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Report on the Environment

Greenhouse Gases

Other Air Topics

What are the trends in greenhouse gas emissions and concentrations?

Greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and certain synthetic chemicals, trap some of the Earth’s outgoing energy, thus retaining heat in the atmosphere.11 Changes in the radiative balance of the Earth—the balance between energy received from the sun and emitted from Earth—as a result of this heat trapping alter weather patterns and climates at global and regional scales.12 Natural factors, such as variations in the sun’s output, volcanic activity, the Earth’s orbit, the carbon cycle, and others, also affect the radiative balance.13 However, increasing concentrations of greenhouse gases due to human activity are affecting various aspects of climate, such as surface air temperature and subsurface ocean temperature. Since 1750, the net global effect of human activities has been one of warming.14 Human health, agriculture, water resources, forests, wildlife, and coastal areas all are vulnerable to climate change.15 The purpose of this section is to evaluate long-term trends in air emissions and ambient concentrations of greenhouse gases that are contributing to climate change, but not to evaluate the effects that these emissions and concentrations cause.16

Though the focus of this question is on greenhouse gases, related factors can also alter the Earth’s climate. Certain radiatively important substances, like black carbon (soot), are technically not greenhouse gases due to their physical state, but they nonetheless affect the flow of energy through the atmosphere. Some of these substances, such as sulfate aerosols, have negative radiative forcings that can lead to cooling effects. Another related factor is albedo (the reflectivity of the Earth’s surface), which affects the portions of absorbed and outgoing energy. Natural and human factors can affect albedo on a global scale (through changes in large-scale features like the polar ice caps) or on a local or regional scale (e.g., by increased amounts of dark paved surfaces that absorb energy). Although this question does not address radiatively important substances that are not greenhouse gases or non-chemical factors like albedo, these influences are also important to understanding the planet’s energy balance and the ways human activities may affect that balance.17 Quantitative information on the relative radiative forcings from greenhouse gases, other radiatively important substances, and selected non-chemical factors is available in other publications.18

Some greenhouse gases are emitted exclusively from human activities (e.g., synthetic halocarbons). Others occur naturally but are found at elevated levels due to human inputs (e.g., carbon dioxide). The anthropogenic sources result from energy-related activities (e.g., combustion of fossil fuels in the electric utility and transportation sectors), agriculture, land-use change, waste management and treatment activities, and various industrial processes. Major greenhouse gases and emissions sources include:

  • Carbon dioxide, widely reported as the most important anthropogenic greenhouse gas.19 Carbon dioxide occurs naturally as part of the global carbon cycle, but human activities have increased atmospheric loadings through combustion of fossil fuels and other emissions sources.20 Natural sinks that remove carbon dioxide from the atmosphere (e.g., oceans, plants) help regulate carbon dioxide concentrations, but human activities can disturb these processes (e.g., deforestation) or enhance them.

  • Methane, which comes from many sources, including human activities such as coal mining, natural gas distribution, waste decomposition in landfills, and digestive processes in livestock and agriculture.21 Natural sources include wetlands and termite mounds.

  • Nitrous oxide, which is emitted during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels.

  • Various synthetic chemicals, such as hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and other synthetic gases, which are released as a result of commercial, industrial, or household uses.

  • Many other gases that are known to trap heat in the atmosphere. Examples include water vapor, which occurs naturally as part of the global water cycle, and ozone, which occurs naturally in the stratosphere and is found in the troposphere largely due to human activities.

Each gas has a different ability to absorb heat in the atmosphere, due to differences in its atmospheric half-life and the amount and type of energy that it absorbs. For example, it would take thousands of molecules of carbon dioxide to equal the warming effect of a single molecule of sulfur hexafluoride—the most potent greenhouse gas, in terms of ability to absorb heat, evaluated by the Intergovernmental Panel on Climate Change.22 To facilitate comparisons between gases that have substantially different properties, the Panel has developed a set of scaling factors called “global warming potentials,” as discussed further in the indicator write-ups.

The remainder of this section focuses on greenhouse gas emissions and concentrations, given that greenhouse gases can affect radiative forcings, thus leading to climate change. However, climate change can also affect atmospheric concentrations of many substances through various feedback mechanisms. Other publications provide detailed information on the broader issues of how climate change can affect air quality.23

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