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Climate Change

Climate Change Indicators in the United States

Gases and Substances Not Included in This Report

This report addresses most of the major, well-mixed greenhouse gases that contribute to warming of the climate. The report does not address trends in emissions or concentrations of substances with shorter atmospheric lifetimes (i.e., less than a year) that are also relevant to climate change, such as ozone in the lower atmosphere, pollutants that lead to ozone formation, water vapor, and aerosols (atmospheric particles) such as black carbon and sulfates. These substances may be considered for future editions of this report.

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, 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. While this “greenhouse effect” occurs naturally, making life as we know it possible, human activities in the past century have substantially increased the amount of greenhouse gases in the atmosphere, causing the atmosphere to trap more heat and leading to changes in the Earth's climate.

What is happening?

The major greenhouse gases emitted into the atmosphere through human activities are carbon dioxide, methane, nitrous oxide, and fluorinated gases (see Major Greenhouse Gases Associated With Human Activities). Some of these gases are produced almost entirely by human activities; others come from a combination of natural sources and human activities.

Many of the major greenhouse gases can remain in the atmosphere for tens to hundreds of years after being released. They become globally mixed in the lower atmosphere, reflecting contributions from emissions sources worldwide.

Several factors determine how strongly a particular greenhouse gas will affect the Earth's climate. One factor is the length of time that the gas remains in the atmosphere. A second factor is each gas’s unique ability to absorb energy. By considering both of these factors, scientists calculate a gas's global warming potential, as compared to an equivalent mass of carbon dioxide (which is defined by a global warming potential equal to 1).

Why does it matter?

As greenhouse gas emissions from human activities increase, they contribute to more warming of the climate, leading to many other changes around the world—in the atmosphere, on land, and in the oceans. These changes will have both positive and negative effects on people, plants, and animals. Because many of the major greenhouse gases can stay in the atmosphere for tens to hundreds of years after being released, their warming effects on the climate will persist over a long time.

Major Greenhouse Gases Associated With Human Activities

Greenhouse gas How it's produced Average lifetime in the atmosphere 100-year global warming potential
Carbon dioxide Emitted primarily through the burning of fossil fuels (oil, natural gas, and coal), solid waste, and trees and wood products. Changes in land use also play a role. Deforestation and soil degradation add carbon dioxide to the atmosphere, while forest regrowth takes it out of the atmosphere. see below* 1
Methane Emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and agricultural practices and from the anaerobic decay of organic waste in municipal solid waste landfills. 12 years 21
Nitrous oxide Emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste. 114 years 310
Fluorinated gases A group of gases that includes hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride, among other chemicals. These gases are emitted from a variety of industrial processes and commercial and household uses, and do not occur naturally. Sometimes used as substitutes for ozone-depleting substances such as chlorofluorocarbons (CFCs). A few weeks to thousands of years Varies (the highest is sulfur hexafluoride at 23,900)

This table shows 100-year global warming potentials, which describe the effects that occur over a period of 100 years after a particular mass of a gas is emitted. EPA uses global warming potentials from the Intergovernmental Panel on Climate Change's (IPCC's) Second Assessment Report, 1 as countries have agreed to do under current international guidelines within the United Nations Framework Convention on Climate Change (UNFCCC). Lifetimes come from the IPCC's Fourth Assessment Report. 2

* Carbon dioxide's lifetime is poorly defined because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some of the excess carbon dioxide will be absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.

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