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Energy and the Environment

Greenhouse Gases

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Greenhouse Gases Basics

Did you know?

If it were not for 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 chemical compounds found in the earth’s atmosphere act as greenhouse gases. When sunlight strikes the earth’s surface, some of it is reradiated back toward space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap its heat in the atmosphere.

Many gases have greenhouse properties. Some gases occur naturally. Some gases are also produced by human activities and others, like 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)

There are several major greenhouse gases emitted as a result of human activity that 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

There are other greenhouse gases that are not counted in U.S. or international greenhouse gas inventories:

  • Water vapor is the most abundant greenhouse gas, but most scientists believe that water vapor produced directly by human activity contributes very little to the amount of water vapor in the atmosphere, so the U.S. Energy Information Administration (EIA) does not estimate emissions of water vapor. Research by NASA suggests a stronger impact from the indirect human effects on water vapor concentrations.
  • Ozone is technically a greenhouse gas because it has an effect on global temperature. At higher elevations in the atmosphere (the stratosphere), where ozone occurs naturally, ozone is needed to block harmful ultraviolet (UV) light. At lower elevations of the atmosphere (the troposphere), ozone is harmful to human health and is a pollutant regulated independently of its warming effects.
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Where Greenhouse Gases Come From

A pie chart showing the anthropogenic (gaseous emissions that are the result of human activities) greenhouse gas emissions by gas for 2011.
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Note: Values expressed as carbon dioxide equivalents (CO2e) are calculated based on their 100-year global warming potential (GWP).

In the United States, greenhouse gas (GHG) emissions come primarily from the burning of fossil fuels in energy use. Energy use is largely driven by economic growth (with short-term fluctuations in growth rate) and by weather patterns that affect heating and cooling needs. {/ee}

Carbon dioxide

Carbon dioxide is the main greenhouse gas.

Other greenhouse gases

There are several other greenhouse gases emitted by the United States as a result of human activity that are included in U.S. and international emissions estimates:

  • Methane (CH4), which comes from landfills, coal mines, agriculture, and oil and natural gas operations
  • Nitrous oxide (N2O), which comes from the use of nitrogen fertilizers, from burning fossil fuels, and from certain industrial and waste management processes
  • High global warming potential (GWP) gases, which are human-made industrial gases
    • Hydrofluorocarbons (HFCs)
    • Perfluorocarbons (PFCs)
    • Sulfur hexafluoride (SF6)
    • Nitrogen trifluoride (NF3)


The energy connection

Fossil fuels consist of hydrogen and carbon. When fossil fuels are burned, the carbon combines with oxygen to create CO2. The amount of CO2 produced depends on the carbon content of the fuel. For example, for the same amount of energy produced, natural gas produces about half and petroleum produces about three-fourths of the amount of CO2 produced by coal.

2 pie charts

Most carbon dioxide emissions come from petroleum and coal use

In 2014, 42% of U.S. energy-related carbon dioxide emissions came from the use of petroleum fuels, 32% came from coal, and about 27% came from natural gas. Although the industrial sector is the largest consumer of energy (including direct fuel use and purchased electricity), the transportation sector emits more CO2 because of its near complete dependence on petroleum fuels.

The residential sector and the commercial sector have lower emission levels than the transportation sector and the industrial sector. Most of the CO2 emissions for the residential sector and the commercial sector are from fossil fuel combustion to produce electricity.

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Effect on the Climate

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

World carbon dioxide emissions and atmospheric concentrations (1751–2011)
Line graph showing how carbon dioxide emissions and carbon dioxide concentrations are rising and are much closer together, now, than they were in the year 1751.
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Emissions of several important greenhouse gases have increased substantially since large-scale industrialization began in the mid-1800s. During the past 20 years, about three-fourths of human-caused (anthropogenic) emissions came from burning fossil fuels. Concentrations of carbon dioxide (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), emissions have exceeded 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.

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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 2013 Reference case (July 25, 2013)

In the U.S. Energy Information Administration's International Energy Outlook 2013 Reference case, which does not assume new policies to limit greenhouse gas emissions, world energy-related carbon dioxide emissions increase from 31.2 billion metric tons in 2010 to 36.4 billion metric tons in 2020 and 45.5 billion metric tons in 2040. Much of the growth in emissions is attributed to developing countries that are not members of the Organization for Economic Cooperation and Development (OECD) that continue to rely heavily on fossil fuels to meet fast-paced growth in energy demand. Non-OECD carbon dioxide emissions total 31.6 billion metric tons in 2040, or 69% of the world total. In comparison, OECD emissions total 13.9 billion metric tons in 2040, or 31% of the world total.

The United States, with 4% of the world's population, produced about 17% of global carbon dioxide emissions from burning fossil fuels in 2011, the most recent year for which global data are available. The United States has the world's largest economy and meets 83% of its energy needs by burning fossil fuels.