The main greenhouse gases are carbon dioxide, methane, nitrous oxide and the fluorinated gases, with water vapor amplifying their effect. Each is released by different activities and each warms the planet to a different degree.
These gases are the working core of climate change explained in the wider guide, which also covers the evidence and outlook.
How greenhouse gases trap heat
The US EPA defines the term simply: gases that trap heat in the atmosphere are called greenhouse gases. They act as a blanket. NASA explains that sunlight is absorbed at the surface and radiated back as infrared heat, and that about 90% of this heat is then absorbed by greenhouse gases and re-radiated, slowing heat loss to space.
Some of this is natural and helpful; without it the planet would be far colder. The concern is the extra load from human activity. The IPCC reports that well-mixed greenhouse gases contributed a warming of 1.0°C to 2.0°C, within a total human-caused warming that brought global surface temperature to 1.1°C above 1850-1900 in 2011-2020, as set out in the Sixth Assessment Report synthesis.
Three properties decide how much a given gas matters. The first is how much of it is in the air. The second is how long it stays there, which the US EPA says ranges from a few years to thousands of years. The third is how strongly each molecule absorbs energy. All of these gases remain in the atmosphere long enough to become well mixed, so measurements taken in one place are roughly representative of the whole planet.
Carbon dioxide
Carbon dioxide is the gas most associated with climate change, mainly because of the quantities involved. According to the US EPA, it enters the atmosphere through burning fossil fuels (coal, natural gas and oil), solid waste, trees and other biological materials, and also through certain chemical reactions such as cement production. The agency’s global overview adds that fossil fuel use is the primary source, and that deforestation and the clearing of land can also release carbon dioxide.
The IPCC Working Group I summary records an annual average concentration of 410 ppm in 2019, a rise of 47% since 1750, and states that this was higher than at any time in at least 2 million years. It also notes that land and ocean have taken up a near-constant proportion, globally about 56% per year, of carbon dioxide emissions from human activities over the past six decades.
Carbon dioxide is also the reference gas. The US EPA explains that increases in its atmospheric concentration will last thousands of years, which is why total cumulative emissions matter so much for long-run warming.
Methane
Methane has both natural and human sources. NASA lists the breakdown of plant matter in wetlands, landfills and rice farming, livestock digestion and manure, and leaks from fossil fuel production and transport, adding that natural gas is 70% to 90% methane. The US EPA names agricultural activities, waste management, energy production and use, and biomass burning as contributors.
Methane is the second most abundant human-produced greenhouse gas after carbon dioxide, accounting for about 11 percent of global emissions, according to the US EPA’s Global Methane Initiative. The IPCC puts the 2019 annual average at 1866 ppb, an increase of 156% since 1750, with methane and nitrous oxide both higher than at any time in at least 800,000 years, as the Working Group I summary reports.
Methane absorbs much more energy than carbon dioxide but does not last as long. The US EPA’s global warming potential page estimates a 100-year global warming potential of 27 to 30, and says methane emitted today lasts about a decade on average. That combination makes methane cuts a fast-acting lever, though they do not replace cuts in carbon dioxide.
Nitrous oxide and fluorinated gases
Nitrous oxide comes mainly from farming. The US EPA’s global overview calls agricultural activities, such as fertilizer use, the primary source, and NASA notes that it is released during commercial and organic fertilizer production and use. The IPCC records a 2019 level of 332 ppb, up 23% since 1750. Per the US EPA, it has a 100-year global warming potential 273 times that of carbon dioxide and remains in the atmosphere for more than 100 years on average.
The fluorinated gases are a different group. They include hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride. The US EPA links their emissions to industrial processes, refrigeration and a variety of consumer products. Chlorofluorocarbons, which NASA says do not exist in nature and are entirely of industrial origin, were used as refrigerants, solvents and spray can propellants.
These gases are typically emitted in smaller quantities than the others but are potent. The EPA’s global warming potential page says their warming potentials can be in the thousands or tens of thousands, and that some, such as perfluorocarbons, persist for hundreds or thousands of years. The IPCC Working Group III summary records that the highest relative growth in emissions since 1990 occurred in fluorinated gases, starting from low levels.
Water vapor, ozone and aerosols
Water vapor is a greenhouse gas but behaves differently. According to the US EPA, warmer air holds more moisture, so as greenhouse gas concentrations increase and temperatures rise, the amount of water vapor in the atmosphere also increases, further amplifying the warming. Direct human releases, such as evaporation from irrigation and power plant cooling, have been determined to have a negligible effect on global climate. Water vapor is therefore a feedback rather than a driver.
Ground-level ozone is created by reactions between nitrogen oxides and volatile organic compounds in sunlight. Besides trapping heat, it is a pollutant that can cause respiratory health problems and damage crops and ecosystems.
Aerosols are not greenhouse gases but they alter the balance. Those that reflect sunlight, such as sulfur emissions from burning coal, have a cooling effect, while those that absorb sunlight, such as black carbon, have a warming effect. This is a reminder that a gas-by-gas view is a simplification of a more complex system.
Comparing the gases and where emissions arise
To compare gases on one scale, analysts use the global warming potential, which the US EPA defines as a measure of how much energy the emission of 1 ton of a gas will absorb over a given period of time, relative to the emission of 1 ton of carbon dioxide. The time period usually used is 100 years. A deeper treatment is given under global warming potential, but the table below gives the main comparison.
| Gas | 100-year global warming potential | Typical lifetime in the atmosphere |
|---|---|---|
| Carbon dioxide | 1 (the reference) | Concentration increases last thousands of years |
| Methane | 27 to 30 | About a decade on average |
| Nitrous oxide | 273 | More than 100 years on average |
| Fluorinated gases | Thousands or tens of thousands | Some persist for hundreds or thousands of years |
The values in this table come from the US EPA’s page on global warming potentials, which also notes that carbon dioxide has a value of 1 by definition regardless of the time period used.
The same US EPA global overview shows how emissions break down by sector for 2019: electricity and heat production 34%, industry 24%, agriculture, forestry and other land use 22%, transportation 15% and buildings 6%. Because each sector emits a different mix of gases, the best route to cutting emissions differs from one to the next.
The stakes of these emissions are economic as well as physical. Pages on the cost of climate inaction and on the social cost of carbon show how analysts translate extra tonnes of gas into damages that can be compared with the cost of reducing them.
Frequently asked questions
Which greenhouse gas is the most important?
Carbon dioxide is the most discussed because fossil fuel use is its primary source and its concentration increases last thousands of years. Methane, nitrous oxide and fluorinated gases add to the total, and the IPCC counts them all within the contribution of well-mixed greenhouse gases.
Why is water vapor not regulated like the other gases?
Human releases of water vapor are determined to have a negligible effect on global climate, while its amount in the air rises in response to warming. It is treated as a feedback, so policy focuses on the gases people emit in significant volumes.
What does CO2 equivalent mean?
It is a way to express different gases in one unit by scaling each according to its global warming potential. The US EPA notes this lets analysts add up emissions estimates of different gases, for example in a national inventory.
Are all greenhouse gases man-made?
No. Carbon dioxide and methane also come from natural sources such as volcanic eruptions and wetlands, but the IPCC states that the rise in concentration since 1750 is unequivocally caused by human activities. Some fluorinated compounds, including chlorofluorocarbons, are purely industrial.
