The greenhouse effect was discovered by Joseph Fourier in 1824, and was first investigated quantitatively by Svante Arrhenius in 1896. Radiation from the Sun, when measured at the top of the Earth's atmosphere, can be seen to cover a wide spectrum. This spectrum is consistent with the blackbody radiation associated with the temperature of the Sun's surface (5250 K). The spectrum observed from the Earth's surface is consistent with this once the adsorption of atmospheric species has been accounted for. The peak intensity of the solar radiation is around 500 nm., or in the UV-Visible region.
The Earth absorbs this radiation and reradi-ates at a lower wavelength, again consistent with a blackbody radiation at its temperature. The peak radiation emitted by the Earth is around 1,000 nm., which corresponds to infrared. Different atmospheric gases absorb infrared radiation, and are collectively referred to as greenhouse gases. This absorption traps the outgoing radiation, maintaining the planet at habitable temperatures.
Greenhouse gases absorb IR radiation at characteristic frequencies that correspond to the normal modes of vibration of the molecule. Greenhouse gases of notable importance include water (H2O), which causes about 36-70 percent of the greenhouse effect (not including clouds); CO2, which causes 9-26 percent; methane (CH4), which causes 4-9 percent; and ozone, which causes 3-7 percent, according to Kiehl et al. These gases all absorb radiation, as they have a vibrational mode that results in a change in dipole of the molecule. These vibrational modes are normally investigated using IR spectroscopy. The IR spectrum of CO2 reveals that it absorbs at two characteristic frequencies; the Bending mode at 14 |im, and the Asymmetric Stretch at four |im.
Greenhouse gases are rated with a greenhouse potential, which ranks their ability to absorb the Earth's radiation relative to CO2 and gives some idea of the relative contributions of each component. Although water is a powerful greenhouse gas, the contribution of additional water is hard to quantify, as its concentration varies greatly throughout the atmosphere. In many areas of the globe, the water vapor concentration is such that all IR radiation of the relevant wavelength is absorbed. This highlights the importance of CO2 as a greenhouse gas, which absorbs wavelengths that water does not.
The amount of time a chemical persists in the atmosphere clearly has great importance for many compounds such as CO2, whereas the fastest loss of CO2 is to the oceans, the main chemical sink in the atmosphere is oxidation. Oxidants are present in the atmosphere as a result of the evolution of photosyn-thetic organisms around 2-3 billion years ago. The primary oxidants present include oxygen (O2), ozone (O3), and the hydoxy radical (OH).
Global warming is proposed to occur because of growing atmospheric concentration of the greenhouse gases; these can be increased by a variety of human processes. The concentration of CO2 in the atmosphere has risen from less than 250 ppm (parts per million) to over 380 ppm since pre-industrial times. This is as a consequence of the burning of fossil fuels such as coal, oil, and gas. Thus carbon that was stored in the ground is released in the form of CO2.
An aerosol is a colloidal suspension of either liquid or solid particles in the gas phase. Aerosols are created in the atmosphere by the action of the winds suspending dust, sea salt, and organic matter, and from biogenic sources such as plant pollen, as well as from anthropogenic sources such as fossil fuel burning. Aerosols are of interest, as one of the key areas of uncertainty in climate change predictions involves the role of aerosols.
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