The concentration of carbon dioxide (CO2) in Earth's atmosphere is of interest because
of its impact on the greenhouse effect. arbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In 2011, CO2
accounted for about 84% of all U.S. greenhouse gas emissions from human
activities. Carbon dioxide is naturally present in the atmosphere as
part of the Earth's carbon cycle (the natural circulation of carbon
among the atmosphere, oceans, soil, plants, and animals). Human
activities are altering the carbon cycle—both by adding more CO2 to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO2 from the atmosphere. While CO2
emissions come from a variety of natural sources, human-related
emissions are responsible for the increase that has occurred in the
atmosphere since the industrial revolution.
U.S. Carbon Dioxide Emissions, By Source
Note: All emission estimates from the
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011.
The main human activity that emits CO
2 is the combustion
of fossil fuels (coal, natural gas, and oil) for energy and
transportation, although certain industrial processes and land-use
changes also emit CO
2. The main sources of CO
2 emissions in the United States are described below.
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Electricity.
Electricity is a significant source of energy in the United States and
is used to power homes, business, and industry. The combustion of fossil
fuels to generate electricity is the largest single source of CO2 emissions in the nation, accounting for about 38% of total U.S. CO2 emissions and 32% of total U.S. greenhouse gas emissions in 2011. The type of fossil fuel used to generate electricity will emit different amounts of CO2. To produce a given amount of electricity, burning coal will produce more CO2 than oil or natural gas.
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Transportation. The combustion of fossil fuels such as gasoline and diesel to transport people and goods is the second largest source of CO2 emissions, accounting for about 31% of total U.S. CO2 emissions and 26% of total U.S. greenhouse gas
emissions in 2011. This category includes transportation sources such
as highway vehicles, air travel, marine transportation, and rail.
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Industry. Many industrial processes emit CO2 through fossil fuel combustion. Several processes also produce CO2
emissions through chemical reactions that do not involve combustion,
for example, the production and consumption of mineral products such as
cement, the production of metals such as iron and steel, and the
production of chemicals. Fossil fuel combustion from various industrial
processes accounted for about 14% of total U.S. CO2 emissions and 12% of total U.S. greenhouse gas
emissions in 2011. Note that many industrial processes also use
electricity and therefore indirectly cause the emissions from the
electricity production.
Carbon dioxide is constantly being exchanged among the atmosphere,
ocean, and land surface as it is both produced and absorbed by many
microorganisms, plants, and animals. However, emissions and removal of
CO
2 by these natural processes tend to balance. Since the
Industrial Revolution began around 1750, human activities have
contributed substantially to climate change by adding CO
2 and other heat-trapping gases to the atmosphere.
In the United States, since 1990, the management of forests and non-agricultural land has acted as a net sink of CO
2, which means that more CO
2
is removed from the atmosphere, and stored in plants and trees, than is
emitted. This sink offset about 14% of total emissions in 2011 and is
discussed in more detail in the Land Use, Land-Use Change, and Forestry section.
To find out more about the role of CO
2 warming the atmosphere and its sources, visit the Causes of Climate Change page and the Greenhouse Gas Indicators page in the Science section.
A recent report on global atmospheric CO2 level has created a concern
among the scientific community associated with global warming and
climate change research. The report released by National Oceanographic
and Atmospheric Administration (NOAA) on 9th of this month has stated
that the daily average CO2 concentration has surpassed the level of 400
ppm as observed in the Mauna Loa Observatory at Hawaii Islands. As
stated by NOAA, such a higher level has been reached for the first time
in the last 800,000 years. The same has also been reported by another
climate research group ‘Scripps Institute of Oceanography’ who has also
added that similar concentration of the green house gas was present
during the Pilocene Age, 3-5 million years ago.Daily measurements of CO2 at a US government agency lab on Hawaii have topped 400 parts per million for the first time.
The station, which sits on the Mauna Loa volcano, feeds its
numbers into a continuous record of the concentration of the gas
stretching back to 1958.The last time CO2 was regularly above 400ppm was three to five million years ago - before modern humans existed. Scientists say the climate back then was also considerably warmer than it is today.
Carbon dioxide is regarded as the most important of the
manmade greenhouse gases blamed for raising the temperature on the
planet over recent decades.Human sources come principally from the burning of fossil fuels such as coal, oil and gas.
The usual trend seen at the
volcano is for the CO2 concentration to rise in winter months and then
to fall back as the northern hemisphere growing season kicks in. Forests
and other vegetation pull some of the gas out of the atmosphere.This means the number can be expected to decline by a few ppm
below 400 in the coming weeks. But the long-term trend is upwards.
