The idea that some gases trap thermal radiation like the glass of a greenhouse was first proposed by Jean Baptiste-Joseph Fourier in 1827. However, this analogy is not strictly correct, because greenhouses warm the air in their interior mainly by blocking con-vective mixing with the outside, not by trapping thermal radiation. However, in spite of this, gases that are relatively transparent to solar radiation and relatively opaque to thermal radiation are known as greenhouse gases. On Earth, water vapor (H2O) is the most important greenhouse gas. Other important gases are carbon dioxide (CO2), nitrous oxide (N2O), chlorofluorocarbons (CFCs), methane (CH4), and ozone (O3). Anthropogenic emissions are important sources of all greenhouse gases, except water vapor. However, the atmospheric water vapor content increases with increases in surface temperature. Therefore, the atmospheric concentration of water vapor increases with anthropogenic emission of greenhouse gases. The increase in atmospheric water vapor content induced by anthropogenic emissions in responsible for a large fraction of the greenhouse effect.
Carbon dioxide
Carbon dioxide (CO2) is a colorless and odorless gas consisting of one carbon and two oxygen atoms. CO2 is produced when carbon compounds are burned in the presence of oxygen. It is produced during forest fires and the combustion of fossil fuels. CO2 is also produced by the decay of organic matter and volcanic eruptions. Other important sources of carbon dioxide are emission by the oceans and respiration by humans and animals. Important sinks of carbon dioxide are absorption by oceans, photosynthesis, and plankton and plant growth. Because the sources and sinks of carbon dioxide varied significantly during the Earth’s history, its atmospheric concentration also varied significantly. There is evidence that the concentration of carbon dioxide was very high in the Earth’s early history, then declined steadily and reached a quasi-steady value of about 280 parts per million. The concentration varied around this value with climate fluctuations.
However, the huge increase in man-made emissions of carbon dioxide that came with the Industrial Revolution has been making its concentration to increase steadily since the end of the 18th century. Today, the concentration of carbon dioxide has already exceeded 350 parts per billion and continues to increase at the rate of more than one part per million per year. The atmospheric concentration of carbon dioxide has been continuously measured at Mauna Loa, Hawaii, since 1957. It shows a clear annual cycle, resulting mainly from changes photosynthesis and a steady increase since the measurements started. The atmospheric concentration of CO2 before this record started can be measured from bubbles of air trapped in ice cores. They show a steady increase since the beginning of the Industrial Revolution.
Human-induced emissions of carbon dioxide have increased dramatically and steadily since the end of the 18th century.
Toxic gases
Nitrous Oxide is a non-toxic gas consisting of one oxygen and two nitrogen atoms (N2O). The main sources of nitrous oxide are natural processes in soils and the ocean, chemical fertilizers, and deforestation. Ice core data suggests that the atmospheric concentration of N2O was approximately constant before the Industrial Revolution started. The annual increase in the atmospheric concentration of N2O is estimated to be about 0.3 percent, according to the IPCC.
Chlorofluorocarbons (CFCs) are non-toxic gases composed primarily of carbon, hydrogen, chlorine, and fluorine. They are used mainly as cleaning agents, refrigerants, fire retardants, and in aerosol sprays. Since they are chemically inert in the troposphere, they have a long residence time, and reach the stratosphere where they are photodissociated by ultraviolet radiation. Then, free chlorine atoms destroy ozone catalytically. Thus, besides being important greenhouse gases, CFCs are responsible for destroying the ozone layer that protects humans from harmful ultraviolet radiation.
Methane is an odorless, non-toxic, flammable gas consisting of one carbon and four hydrogen atoms (CH4). Mainly anaerobic or oxygen-deprived processes produce methane. It forms when the digestion of organic material by bacteria releases single carbon atoms. The main sources of methane are wetlands, the combustion of fossil fuels, animals, rice plantations, biomass burning, landfills, termites, and the oceans. Methane is removed from the atmosphere primarily by reaction with hydroxyl radicals (OH). The atmospheric concentration of methane has also been steadily increasing since the beginning of the Industrial Revolution. The atmospheric concentration of CH4 has been increasing at the rate of about 1 percent per year.
Ozone (O3) is formed via the dissociation of molecular oxygen by ultraviolet radiation. It is the most photochemically active greenhouse gas. Ozone is an important greenhouse gas because it strongly absorbs ultraviolet radiation. About 90 percent of the atmospheric ozone is found on the stratospheric ozone layer. However, the ozone concentration in very polluted areas can exceed the stratospheric levels.
The concentration of water vapor increases exponentially with temperature. This fact and the presence of water reservoirs such as lakes and oceans make the opacity of the atmosphere a strong function of surface temperature. Thus, global increases in temperature (global warming) produce a positive feedback in the climate system. Increases in the surface temperature causes increases in the amount of atmospheric water vapor that produce further increases in the surface temperature. This positive feedback makes the greenhouse effect increase dramatically with temperature. Above a critical value of the solar forcing, the atmospheric water vapor feedback becomes so large that equilibrium is not possible and a runaway greenhouse occurs. In fact, there is evidence that the dependence of the concentration of water vapor on temperature might lead to multiple climate equilibria.
