U.S. Greenhouse Gas Emissions

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 U.S. 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: 1990–2013 (U.S. EPA, 2015). This indicator is restricted to emissions associated with human activities.

Each greenhouse gas has a different lifetime (how long it stays in the atmosphere) and a different ability to trap heat in our atmosphere. To allow different gases to be compared and added together, emissions are converted into carbon dioxide equivalents. This step uses each gas’s 100-year global warming potential, which measures how much a given amount of the gas is estimated to contribute to global warming over a period of 100 years after being emitted. Carbon dioxide is assigned a global warming potential equal to 1. This analysis uses global warming potentials from the Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report. In that report, methane has a global warming potential of 25, which means a ton of methane emissions contributes 25 times as much warming as a ton of carbon dioxide emissions over 100 years, and that ton of methane emissions is therefore equal to 25 tons of carbon dioxide equivalents. See the table of key gases for comparison with global warming potentials from IPCC’s Fifth Assessment Report. For additional perspective, this indicator also shows greenhouse gas emissions in relation to economic output and population.

In 2013, U.S. greenhouse gas emissions totaled 6,673 million metric tons (14.7 trillion pounds) of carbon dioxide equivalents. This 2013 total represents a 6 percent increase since 1990 but a 9 percent decrease since 2005 (see Exhibit 1).

For the United States, during the period from 1990 to 2013 (see Exhibit 1):

• Emissions of carbon dioxide, the primary greenhouse gas emitted by human activities, increased by 7 percent.
• Methane emissions decreased by 15 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, 2015).
• Nitrous oxide emissions, predominantly from agricultural soil management practices, such as the use of nitrogen as a fertilizer, increased by nearly 8 percent.
• Emissions of fluorinated gases (hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride), released as a result of commercial, industrial, and household uses, increased by 73 percent.

Among the various sectors of the U.S. economy, electricity generation (power plants) accounts for the largest share of emissions—32 percent of total greenhouse gas emissions since 1990. Transportation is the second-largest sector, accounting for 26 percent of emissions since 1990 (Exhibit 2).

Emissions sinks, the opposite of emissions sources, absorb carbon dioxide from the atmosphere. In 2013, 13 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.

Total emissions and emissions per capita declined from 2007 to 2009, due in part to a drop in U.S. economic production during this time. Emissions decreased again from 2010 to 2012, largely due to the growing use of natural gas to generate electricity in place of more carbon-intensive fuels (U.S. EPA, 2015).

Overall, with less than one-twentieth of the world’s population (U.S. Bureau of the Census, 2015), 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, 2014).

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. This indicator also excludes black carbon and aerosols, which most greenhouse gas emissions inventories do not cover. There are also many natural greenhouse gas emission sources; however, this indicator includes only emissions that are associated with human activities—those that are most responsible for the observed buildup of these gases in our atmosphere.

Data for this indicator came from EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2013 (U.S. EPA, 2015). This report is available online at http://www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html.