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Carbon Dioxide (CO2): Carbon dioxide enters the atmosphere through the burning of fossil fuels (oil, natural gas, and coal), solid waste, trees
and wood products, and also as a result of other chemical reactions (e.g., manufacture of cement). Carbon dioxide is also
removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon
cycle.
Methane (CH4): Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from
livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.
Nitrous Oxide (N2O): Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and
solid waste.
Fluorinated Gases: Hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are synthetic, powerful greenhouse gases that are emitted
from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for ozone-depleting substances (i.e., CFCs, HCFCs, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse
gases, they are sometimes referred to as High Global Warming Potential gases (“High GWP gases”).
US EPA Carbon Dioxide Inventory, Human Caused Sources and Sinks
Fossil Fuel Combustion | Industrial Process and Products Uses | Carbon Sequestration | Geologic Sequestration
A variety of human activities lead to the emission (sources) and removal (sinks) of carbon dioxide (CO2):
- The largest source of CO2 emissions globally is the combustion of fossil fuels such as coal, oil and gas in power plants, automobiles, industrial facilities and other sources.
- A number of specialized industrial production processes and product uses such as mineral production, metal production and the use of petroleum-based products can also lead to CO2
emissions.
- Carbon sequestration is the process by which growing trees and plants absorb or remove CO2 from the atmosphere and
turn it into biomass (e.g., wood, leaves, etc.). Deforestation, conversely, can lead to significant levels of CO2
emissions in some countries.
- Carbon dioxide can be captured from power plants and industrial facilities before it is released into the atmosphere, and then injected deep underground.
The figure below displays a breakdown of sources of CO2 emissions in the U.S. in 2005. By
far the largest source is fossil fuel combustion:
Natural Sources and Sinks of Carbon Dioxide
Greenhouse gases occur naturally in the Earth’s atmosphere in addition to being emitted through human activities.
This natural “carbon cycle” includes carbon dioxide used in plants during photosynthesis and the exchange of carbon
dioxide between the atmosphere and the oceans.
The primary natural processes that release CO2 into the atmosphere (sources) and that remove
CO2 from the atmosphere (sinks) are:
- Animal and plant respiration, by which oxygen and nutrients are converted into CO2 and energy,
and plant photosynthesis by which CO2 is removed from the atmosphere and stored as carbon in
plant biomass;
- Ocean-atmosphere exchange, in which the oceans absorb and release CO2 at the sea surface;
and
- Volcanic eruptions, which release carbon from rocks deep in the Earth’s crust (this source is very small).
Here are some additional resources on the Carbon Cycle:
Individual Emissions
In the Home
On the Road
Personal Emissions Calculator
Individual Emissions
In the home or on the road, daily activities cause emissions of greenhouse gases. Individuals can produce greenhouse gas emissions directly by burning
oil or gas for home heating or indirectly by using electricity generated from fossil fuel burning.
Within the United States, per person emissions can vary depending on a person’s location, habits and personal choices.
For example, the types of fuel used to generate the electricity a person uses can lead to different levels of emissions. A
power plant run on coal emits more greenhouse gases per unit of electricity than a power plant that uses natural gas. How
much you drive and your vehicle's fuel efficiency, as well as time spent idling in traffic, also affect the level of emissions.
In addition, the amount of recycling done by a person in his or her home can affect emissions by reducing the amount of methane-generating
waste sent to landfills.
Your Emissions
Through actions at home, at work, and on the road, individuals can affect their greenhouse gas emissions levels. The first
step in reducing your greenhouse gas emissions is to identify how much your household emits. If you have old copies of energy bills, EPA’s personal greenhouse gas emissions calculator can help estimate your household’s annual emissions.
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