EPA's Report on the Environment: External Review Draft
U.S. Greenhouse Gas Emissions
Note to reviewers of this draft revised ROE: This indicator reflects data through 2011. EPA anticipates updating this indicator in 2014.
Energy from the sun drives the Earth's weather and climate. The Earth absorbs some of the energy it receives from the sun and radiates the rest back toward space. However, certain gases in the atmosphere, called greenhouse gases (GHGs), absorb some of the energy radiated from the Earth and trap it in the atmosphere. These gases essentially act as a blanket, making the Earth’s surface warmer than it otherwise would be. This “greenhouse effect” occurs naturally, making life as we know it possible. Since the Industrial Revolution began in the late 1700s, however, people have added GHGs into the atmosphere by burning fossil fuels, cutting down forests, and conducting other activities (e.g., agriculture). The resulting substantial increase in GHG concentrations (see the GHG Concentrations indicator) is causing the atmosphere to trap more heat and leading to changes in the Earth’s climate.
A number of factors influence the quantities of GHGs released into the atmosphere by human activities, including economic activity, population, consumption patterns, energy prices, land use, and technology. There are several ways to track these emissions over time.
This indicator focuses on emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gases—all important greenhouse gases that are influenced by human activities (see table of key gases and sources). These particular gases are covered under the United Nations Framework Convention on Climate Change, an international agreement that requires participating countries to develop and periodically submit an inventory of greenhouse gas emissions. Data and analysis for this indicator come from EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011 (U.S. EPA, 2013). This indicator is restricted to emissions associated with human activities.
This indicator reports emissions of greenhouse gases according to their 100-year global warming potential, a measure of how much a given amount of the greenhouse gas is estimated to contribute to global warming over a period of 100 years after being emitted (see table). For purposes of comparison, global warming potential values are calculated in relation to carbon dioxide and are expressed in terms of carbon dioxide equivalents.
What the Data Show
In 2011, U.S. greenhouse gas emissions totaled 6,702 million metric tons of carbon dioxide equivalents, an 8 percent increase from 1990 (Exhibit 1). For the United States, during the period from 1990 to 2011:
- Emissions of carbon dioxide, the primary greenhouse gas emitted by human activities, increased by 10 percent.
- Methane emissions decreased by 8 percent, as reduced emissions from landfills, coal mines, and natural gas systems were greater than increases in emissions from activities such as livestock production (U.S. EPA, 2013).
- Nitrous oxide emissions, largely derived from vehicle emissions and agricultural soil management practices, such as the use of nitrogen as a fertilizer, increased by nearly 4 percent.
- Emissions of fluorinated gases (hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride), released as a result of commercial, industrial, and household uses, increased by 61 percent.
Electricity generation is the largest U.S. emissions source, accounting for about 32 percent of total U.S. greenhouse gas emissions since 1990 (Exhibit 2). Transportation is the second-largest source of greenhouse gas emissions, accounting for 27 percent of emissions since 1990. While emissions from most sectors have increased since 1990, emissions from industry have declined by 13 percent, reflecting improved energy efficiency, the switch from traditional to less carbon-intensive fuels, and the shift from a manufacturing-based economy to a service-based economy (U.S. EPA, 2013).
Emissions sinks, the opposite of emissions sources, absorb and store emissions. In 2011, 14 percent of U.S. greenhouse gas emissions were offset by sinks resulting from land use and forestry practices (Exhibit 2). One major sink is the net growth of forests, which remove carbon from the atmosphere. Other carbon sinks are associated with how people use the land, including the practice of depositing yard trimmings and food scraps in landfills.
Despite increases in U.S. GHG emissions from 1990 to 2011 overall, emissions decreased between 2007 and 2011 (Exhibits 1 and 2). This decline was seen for nearly all gases and sectors. The decrease in emissions can largely be attributed to lowered energy use due to slower economic growth and to fuel switching from coal to natural gas—a less carbon intensive fuel—as the cost of natural gas decreased compared with the cost of coal (U.S. EPA, 2013).
Overall, with less than one-twentieth of the world’s population (U.S. Bureau of the Census, 2013), the U.S. currently accounts for nearly one-fifth of total global emissions of carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride (World Resources Institute, 2012).
While this indicator addresses the major GHGs emitted by human activities, it does not include other greenhouse gases and substances that are not covered under the United Nations Framework Convention on Climate Change but that still affect the Earth’s energy balance and climate. For example, this indicator excludes ozone-depleting substances such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which have high global warming potentials, as these gases are being phased out under an international agreement called the Montreal Protocol. There are also many natural greenhouse gas emission sources; however, this indicator includes only emissions that are associated with human activities.
Data for this indicator came from EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011 (U.S. EPA, 2013). This report is available online at www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.
For More Information
- EPA's U.S. Greenhouse Gas Inventory Report
- EPA's Greenhouse Gas Reporting Program (includes a mapping tool to identify major sources of greenhouse gas emissions)
- This indicator also appears in EPA's Climate Change Indicators in the United States
- This indicator relates to: Greenhouse Gases
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