The atmosphere surrounding our Earth is essential for life. The chemical composition of the atmosphere changes with altitude and gases occurring in different proportions and sometimes in traces in the different strata have very important roles in the equilibrium between the Earth and the Sun. They therefore affect the climate and the radiation reaching the Earth's surface. The stratospheric ozone layer filters part of the ultraviolet component of the Sun's radiations. The presence of a certain concentration of carbon dioxide maintains climate.

Like biological equilibria, the chemical equilibria of the different layers ofthe atmosphere are extremely sensitive to changes. Those induced by humans can trigger irreversible processes and chain reactions. Anthropogenic emissions ofgreenhouse gases have global implications in terms of sustainability. As indicated by the 1997 Kyoto Protocol, the principal greenhouse gases are CO2, CH4, N2O, HFC, PFC, and SF6. Further species are NOm CO, non-methane volatile organic compounds (NMVOC), and SO2.

The surprising climatic importance ofa gas like carbon dioxide present in the atmosphere in traces (0.03%), is due to a special property: it absorbs a part ofthe spectrum of radiant energy to which the other atmospheric gases are transparent. In other words, radiation emitted by the Earth which would normally pass through the atmosphere into space, is trapped by carbon dioxide.

The Sun emits a spectrum of radiation ranging from ultraviolet, through visible to infrared (IR). Some of these wavelengths are partially absorbed, diffused, or reflected by atmospheric gases, suspended particles, and clouds. The main components of atmosphere, oxygen and nitrogen, are essentially transparent to terrestrial radiation. Stratospheric ozone absorbs strongly in the ultraviolet and visible regions. Water vapor absorbs mainly in the IR. Clouds can reflect up to 70% of the total incident radiation, depending on its altitude and thickness. The portion of radiant energy absorbed by the atmosphere adds to the global thermal content of the atmosphere. Energy reaching the surface of the Earth is absorbed and partly reflected and returned to space; this depends on the reflective power of the surface, known as albedo. Absorbed energy heats the surface of the Earth.

For the surface temperature to remain stable over the period of a year, the Earth and its atmosphere must radiate into space as much energy as they absorb from the Sun. This is the only way to maintain the natural equilibrium. The Earth and atmosphere can lose energy to space only by the emission of radiation in the IR region. The wavelength of the IR energy returned is different from the wavelength of the radiation coming from the Sun. This is why gases like carbon dioxide and water vapor do not absorb the incoming radiation but absorb the outgoing IR radiation. Increasing concentrations of these gases in the atmosphere absorb increasing quantities of certain bands of IR. The energy absorbed by these gases is returned to the Earth as IR radiation, increasing the total flux of radiant energy on the Earth's surface.

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