inviting the question: Why are
these years so different from each other?
It is difficult to estimate the relative importance of the different factors
that affect year-to-year changes in emissions. Since 1990, energy-related
emissions have grown on average by about 1.0 percent per year. At current
emissions levels that would mean about 60 MMT added per year. With U.S.
GDP growing by 2.8 percent in 2006 but by only 2.0 percent in 2007, why was
2006 emissions growth so far below average (-69 MMT) and 2007 growth above
average (+84 MMT)?
One of the most important factors causing deviations from average emissions
growth is weather. The table on the right shows emissions from energy use for
heating and cooling in the residential and commercial sectors for 2006
and 2007, and weather adjustments for both sectors, based on estimates
from EIAs Annual Energy Outlook 2008.7 In 2006, emissions attributable
to space heating and space cooling both were lower than in 2005; in 2007,
both were higher than in 2006.8 The annual variations can be estimated
more precisely by using changes in heating degree-days (HDD) and cooling
degree-days (CDD).9 In 2006, both HDD and CDD were down relative to 2005,
reducing both space heating and cooling requirements; in 2007, both HDD
and CDD were up relative to 2006, increasing demand for both heating and
cooling.
In the residential sector, CO2 emissions related to heating and cooling
in 2006 were estimated to be about 48 and 13 MMT lower than in 2005, respectively,
for a total weather effect of -61 MMT. In the commercial sector, emissions
related to heating and cooling in 2006 were 10 and 4 MMT lower than in
2005. Thus, for both sectors, emissions in 2006 were about 76 MMT lower
than they would have been without the weather effect. The effect is not
insignificant: when 2006 emissions in the two sectors are adjusted for
the weather effect, the result shows an increase of 7 MMT from 2005 to
2006. Similarly, for 2007, the weather effect accounts for about two-thirds
(56 MMT) of the total increase in residential and commercial sector CO2 emissions relative to 2006.
With the adjustments for weather, the growth of total energy-related CO2 emissions in both 2006 and 2007 is below the 1.0-percent average growth
rate for the 1990-2007 period. Other important factors in year-to-year
changes in CO2 emissions include total and relative fuel prices, efficiency
of electricity generation and the energy-intensive industries, and the
availability of hydropower, nuclear power, and other low-carbon energy
sources, as well as overall economic growth.
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Residential Sector |
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Residential sector carbon dioxide emissions originate primarily from:
-Direct fuel consumption (principally, natural gas) for heating and cooking
-Electricity for cooling (and some heating), for lighting, and increasingly
for televisions, computers, and other
household electronic devices (see Table
7 below).
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Energy consumed for heating in homes and businesses has a large influence
on the annual fluctuations in energy-related carbon dioxide emissions.
-The 6.5-percent increase in heating degree-days in 2007 was the second-largest
year-to-year increase over the
period from 1990 to 2007 (see Figure 7 on right).
-Although annual changes in cooling degree-days have a smaller impact on
energy demand, the 2.2-percent
increase in 2007 contributed to the upward
pressure on electricity demand.
-Weather was a key factor behind the 4.3-percent increase in carbon dioxide
emissions from the residential
sector in 2007. |
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In the longer run, residential emissions are affected by population growth
and income. From 1990 to 2007:
-Residential sector carbon dioxide emissions grew by an average of 1.6 percent
per year.
-U.S. population grew by an average of 0.9 percent per year.
-Per-capita income (measured in constant dollars) grew by an average of
1.7 percent per year. |
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figure data
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Commercial Sector |
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Commercial sector emissions (see Table 8 below) are largely the result of energy
use for lighting, space heating, and space cooling in commercial structures,
such as office buildings, shopping malls, schools, hospitals, and restaurants.
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Lighting accounts for a larger component of energy demand in the commercial
sector (approximately 20 percent of total demand in 2006) than in the residential
sector (approximately 11 percent of the total). |
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Commercial sector emissions are affected less by weather than are residential
sector emissions: heating and cooling accounted for approximately 37 percent
of energy demand in the residential sector in 2006 but only about 19 percent
in the commercial sector.10 |
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Although weather has a smaller effect on emissions in the commercial sector
than in the residential sector, it contributed, along with 2-percent economic
growth, to the 4.3-percent increase in 2007well above the average annual
increase from 1990 to 2006. |
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In the longer run, trends in emissions from the commercial sector parallel
economic trends. Commercial sector emissions grew at an average annual
rate of 2.0 percent from 1990 to 2007about the same rate as growth in
real per capita income (see Figure 8 on right). |
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Emissions from direct fuel consumption in the commercial sector declined
from 1990 to 2007, while the sectors electricity-related emissions increased
by an average of 2.6 percent per year. |
Data for all years 1990-2007
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figure data
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Industrial Sector |
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Unlike commercial sector emissions, trends in U.S. industrial sector emissions
(see Table 9 below) have not followed economic growth trends. In 2007, industrial
carbon dioxide emissions fell by 0.8 percent from 2006 and were 2.8 percent
below their 1990 level.
