What are the main greenhouse gases? The contribution of a gas to the variation of the greenhouse effect is determined by the radiative forcing of the gas, by its concentration in the atmosphere and by its residence time in the atmosphere.
The index known as Global Warming Potential, which represents the combined effect of the residence time in the atmosphere of each gas and the relative specific effectiveness in the absorption of infrared radiation emitted by the Earth, is a measure of how much a given greenhouse gas contributes to the global warming, commensurate with CO2, taken as a benchmark and whose GWP has by definition the value 1.
Water vapor, carbon dioxide, nitrous oxide, methane and sulfur hexafluoride are the main greenhouse gases in the earth's atmosphere. In addition to these gases of both natural and man-made origin, there is a wide range of greenhouse gases released into the atmosphere of exclusively man-made origin, such as chlorofluorocarbons, bromofluorocarbons and many other substances whose molecules contain halogens, whose emissions are regulated by the Protocol of Montreal.
The contribution of a gas to the variation of the greenhouse effect is determined by the radiative forcing of the gas, by its concentration in the atmosphere and by its residence time in the atmosphere. The main greenhouse gas is water vapor (H2O), responsible for about two thirds of the natural greenhouse effect, although there are opinions according to which water vapor is responsible for up to 98% of the greenhouse effect.
In the atmosphere, water molecules capture the heat radiated from the Earth and radiate it in all directions, thus heating the Earth's surface before being radiated back out into space. Methane (CH4) is typically considered responsible for about 8%, even if its real impact strongly depends on its Global Warming Potential, which if properly calculated on the average life of the gas in the atmosphere (about 12 years) passes from the commonly used about 30 to about 110, greatly increasing its radiative forcing.
Among these gases, the best known are chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). The concentration of these gases in the atmosphere is very low, but their heating potential is 3,000 to 13,000 times higher than CO2.
Halocarbons do not derive from natural processes; their presence in the atmosphere is mostly attributable to human activities. Until the mid-1970s, CFCs were widely used as propellants for spray cans, solvents and some adhesives.
Carbon dioxide, whose molecule has the formula CO2, is responsible for 5-20% (the most accredited theory is 15%) of the natural greenhouse effect and interacts with the atmosphere for natural and anthropic causes. The natural reservoirs of CO2 are the oceans, fossil sediments, the terrestrial biosphere, the atmosphere.
Much of the carbon dioxide from ecosystems is emitted into the atmosphere. A number of organisms have the ability to assimilate atmospheric CO2. Nitrous oxide makes up a very small part of the atmosphere, and is a thousand times less present than CO2, but nearly 300 times more powerful at retaining heat.
Ozone is minimally contained in the atmosphere and is mainly concentrated around a height of 45 km where it is formed by the reaction between UVA solar rays and atmospheric oxygen.
Stratospheric ozone acts as a filter against ultraviolet radiation from the Sun, but in the troposphere it acts as a greenhouse gas, although its contribution is minimal.