The green house effect, first discovered by Joseph Fourier in 1824, and first investigated quantitatively by Svante Arrhenius in 1896, is the radioactive forcing process by which an atmosphere warms a planet The name is from the similar effect which greenhouses utilize in order to facilitate plant growth.
Presently there is general acceptance by the scientists that the average earth temperature is increasing due to green house effect.
The earth receives an enormous amount of solar radiation. The direct solar radiation flux just above the atmosphere, averages about 1366 watt per square meter or 1.740 X 1017 watt after being distributed over the entire earth, which is much more than the power generated by human activities. Most of the solar radiation leaves the earth and the amount of energy from the sun that is absorbed by the earth is small.
Certain gases such as CO2, CFCs in the upper atmosphere trap infrared radiation preventing it from being rejected into the Space and these gases consequently enhance a radiative forcing effect that contributes to the global warming. The degree of the green house effect is dependent primarily on the concentration of green house gases in the earth’s atmosphere.
The earth will be warmer with gases in the upper atmosphere than it would be without them. The warming effect of these gases over time depends upon their quantity, their chemical and radioactive properties, their lifetime in the atmosphere and the time required to ascend into Stratosphere.
With industrial revolution, use of fossil fuel has increased considerably there by CO2 emission has increased on a large scale whereas CO2 absorption by the plants has reduced due to felling of trees to a large extent. It is obvious that the emission of CO2 and CFCs into atmosphere creates global Warming because of green house effect.
OZONE DEPLETION DUE TO GREEN HOUSE EFFECT
The gases nitrogen and oxygen together make up about 99% of the volume of the dry atmosphere. The remaining 1% is comprised of a number of different gases of which ozone, carbon dioxide, nitrous oxide, and , methane are most important to life on the earth. Ozone is concentrated in a layer that extends from 15 to 55 km above the earth’s surface. Ozone is important to life because it absorbs harmful ultraviolet radiation from the sun. Recent investigations of the ozone layer have revealed areas of severe thinning located primarily at the South Pole.
Researchers have determined that chlorofluorocarbons (CFCs) along with other chlorine and bromine containing compounds have been largely responsible for accelerated depletion of ozone in the earth’s stratosphere.
Not until 1973, was chlorine found to be catalytic agent in ozone destruction. CFCs in stratosphere are destroyed by Solar ultraviolet radiation, thereby releasing chlorine atoms that do react with O3. Global monitoring of ozone level has shown statistically significant downward trends at all latitudes outside the tropics. Measurements at several ground based stations have shown corresponding upward trend in CFCs in both the northern as well as southern hemisphere. Despite rapid-phase out of CFCs, ozone levels are expected to be lower than pre-depletion levels for several decades due to long tropospheric lifetime of CFCs. Hydro chlorofl-uorocarbons (HCFCS) also contain chlorine atoms, but the presence of hydrogen makes them reactive with chemicals in the troposphere. And hence ozone depletion potential of HCFCs, as chlorine will be removed by chemical processes in the lower atmosphere.
Ozone Depletion Potential (ODP) and Global Warming potential (GWP) of some important refrigerants on a comparative scale is shown in the table below:
|Refrigerant||Chemical Formulae||ODP||GWP||Estimated Atmospheric Life in years|
|HYDRO FLUORO CARBONS|
|R-502||48.8% R-22 & 51.2% R-115||0.22||7300||–|
The Montreal protocol on substances that deplete the ozone layer in an international treaty designed to protect the ozone layer by phasing out the production of a number of substances believed to be responsible for ozone depletion.
The treaty is structured around several groups of halogenated hydrocarbons that have been shown to play a role in ozone depletion. For each group, the treaty provides a time table on which the production of those substances must be phased out and eventually eliminated.
- Construction and production of CFCs and R-ll, R-12, R-113,R-114 and R-115 has been phased out by 1996. .
- There is a slower phase out (to zero by 2010) of other substances ( Halon 1211, 1301, 2402,CFCs, R-13, 111, 112 etc.) and some chemicals (carbon tetrachloride,1,1,1-trichloroethane) the phasing out of the less active HCFCs started only in 1996 and will go on till a complete phasing out is achieved in 2030.
- A fund has been created to help the developing countries for the implementation of the Montreal protocol. The fund is utilized for phasing out the use of ozone depleting substances, for _ example, to finance the conversion of existing manufacturing processes, train personnel, pay loyalties and patent rights on new technologies etc.
This treaty has been a historic step and due to this widespread adoption and implementation, has been hailed as one of the most successful international agreement to date. The treaty was opened for signature on September 16,1987 and entered into force on January 1,1989. There have been five revisions to the treaty. At present, 189 nations have become party to this treaty.
Since the Montreal protocol came into effect, the atmospheric concentrations of CF Cs and other related chlorinated hydrocarbons have either leveled off or decreased whereas the concentration of HCFCs has increased.