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Decreases in industrial sector carbon dioxide emissions have resulted largely
from erosion of the older energy-intensive (specifically, coal-intensive)
U.S. industrial base. |
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Coke plants consumed 22.7 million short tons of coal in 2007, down from
38.9 million short tons in 1990. Other industrial coal consumption declined
from 76.3 million short tons in 1990 to 56.5 million short tons in 2007,
as reflected by the drop in emissions from coal shown in Figure 9 on right. |
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The share of manufacturing activity represented by less energy-intensive
industries, such as computer chip and electronic component manufacturing,
has increased, while the share represented by the more energy-intensive
industries has fallen. |
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By fuel, only petroleum and purchased electricity use in 2007 were above
1990 levels for the commercial sector. Coal use has fallen since 1990,
and natural gas use, which rose in the 1990s, has fallen since 2000. |
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figure data
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Transportation Sector |
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Transportation sector carbon dioxide emissions in 2007 were 431.8 million
metric tons higher than in 1990 (see Table 10 below), an increase that represents
44 percent of the growth in unadjusted energy-related carbon dioxide emissions
from all end-use sectors over the period.
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The transportation sector has led all U.S. end-use sectors in emissions
of carbon dioxide since 1999; however, with higher fuel prices and slower
economic growth in 2007, emissions from the transportation sector were
essentially unchanged from their 2006 level. |
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Petroleum combustion is the largest source of carbon dioxide emissions
in the transportation sector, as opposed to electricity-related emissions
in the other end-use sectors. |
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Increases in ethanol fuel consumption in recent years have mitigated the
growth in transportation sector emissions somewhat (emissions from energy
inputs to ethanol production plants are counted in the industrial sector). |
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Transportation sector emissions from gasoline and diesel fuel combustion
generally parallel total vehicle miles traveled (see Figure 10 on right). |
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figure data
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Electric Power Sector |
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The electric power sector transforms primary energy inputs into electricity.
The sector consists of companies whose primary business is the generation
of electricity.
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Carbon dioxide emissions from electric power generation rose by 2.9 percent
in 2007 (see Table 11 below). The increase resulted from growth in total electricity
generation (2.5 percent) and an increase in the carbon intensity of the
electricity supply (0.4 percent). |
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Higher overall carbon intensity of power generation in 2007 was the result
of a large drop in generation from hydropower resources (down by 40 billion
kilowatthours), which more than offset increases in generation from wind
and nuclear power plants (up by 6 and 19 billion kilowatthours, respectively).
While nuclear generation rose by 2.4 percent from 2006 to 2007, generation
from renewable fuelsincluding hydropowerfell by 9.6 percent, and generation
from fossil fuels increased by 4.1 percent. |
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From 2000 to 2007, as the overall efficiency of U.S. electricity generation
has increased, there has been a decline in electric power sector energy
losses11 relative to total sales, which has helped to mitigate the sectors
carbon dioxide emissions (see Figure 11 on right). For example, generation from natural
gas rose by 57 percent from 2000 to 2007, but emissions from natural-gas-fired
generation rose by only 33 percent over the same period. |
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figure data
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Nonfuel Uses of Energy Inputs |
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Adjustments to Energy Consumption |
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EIA's greenhouse gas emissions inventory includes two adjustments to energy
consumption (see Table 14 below). First, the energy consumption and carbon dioxide
emissions data in this report correspond to EIAs coverage of energy consumption,
which includes the 50 States and the District of Columbia, but under the
UNFCCC the United States is also responsible for emissions emanating from
its Territories; therefore, their emissions are added to the U.S. total.
Second, because the IPCC definition of energy consumption excludes international
bunker fuels, emissions from international bunker fuels are subtracted
from the U.S. total. Similarly, because the IPCC excludes emissions from
military bunker fuels from national totals, they are subtracted from the
U.S. Total.
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The net adjustment in emissions has been negative in every year from 1990
to 2007, because emissions from bunker fuels have always exceeded emissions
from U.S. Territories. The net negative adjustment for 2007 was larger
(-74.2 MMT), because emissions from the U.S. Territories fell while emissions
from bunker fuels increased. |
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Other Sources |
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Other emissions sources in total accounted for 1.7 percent (105.1 MMT)
of all U.S. carbon dioxide emissions in 2007 (see Figure 12 on right).
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The largest source of U.S. carbon dioxide emissions other than fossil fuel
consumption is cement manufacture (see Table 15 below), where most emissions result
from the production of clinker (consisting of calcium carbonate sintered
with silica in a cement kiln to produce calcium silicate). |
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Limestone consumption, especially for lime manufacture, is the source of
15 to 20 MMT of carbon dioxide emissions per year. |
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In addition, other sources include: soda ash manufacture and consumption;
carbon dioxide manufacture; aluminum manufacture; flaring of natural gas
at the wellhead; carbon dioxide scrubbed from natural gas; and waste combustion
in the commercial and industrial sectors. |
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figure data
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Notes and Sources |
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