school energy&environment

Greenhouse gases

Natural greenhouse gases include water vapour, carbon dioxide, methane, nitrogen protoxide and ozone. Some human activities contribute to the increase in the concentration of these gases in the atmosphere and, furthermore, they free other greenhouse gases that are exclusively anthropogenic in the air.
Let us now examine the characteristics of the principal gases which have a greenhouse effect, in detail:
Water vapour
Water vapour is the principal greenhouse gas, it is responsible for 2/3 of the natural greenhouse effect, due to its abundance in the atmosphere and due to its efficacy. Atmospheric water vapour is part of the water cycle, a closed system of water circulation from the oceans and the continents to the atmosphere in a continuous cycle of evaporation, transpiration, condensation and precipitation. Its concentration is very variable in space, but relatively stable in time. Furthermore it is not directly influenced by human activities, but depends exclusively on the air temperature. For every degree Celsius more, the content of water vapour in the air increases 7%.
Carbon dioxide
Carbon dioxide (CO₂) is the principal greenhouse gas deriving from human activities, and it is the one that contributes most to the anthropic greenhouse effect. CO₂ is one of the main compounds of carbon and is the principal vehicle for carbon exchange in nature between reservoirs (or pools) that are present in the atmosphere, hydrosphere, geosphere and biosphere, through processes that form the carbon cycle:

• in the biosphere, carbon is present in organic molecules (e.g. lipids, glucides, etc.) amounting to about 2,000 billion tons or gigatons;

• in the oceans,  carbon is dissolved in the form of carbonates and bicarbonates, amounting to about 39,000 gigatons;

• in the geosphere, carbon can be found in the form of limestone, amounting to  90,000,000 gigatons and fossil fuel amounting to 6,000 gigatons;

• in the atmosphere,  carbon is found in the form of carbon dioxide, amounting to 750 gigatons.

These reservoirs are tied to each other by exchanges, and their natural balance, in absence of human activities, is practically equal. Starting from the Industrial Revolution, with the intensification of human activities, the concentrations of carbon dioxide in the atmosphere increased, and today CO₂ is responsible for 65% of the anthropic greenhouse effect. The carbon dioxide deriving from human activities is tied principally to the fossil fuels combustion reaction, to deforestation and changes in land use. CO₂ can persist in the atmosphere for a period that varies between 50 and 250 years before returning to the ground.
Methane
Methane (CH₄) is derived from anaerobic fermentation (i.e. from decomposition) of organic substances. In nature, methane is emitted by mangrove swamps, while the anthropogenic emissions derive mainly from the use of fossil fuels, animal farming, agriculture (paddy fields) and city dumps. Even though it is more powerful than CO₂, methane contributes to 17% of the anthropic greenhouse effect due to the minor concentration and the shorter period in which it remains in the atmosphere, compared to carbon dioxide.
Nitrogen protoxide
Nitrogen protoxide (N₂O) is a very powerful greenhouse gas, with a very long time of persistence in the atmosphere (120 years). Natural sources of N₂O are the oceans, the rainforests and the bacteria that are present in the ground. With regard to human activities, instead, it derives mainly from nitrogen fertilizers used in agriculture and some industrial productions.
Chlorofluorocarbons 
Chlorofluorocarbons or CFC, unlike the gases described here above, which can all be found in nature, are produced artificially by humans and used as refrigerants, spray bottle propellants, and fire extinguishers.
Besides being responsible for destroying the ozone in the stratosphere (Ozone Hole) they are powerful greenhouse gases and they persist in the atmosphere for thousands of years. Their emissions have been remarkably reduced with the application of the Montreal Protocol (1987) which prohibited their use. However even the gases that they have been replaced with (hydrofluorocarbons and perfluorocarbons) are powerful greenhouse gases.