Climate Change Indicators in the United States
* HFCs are hydrofluorocarbons, PFCs are perfluorocarbons, and SF6 is sulfur hexafluoride.
Data source: U.S. EPA, 2014 4
Data source: U.S. EPA, 2014 5
Data source: U.S. EPA, 2014 6
- In 2012, U.S. greenhouse gas emissions totaled 6,526 million metric tons (14.4 trillion pounds) of carbon dioxide equivalents. This 2012 total represents a 5 percent increase since 1990 but a 10 percent decrease since 2005 (see Figure 1).
- For the United States, during the period from 1990 to 2012 (see Figure 1):
- Emissions of carbon dioxide, the primary greenhouse gas emitted by human activities, increased by 5 percent.
- Methane emissions decreased by 11 percent, as reduced emissions from landfills, coal mines, and natural gas systems were greater than increases in emissions from activities such as livestock production. 2
- Nitrous oxide emissions, predominantly from agricultural soil management practices such as the use of nitrogen as a fertilizer, increased by nearly 3 percent.
- Emissions of fluorinated gases (hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride), released as a result of commercial, industrial, and household uses, increased by 83 percent.
- Electricity generation is the largest U.S. emissions source, accounting for 32 percent of total greenhouse gas emissions since 1990. Transportation is the second-largest source of greenhouse gas emissions, accounting for 27 percent of emissions since 1990 (see Figure 2).
- Emissions sinks, the opposite of emissions sources, absorb carbon dioxide from the atmosphere. In 2012, 15 percent of U.S. greenhouse gas emissions were offset by sinks resulting from land use and forestry practices (see Figure 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.
- Emissions increased at about the same rate as the population from 1990 to 2007, which caused emissions per capita to remain fairly level (see Figure 3). 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.3
- From 1990 to 2012, greenhouse gas emissions per dollar of goods and services produced by the U.S. economy (the gross domestic product or GDP) declined by 39 percent (see Figure 3). This change may reflect a combination of increased energy efficiency and structural changes in the economy.
A number of factors influence the quantities of greenhouse gases released into the atmosphere, including economic activity, population, consumption patterns, energy prices, land use, and technology. There are several ways to track these emissions, such as by measuring emissions directly, calculating emissions based on the amount of fuel that people burn, and estimating other activities and their associated emissions. EPA has two key programs that provide data on greenhouse gas emissions in the United States: the Inventory of U.S. Greenhouse Gas Emissions and Sinks and the Greenhouse Gas Reporting Program. See the Greenhouse Gases overview to learn more about these programs.
About the Indicator
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. 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 annual inventory submission, the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2012.1 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) Second Assessment Report. In that report, methane has a global warming potential of 21, which means a ton of methane emissions contributes 21 times as much warming as a ton of carbon dioxide emissions over 100 years, and that ton of methane emissions is therefore equal to 21 tons of carbon dioxide equivalents. See the table 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.
While this indicator includes the major greenhouse gases 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 (see the Climate Forcing indicator for more details). For example, this indicator excludes ozone-depleting substances such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which have high global warming potentials, as these gases have been or are currently 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 emissions 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: 1990–2012. This report is available online at: www.epa.gov/climatechange/ emissions/
usinventoryreport.html. The calculations in Figure 3 are based on GDP and population data provided by the U.S. Bureau of Economic Analysis and the U.S. Census, respectively.
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