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Energy and the Environment

Greenhouse Gases

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Greenhouse Gases Basics

Did you know?

If there were no naturally occurring greenhouse gases, the earth would be too cold to support life as we know it. Without the greenhouse effect, the average temperature of the earth would be about -2°F rather than the 57°F we currently experience.

Many of the chemical compounds in the earth’s atmosphere act as greenhouse gases. When sunlight strikes the earth’s surface, some of it radiates back toward space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap its heat in the atmosphere, creating a greenhouse effect that results in global warming and climate change.

Many gases have greenhouse properties. Some gases occur naturally and are also produced by human activities. Some, such as industrial gases, are only human made.

What are the types of greenhouse gases?

Image of the Earth showing the steps involved in the Greenhouse Effect. 1. Solar radiation passes through the clear atmosphere.  2. Most radiation is absorbed by the Earth's surface and warms it.  3. Some solar radiation is reflected by the Earth and the atmosphere.  4. Some of the infrared radiation passes through the atmosphere, and some is absorbed and re-emitted in all directions by greenhouse gas molecules. The effect of this is to warm the Earth's surface and the lower atmosphere.  5. Infrared radiation is emitted from the Earth's surface.
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Source: Adapted from U.S. Environmental Protection Agency (public domain)

Several major greenhouse gases that are emitted as a result of human activity are included in U.S. and international estimates of greenhouse gas emissions:

  • Carbon dioxide (CO2)
  • Methane (CH4)
  • Nitrous oxide (N2O)
  • Industrial gases:
    • Hydrofluorocarbons (HFCs)
    • Perfluorocarbons (PFCs)
    • Sulfur hexafluoride (SF6
    • Nitrogen trifluoride (NF3



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Where Greenhouse Gases Come From

In the United States, most of the emissions of human-caused (anthropogenic) greenhouse gases (GHG) come primarily from burning fossil fuels (coal, hydrocarbon gas liquids, natural gas, and petroleum) for energy use. Economic growth (with short-term fluctuations in growth rate) and weather patterns that affect heating and cooling needs are the main factors that drive the amount of energy consumed. Energy prices and government policies can also affect the sources or types of energy consumed.

Carbon dioxide

In 2015, emissions of carbon dioxide (CO2) resulting from the combustion of fossil fuels for energy were equal to 77% of total U.S. anthropogenic GHG emissions (based on global warming potential) and about 93% of total U.S. anthropogenic CO2 emissions. Carbon dioxide emissions from other anthropogenic sources and activities were about 5% of total GHG emissions and about 7% of total CO2 emissions.1

The energy connection

Fossil fuels consist mainly of carbon and hydrogen. When fossil fuels are burned, oxygen combines with carbon to form CO2 and with hydrogen to form water (H2O). These reactions release heat that we use for energy. The amount of CO2 produced depends on the carbon content of the fuel, and the amount of heat produced depends on the carbon and hydrogen content. Because natural gas, which is mostly methane (CH4), has a high hydrogen content, combustion of natural gas produces less CO2 for the same amount of heat produced from burning other fossil fuels. For example, for the same amount of energy produced, burning natural gas produces about half of the amount of CO2 produced by burning coal.

About half of U.S. energy-related CO2 emissions are from petroleum use

In 2016, about 45% of U.S. energy-related carbon dioxide emissions came from burning petroleum fuels, 29% came from burning natural gas, and 28% came from burning coal.2

2 pie charts. The first pie chart shows U.S. energy consumption by major fuel/energy sources in 2016: Coal 15%; Non-fossil fuels 19%, natural gas 29%, and petroleum 37%. The second pie chart shows the resulting carbon dioxide emissions by type of fossil fuel: Coal 26%, natural gas 29%, and petroleum 45%.

1 U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2015, Executive Summary, April 2017.
2 U.S. Energy Information Administration, Monthly Energy Review, Environment, April 2017, preliminary data.

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Effect on the Climate

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Greenhouse gas emissions and atmospheric concentrations have increased over the past 150 years


Emissions of several important greenhouse gases that result from human activity have increased substantially since large-scale industrialization began in the mid-1800s. Most of these human-caused (anthropogenic) greenhouse gas emissions were carbon dioxide (CO2) from burning fossil fuels.

Concentrations of CO2 in the atmosphere are naturally regulated by many processes that are part of the global carbon cycle. The flux, or movement, of carbon between the atmosphere and the earth's land and oceans is dominated by natural processes like plant photosynthesis. Although these natural processes can absorb some of the anthropogenic CO2 emissions produced each year (measured in carbon equivalent terms), starting in about 1950, emissions began exceeding the capacity of these processes to absorb carbon.

This imbalance between greenhouse gas emissions and the ability for natural processes to absorb those emissions has resulted in a continued increase in atmospheric concentrations of greenhouse gases. Concentrations of CO2 in the atmosphere have increased by about 40% since the mid-1800s.

Greenhouse gases warm the planet

Scientists know with virtual certainty that increasing greenhouse gas concentrations tend to warm the planet.

Did you know?

Fossil fuels supplied about 81% of the primary energy consumed in the United States and were responsible for about 93% of total U.S. carbon dioxide emissions from human activity in 2015.

The report later states, "It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together."

The report also states, “Concentrations of CO2, CH4, and N2O now substantially exceed the highest concentrations recorded in ice cores during the past 800,000 years. The mean rates of increase in atmospheric concentrations over the past century are, with very high confidence, unprecedented in the last 22,000 years.”

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Outlook for Future Emissions

Carbon dioxide emissions are expected to increase

Bar graph showing projected greenhouse gas emissions over the next two decades growing faster for non-OECD countries than for OECD countries.
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Source: U.S. Energy Information Administration, International Energy Outlook 2016, Reference case, May 2016

In the U.S. Energy Information Administration's International Energy Outlook 2016 (IEO2016) Reference case, world energy-related carbon dioxide (CO2) emissions are projected to increase from 32.3 billion metric tons in 2012 to 35.6 billion metric tons in 2020 and to 43.2 billion metric tons in 2040.