Greenhouse gases are atmospheric gases that absorb and emit infrared radiation and therefore influence the thermal energy balance of Earth. Important greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, ozone, and several fluorinated gases. Their molecular structures allow them to interact with specific wavelengths of infrared radiation. Although some greenhouse gases occur naturally, human activities have increased the atmospheric concentration of several of them. Their concentrations may be small compared with nitrogen and oxygen, but their radiative properties make them highly important to climate. Greenhouse gases are therefore defined by their physical interaction with radiation rather than by whether they are natural or human made.
Water vapour is the most abundant greenhouse gas in the atmosphere and has a major influence on the natural greenhouse effect. Its concentration varies strongly with temperature, location, and weather because warmer air can contain more water vapour before saturation is reached. Human emissions of water vapour generally have relatively short lived effects compared with long lived greenhouse gases because atmospheric water cycles rapidly through evaporation, condensation, clouds, precipitation, and runoff. Water vapour therefore acts mainly as a feedback rather than as the principal long term forcing driving modern climate change. When other greenhouse gases warm the atmosphere, increased water vapour can amplify that warming.
Carbon dioxide is a long lived greenhouse gas and is the most important human caused contributor to long term warming. It is released through fossil fuel combustion, cement manufacture, deforestation, land use change, and several industrial processes. Carbon dioxide is also naturally exchanged among the atmosphere, oceans, soils, vegetation, and geological reservoirs. Human emissions disturb these natural carbon cycle flows by adding carbon from geological stores to the active surface system. Some emitted carbon is absorbed by oceans and terrestrial ecosystems, but a substantial fraction remains in the atmosphere and contributes to increased radiative forcing.
Methane is a potent greenhouse gas with a much shorter atmospheric lifetime than carbon dioxide. Important sources include livestock, wetlands, rice cultivation, landfills, wastewater, coal mining, oil and gas extraction, and distribution systems. Methane absorbs infrared radiation efficiently and also participates in atmospheric chemical reactions. Its oxidation eventually produces carbon dioxide and water and can influence other atmospheric constituents. Methane is therefore important both because of its direct radiative effect and because of its broader role in atmospheric chemistry. Its relatively short lifetime means that reducing methane emissions can affect atmospheric concentrations comparatively quickly.
Nitrous oxide is a long lived greenhouse gas whose human emissions are strongly associated with agricultural nitrogen use, manure, soil processes, and some industrial activities. Fluorinated gases include hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and related compounds. Many fluorinated gases are highly effective infrared absorbers and can have atmospheric lifetimes ranging from years to thousands of years depending on the substance. Ozone is also a greenhouse gas, although its climate role differs according to altitude and concentration. Stratospheric ozone is especially important because it also absorbs ultraviolet radiation, while tropospheric ozone contributes to warming and air pollution.
Greenhouse gases differ in atmospheric lifetime, concentration, radiative efficiency, sources, and sinks. Atmospheric lifetime is the approximate timescale over which a substance remains influential in the atmosphere, although removal can involve multiple reservoirs and is not always represented by one simple decay time. Radiative efficiency describes how effectively a gas influences radiative transfer for a given change in concentration. Global warming potential is one metric used to compare the integrated warming influence of different gases over a specified time horizon, commonly relative to carbon dioxide. The choice of time horizon matters because gases such as methane have different atmospheric lifetimes from carbon dioxide.
Greenhouse gases are not the same thing as air pollutants in every context. Some greenhouse gases are also harmful pollutants or participate in pollution chemistry, while others can be present at concentrations that are not directly toxic but still influence climate. Carbon dioxide, for example, is a greenhouse gas at atmospheric concentrations far below those normally associated with direct toxicity. Methane is flammable and contributes to atmospheric chemistry but is primarily important climatically because of its radiative and chemical effects. Greenhouse gases should therefore be classified according to their climate relevant physical properties rather than simply being described as pollutants.
Greenhouse gases are fundamental components of the climate system because they regulate how efficiently Earth loses thermal energy to space. Natural greenhouse gases make the planet substantially warmer than an atmosphere without infrared absorbing gases would be. Human activities have increased the concentrations of several important greenhouse gases, strengthening the greenhouse effect and causing global warming. Understanding each gas requires considering its molecular properties, atmospheric lifetime, sources, sinks, radiative efficiency, chemical reactions, and interactions with oceans, land, ecosystems, and other atmospheric constituents. Their combined influence is determined by both concentration and physical effectiveness, rather than simply by which gas is most abundant.
