ENVIRONMENTAL SCIENCE : SOCIAL ISSUES 203
The Greenhouse Effect is a natural phenomenon that plays a central role, in determining
the earth’s climate. The hot surface of the sun radiates heat and light energy. Several gases
in the atmosphere are transparent to light but absorb infrared radiation. These allow sunlight
to pass through the atmosphere and be absorbed by the earth’s surface. This energy ,is
again radiated as heat energy, which is absorbed by the gases. As the effect is similar in
nature to what happens in a’ botanical greenhouse (the glass panes allows the light energy
to enter inside but diminishes the loss of heat), these gases are called greenhouse gases and
the resultant warming from their increase is called the greenhouse effect. Anthropogenic
activities add to the phenomenon accelerating greenhouse gas building process. Global increase
of greenhouse gases in the atmosphere viz., carbon dioxide, nitrous oxide, methane and
chlorofluorocarbons are now well documented. In addition to all these changes, troposphere
and stratospheric chemistry are being modified due to the addition of these gases as well
as emission of carbon monoxide, nitrogen oxides and other compound. The United State
Environmental Protection Agency, Office of Policy, Planning and Evaluation in 1989 have
documented the increase of the different green house gases.
Table 6.1: Major Green House Gases Contributing to Global WarmingSl.No. Gas Contribution to global
warming- Carbon dioxide 57
- Chlorofluorocarbons 25
- Methane 12
- Nitrous oxide 6
The concentration of carbon dioxide into the atmosphere has increased by 25% since the
industrial revolution. Carbon dioxide is increasing at a rate of about 0.4% per year and is
responsible for about half of the current increases in the greenhouse effect. The concentration
of methane has more than doubled during the last three centuries. Agricultural sources
particularly rice cultivation and animal husbandry has probably been the most significant
contributors to historical increase in concentrations. But there is the potential for rapid
growth in emissions from landfills, coal seems, permafrost, natural gas explorations and
pipeline leakage, and biomass burning associated with forest clearings in the future.
The concentrations of nitrous oxide have increased by 5-10% since pre-industrial times.
The cause of this increase is highly uncertain, but it has been understood that the use of
nitrogenous fertilizer, land clearing biomass burning and fossil fuel combustion have all
contributed. Nitrous oxide is currently increasing at a rate of about 0.25% per year, which
represents and imbalance between sources and sinks of about 30%. CFC’s were introduced
into the atmosphere for the first time during the century; the most common species are
CFC-12 and CFC-II. Of major concern because of their potential to deplete stratospheric
ozone, the CFCs also represent about 15% of the current increases in the greenhouse effect.
The chemistry of the atmosphere is changing due to emission of carbon monoxide,
nitrogen oxides, and volatile organic compounds, among other species, in addition to the
changes in the greenhouse gases already described. This alters the amount and distribution
of ozone and the oxidizing power of the atmosphere. which changes the lifetimes of methane