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Emissions
This page provides EPA’s answers to frequently asked questions about greenhouse gas emissions. Click on a question below to view the answer. Links throughout the answers will guide you to further information on EPA’s Climate Change site or from other sources.
- What are the most important greenhouse gases? Where are they coming from and how have they changed?
- Since 1990, how have greenhouse gas emissions in the U.S. changed?
- What are the largest sources of greenhouse gas emissions in the U.S.?
- What are greenhouse gas emissions from the power sector?
- What are greenhouse gas emissions from the transportation sector?
- How are individuals contributing to the build-up of greenhouse gases?
- How much carbon dioxide do humans contribute through breathing?
- What are the most important greenhouse gases? Where are they coming from and how have they changed?
Many greenhouse gases, like water vapor and carbon dioxide (CO2), occur naturally. Fuel burning and other human activities are adding large amounts of carbon dioxide and other gases to the natural mix at a faster rate than at any other time on record. Other important greenhouse gases produced by human activity include methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).
Since 1750, atmospheric concentrations of CO2, CH4 and N20 have increased by over 36 percent, 148 percent and 18 percent, respectively. Scientists have concluded that this is due primarily to human activity.
Source: EPA Greenhouse Gas Emissions
- Since 1990, how have greenhouse gas emissions in the U.S. changed?
Overall, total U.S. emissions have risen by 17 percent from 1990 to 2007. This trend is projected to continue at about 1 percent per year assuming current trends in economic growth and fuel consumption continue. The increase is driven principally by population and economic growth, and the rate of change is affected by energy price fluctuations, technological changes, seasonal temperatures, and other factors. On an annual basis, the overall consumption of fossil fuels in the United States generally fluctuates in response to changes in general economic conditions, energy prices, weather, and the availability of non-fossil alternatives. Future trends will be driven by changes that affect the scale of consumption (e.g., population, number of cars, and size of houses), the efficiency with which energy is used in equipment (e.g., cars, power plants, steel mills, and light bulbs), and consumer behavior (e.g., walking, bicycling, or telecommuting to work instead of driving). For details on U.S. greenhouse gas emissions, visit the U.S. Greenhouse Gas Inventory on EPA’s Climate Change site.
Source: EPA US GHG Trends (PDF) (40 pp, 975KB, About PDF)
Source: Energy Information Agency (EIA) International Energy Outlook (IEO) Table A10. World Carbon Dioxide Emissions by Region, Reference Case, 1990-2030: International Energy Outlook 2007 (PDF)
(230 pp, 1.36MB, About PDF) - What are the largest sources of greenhouse gas emissions in the U.S.?
In the U.S., our energy-related activities account for over three-quarters of our human-generated greenhouse gas emissions, mostly in the form of carbon dioxide emissions from burning fossil fuels. More than half the energy-related emissions come from large sources such as power plants and factories, while about a third comes from transportation. Industrial processes (such as the production of cement, steel, and aluminum), agriculture, other land use, and waste management are also important sources of greenhouse gas emissions in the United States. Forestry is also an important sector — in the U.S., after accounting for tree growth and harvesting, there is a net accumulation of carbon from the atmosphere and into biomass. This net accumulation partially offsets some of the emissions from other sources. For a detailed look at U.S. greenhouse gas emissions, visit the U.S. Greenhouse Gas Inventory on EPA’s Climate Change site.
Source: EPA US GHG Inventory Reports - What are greenhouse gas emissions from the power sector?
The process of generating electricity is the single largest source of emissions in the United States, representing 34 percent of emissions from all sources across the country in 2007. Electricity generation also accounted for the largest share of carbon dioxide emissions from fossil fuel combustion, approximately 42 percent in 2007. Electricity was consumed primarily by users in the residential, commercial, and industrial sectors for lighting, heating, electric motors, appliances, electronics, and air conditioning.
Source: EPA US GHG Inventory Reports - What are greenhouse gas emissions from the transportation sector?
In the U.S., the transportation sector accounts for approximately 33 percent of total carbon dioxide emissions from fossil fuel combustion, the largest share of any end-use economic sector in 2007. Emissions from this sector increased by 29 percent from 1990 to 2007, representing an average annual increase of 1.7 percent. Nearly 60 percent of the emissions resulted from gasoline consumption for personal vehicle use. The remaining emissions came from other transportation activities, including the combustion of diesel fuel in heavy-duty vehicles and jet fuel in aircrafts. Current details on emissions from the transportation and other sectors are available in the U.S. Greenhouse Gas Inventory.
Source: EPA US GHG Inventory Reports - How are individuals contributing to the build-up of greenhouse gases?
Many daily activities produce greenhouse gas emissions. Individuals can produce greenhouse gas emissions directly by driving a car or burning oil or gas for home heating. Individuals can also produce greenhouse gas emissions indirectly by using electricity generated from fossil fuels. In the United States, emissions per person vary depending on 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 that burns coal emits more greenhouse gases per unit of electricity than a power plant that uses natural gas. How much a person drives, the vehicle's fuel efficiency, and the proportion of driving time spent idling in traffic also affect the level of emissions. In addition, a household’s reuse and recycling of materials can affect emissions by reducing the amount of methane-generating waste sent to landfills. EPA’s household greenhouse gas emissions calculator provides a good estimate of emissions generated by individuals. For more information, visit the Individual Emissions page on EPA’s Climate Change site.
Source: EPA Individual Emissions - How much carbon dioxide do humans contribute through breathing?
The average person, through the natural process of breathing, produces approximately 2.3 pounds (1 kg) of carbon dioxide per day. The actual amount depends strongly on the person’s activity level. However, this carbon dioxide is part of a natural closed-loop cycle and does not contribute to the greenhouse gas concentrations in the atmosphere. Natural processes of photosynthesis (in plants) and respiration (in plants and animals) maintain a balance of oxygen and carbon dioxide in the atmosphere. Thus, the carbon dioxide from natural process is not included in greenhouse gas inventories.
In contrast, the burning of fossil fuels upsets this natural equilibrium by adding a surplus of carbon dioxide into the system. The carbon in fossil fuels has been stored underground for millions of years and thus is not part of the current natural carbon cycle. When those fuels are burned, the carbon dioxide generated is over and above the amount circulating from natural sources. Land use changes such as deforestation also upset the natural equilibrium by reducing the amount of carbon dioxide removed from the atmosphere by forests. Thus, both fossil fuel burning and deforestation are accounted for by scientists who develop greenhouse gas inventories to study how greenhouse gases contribute to climate change.
Source: Carbon Dioxide Information Analysis Center FAQs
