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dramatic climatic changes


Methane is produced through both natural and human activities. For example, natural wetlands, agricultural activities, and fossil fuel extraction and transport all emit CH4.

Methane is more abundant in Earth’s atmosphere now than at any time in at least the past 800,000 years.[2] Due to human activities, CHconcentrations increased sharply during most of the 20th century and are now more than two-and-a-half times pre-industrial levels. In recent decades, the rate of increase has slowed considerably.[2]

For more information on CH4 emissions and sources, and actions that can reduce emissions, see EPA’s Methane page in the Greenhouse Gas Emissions website. For information on how methane is impacting the Arctic, see the EPA report Methane and Black Carbon Impacts on the Arctic.

Nitrous oxide 

Nitrous oxide is produced through natural and human activities, mainly through agricultural activities and natural biological processes. Fuel burning and some other processes also create N2O. Concentrations of N2O have risen approximately 20% since the start of the Industrial Revolution, with a relatively rapid increase toward the end of the 20th century.[2]

Overall, N2O concentrations have increased more rapidly during the past century than at any time in the past 22,000 years.[2] For more information on N2O emissions and sources, and actions that can reduce emissions, see EPA’s Nitrous Oxide page in the Greenhouse Gas Emissions website.

Other greenhouse gases

  • Water vapor is the most abundant greenhouse gas and also the most important in terms of its contribution to the natural greenhouse effect, despite having a short atmospheric lifetime. Some human activities can influence local water vapor levels. However, on a global scale, the concentration of water vapor is controlled by temperature, which influences overall rates of evaporation and precipitation.[2] Therefore, the global concentration of water vapor is not substantially affected by direct human emissions.
  • Tropospheric ozone (O3), which also has a short atmospheric lifetime, is a potent greenhouse gas. Chemical reactions create ozone from emissions of nitrogen oxides and volatile organic compounds from automobiles, power plants, and other industrial and commercial sources in the presence of sunlight. In addition to trapping heat, ground-level ozone is a pollutant that can cause respiratory health problems and damage crops and ecosystems.
  • Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), together called F-gases, are often used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. Unlike water vapor and ozone, these F-gases have a long atmospheric lifetime, and some of these emissions will affect the climate for many decades or centuries.

For more information on greenhouse gas emissions, see the Greenhouse Gas Emissions website, including an expanded discussion of global warming potentials and how they are used to measure the relative strengths of greenhouse gases. To learn more about actions that can reduce these emissions, see What You Can Do.

Other climate forcers

Particles and aerosols in the atmosphere can also affect climate. Human activities such as burning fossil fuels and biomass contribute to emissions of these substances, although some aerosols also come from natural sources such as volcanoes and marine plankton.

  • Black carbon (BC) is a solid particle or aerosol, not a gas, but it also contributes to warming of the atmosphere. Unlike GHGs, BC can directly absorb incoming and reflected sunlight in addition to absorbing infrared radiation. BC can also be deposited on snow and ice, darkening the surface and thereby increasing the snow’s absorption of sunlight and accelerating melt. For information on how BC is impacting the Arctic, see EPA assessment Methane and Black Carbon Impacts on the Arctic.
  • Sulfates, organic carbon, and other aerosols can cause cooling by reflecting sunlight.
  • Warming and cooling aerosols can interact with clouds, changing a number of cloud attributes such as their formation, dissipation, reflectivity, and precipitation rates. Clouds can contribute both to cooling, by reflecting sunlight, and warming, by trapping outgoing heat.

For more information on greenhouse gas emissions, see the Greenhouse Gas Emissions website. To learn more about actions that can reduce these emissions, see What EPA is Doing and What You Can Do.