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Energy and the environment

Greenhouse gases

Greenhouse gases basics

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 exhibit these greenhouse properties. Some gases occur naturally and are also produced by human activities. Some, such as industrial gases, are exclusively human made.

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.

Source: Adapted from U.S. Environmental Protection Agency (public domain)

Did you know?

Without 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. However, historical measurements show that the current global atmospheric concentrations of carbon dioxide, methane, and nitrous oxide are higher than they have been within the past 800,000 years.

What are the types of greenhouse gases?

Several major greenhouse gases that result from 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)

Other greenhouse gases not counted in U.S. or international greenhouse gas inventories are water vapor and ozone.

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. Therefore, the U.S. Energy Information Administration (EIA) does not estimate emissions of water vapor.

Ozone is technically a greenhouse gas, but ozone is helpful or harmful depending on where it is found in the earth's atmosphere. Ozone occurs naturally at higher elevations in the atmosphere (the stratosphere) where it blocks ultraviolet (UV) light that is harmful to plant and animal life from reaching the earth’s surface. The protective benefits of stratospheric ozone outweigh its contribution to the greenhouse effect. The United States and countries all around the world ban and control production and use of several industrial gases that destroy atmospheric ozone and create holes in the ozone layer. Learn more about ozone layer protection. At lower elevations of the atmosphere (the troposphere), ozone is harmful to human health. Learn more about ground-level ozone pollution and what is being done to reduce ozone pollution.

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 changes 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.

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, which 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 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.

Carbon dioxide

According to EPA, in 2021, CO2 emissions accounted for about 79% of total U.S. anthropogenic GHG emissions (based on 100-year global warming potential). Burning fossil fuels for energy accounted for 73% of total GHG emissions and for 92% of total U.S. anthropogenic CO2 emissions in the United States. CO2 emissions from other anthropogenic sources and activities were about 6% of total GHG emissions and 8% of total CO2 emissions.1

About half of U.S. energy-related CO2 emissions were from petroleum use in 2022

In 2022, petroleum accounted for about 36% of U.S. energy consumption, but petroleum was the source of 46% of total annual U.S. energy-related CO2 emissions. Natural gas accounted for about 32% of U.S. energy consumption and for about 34% of total annual energy-related CO2 emissions. Coal accounted for about 11% of U.S. energy consumption and for about 21% of total annual energy-related CO2 emissions.2

The transportation sector accounts for the largest share of U.S. energy-related CO2 emissions

Consumption of fossil fuels accounts for most of the energy-related CO2 emissions of the major energy-consuming sectors: commercial, industrial, residential, transportation, and electric power. Although the industrial sector was the highest energy end-use sector in 2022 (includes direct primary energy use and electricity purchases from the electric power sector) sector, the transportation sector emitted more CO2 because of its near complete dependence on petroleum fuels.

1 U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2021, April 2023. Includes U.S. Territories.

2 U.S. Energy Information Administration, Monthly Energy Review, Environment, May 2023, preliminary data.

Effect on the climate

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 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.

An illustration of the global carbon cycle.

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. In computer-based models, rising concentrations of greenhouse gases produce an increase in the average surface temperature of the earth over time. Rising temperatures may produce changes in precipitation patterns, storm severity, and sea level. All together, these effects are commonly referred to as climate change.

Assessments by the Intergovernmental Panel on Climate Change (IPCC) suggest that the earth’s climate warmed 0.92 degrees Centigrade (1.66 degrees Fahrenheit) between 1880 and 2012 and that human activity affecting the atmosphere is likely an important driving factor. The IPCC Sixth Assessment Report, (Chapter 3: Human Influence on the Climate System) states: "It is unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times."

Did you know?

In 2021, fossil fuels were the source of about 73% of total U.S. human-caused (anthropogenic) greenhouse gas emissions.

The report later states: "The likely range of human-induced warming in global-mean surface air temperature in 2010–2019 relative to 1850–1900 is 0.8°C–1.3°C." The report also states: "The current rates of increase of the concentration of the major greenhouse gases (carbon dioxide, methane and nitrous oxide) are unprecedented over at least the last 800,000 years. Several lines of evidence clearly show that these increases are the results of human activities."

Outlook for future emissions

Carbon dioxide emissions are expected to increase

EIA’s International Energy Outlook 2023 Reference case projects that if current policy and technology trends continue, global energy consumption and energy-related CO2 emissions will increase from 2020 through 2050 as a result of population and economic growth. However, projected future growth in energy-related CO2 emissions is not evenly distributed across the world, and the majority of the projected future growth in energy-related CO2 emissions is among the group of countries outside the Organization for Economic Cooperation and Development (OECD).