Which Gases Are Greenhouse Gases?
ACS Climate Science Toolkit | Greenhouse Gases
Some greenhouse gases occur naturally and enter the atmosphere as a result of both natural processes (such as decomposition of organic matter) and human activity (such as burning fossil fuels and agriculture). Greenhouse gases that occur both naturally and from human activities include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3). Other greenhouse gases have essentially no natural sources, but are side products of industrial processes or manufactured for human purposes such as cleaning agents, refrigerants, and electrical insulators. These include the fluorinated gases: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HCFCs), bromofluorocarbons (halons), perfluorcarbons, PFCs, nitrogen trifluoride, NF3, and sulfur hexafluoride, SF6.
These gases contribute to atmospheric warming when they absorb infrared radiation emitted by the solar-warmed Earth and transfer their extra energy to the surrounding atmospheric gases. The figure shows that the temperature at the surface of the Earth has increased by about 0.9 °C during the past century with more than half the change since about 1980. The temperature can change only if there is a change in the Earth’s energy budget as the balance between the energies of incoming and outgoing radiation is upset.
Upsetting the Earth’s energy balance can be the result of changes in many factors, including energy from the sun, greenhouse gases, and cloud cover. A change in any one these factors changes the amount of radiation reaching the Earth’s surface or being emitted into space. The effect of such a change is a radiation imbalance that causes the Earth’s surface either to heat or cool. The size of this imbalance for each factor that affects the changing surface temperature is characterized in terms of a “radiative forcing”, that is, the amount by which it upsets the Earth’s energy balance.
The Intergovernmental Panel on Climate Change (IPCC) defines radiative forcing as the change in net radiation flux (incoming minus outgoing in W·m–2) at the top of the troposphere caused by a change in some forcing factor relative to its pre-industrial state (taken as the mid-eighteenth century). A positive radiative forcing value means the change in the forcing factor increases the energy retained by the planet and leads to warming. A negative value means the change decreases the energy retained by the planet and leads to cooling. This graphical summary from the IPCC Report provides radiative forcing values for greenhouse gases and other factors with an indication of their uncertainties (which help show where more studies could further our understanding of the climate variables).
The “temperature anomaly” plotted on the graph is equivalent to the difference between the annual mean temperature for a year and the mean of the temperatures for the base period 1951-1980. The black line is the annual mean and the red line is the five-year running mean. The green bars show uncertainty estimates.
Changes in Atmospheric Constituents and in Radiative Forcing
These radiative forcings represent the present state of the planet and its atmospheric warming, but the effects of possible future changes in the composition of the greenhouse gases are not captured. Another property of greenhouse gases that is used to assess possible future impacts is their global warming potential. Quantifying radiative forcing and GWP depends on the infrared spectra of greenhouse gases, so infrared spectroscopy has a central role in climate science research.
Several other gases can have indirect effects on atmospheric warming. This occurs when chemical reactions in the atmosphere produce or destroy greenhouse gases, including tropospheric ozone. Indirect effects also occur when a gas influences atmospheric lifetimes of other gases or affects atmospheric processes like cloud formation that alter Earth’s radiative energy balance by increasing Earth’s albedo. Gases that can cause these indirect effects include carbon monoxide (CO), oxides of nitrogen (NOx) and non-methane volatile organic compounds (NMVOC). For more on greenhouse gas sources, see What are the greenhouse changes since the Industrial Revolution? and Greenhouse Gas Sources and Sinks.
The total forcing from all sources cannot be obtained by simply adding the individual values because different sources can interact to amplify or interfere with one another. For example, two greenhouse gases might absorb in the same wavelength region and partially offset one another’s effectiveness when they are present together.