Saturday, 07 February 2015 06:26






Atmosphere is a thick gaseous envelope that surrounds the earth and extends thousands of kilometers above the earth’s surface. Much of the life on the earth exists because of the atmosphere otherwise the earth would have been barren. In fact, atmosphere directly or indirectly influences the vegetation pattern, soil type and topography of the earth.

The atmosphere is a mixture of several gases. The major constant gas components which remain the same over time and location are:  Nitrogen (N2) 78%; Oxygen (O2) 21% and Argon (Ar) 1%.

The major variable gas components that vary over time and location are: Carbon Dioxide (CO2) 0.038% ; Water Vapor (H20) 0-4% ; Methane (CH4) trace ; Sulfur dioxide (SO2) trace ; Ozone (O3) trace and Nitrogen oxides trace (NO, NO2, N2O).

While nitrogen and oxygen comprise 99% of the atmospheric gases, they have little effect on atmospheric processes and consequently little or no effect on weather or climate. The gases which make up far less than 1 percent of the atmosphere have a much greater influence on both short-term weather and long-term climate.

Most of the incoming solar radiation (short wavelength, shown in purple) is absorbed and converted to long wavelength radiation at or near the Earth's surface. Thermal energy (heat) results from the absorption of some long wavelength radiation by atmospheric gases, including water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These gases have the ability to absorb thermal energy (heat) emitted by the earth and thus are able to warm the atmosphere. This warming is popularly called as the "greenhouse effect."

There are obvious benefits to these so-called greenhouse gases as without them the surface of the earth would be about 30 degrees Celsius cooler, and far too cold for life to exist. On the other hand, these greenhouse gases are so thermally potent that even proportionately small amounts can cause Earth’s lower atmospheric temperature to rise.

But the concentration of heat-trapping gases in the atmosphere has climbed to a record high in 2011, according to the analysis of observations from the World Meteorological Organization’s (WMO) Global Atmosphere Watch programme. The atmospheric concentration of carbon dioxide (CO2)-the single most important greenhouse gas- reached 390.9 parts per million (p.p.m.) in 2011 and is now 40% above the pre-industrial level of 280 p.p.m. Methane (1,813 parts per billion) and nitrous oxide (324 parts per billion)-both potent greenhouse gases-also reached new highs. From 1990 to 2011, radiative forcing by long-lived greenhouse gases has increased by almost one-third, with CO2 alone accounting for about 80% of this increase. Since the start of the industrial era in 1750, about 1,377 billion tonnes of CO2 have been released into the atmosphere.

About one half of that amount may have been absorbed by the ocean and by soils and plants on land. The other half lingers in the atmosphere, causing temperatures near the surface to warm.

There are ten primary GHGs; of these, water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are naturally occurring. Perfluorocarbons (CH4, C2F6), hydrofluorocarbons (CHF3, CF3CH2F, CH3 CHF2), and sulfur hexafluoride (SF6), are only present in the atmosphere due to industrial processes.

Some salient characteristics of GHGs:

• Water vapor is the most abundant and dominant GHG in the atmosphere. Its concentration depends on temperature and other meteorological conditions, and not directly upon human activities.

• CO2 is the primary anthropogenic greenhouse gas, accounting for 77% of the human contribution to the greenhouse effect in 2004.

• The second-most important greenhouse gas for the enhanced greenhouse effect is methane (CH4). Since the beginning of the Industrial Revolution, atmospheric methane concentrations have doubled and contributed some 20% to the enhancement of the greenhouse gas effect. In industrialized countries, methane accounts typically for 15% of greenhouse gas emissions.

• Methane is created predominantly by bacteria that feed on organic material where there is a lack of oxygen. It is therefore emitted from a variety of natural and human-influenced sources. Natural sources include wetlands, termites, and oceans. Human-influenced sources include the mining and burning of fossil fuels, livestock husbandry (cattle eat plants that ferment in their stomachs, so they exhale methane and their manure contains it), rice cultivation (flooded paddy fields produce methane since organic matter in the soil decomposes without sufficient oxygen) and landfills (again, organic waste decomposes without sufficient oxygen).

• Nitrous oxide (N2O) is released naturally from oceans and rainforests and by bacteria in soils. Human-influenced sources include nitrogen-based fertilizers, fossil fuel combustion and industrial chemical production using nitrogen, such as sewage treatment.

• Fluorinated greenhouse gases are the only greenhouse gases that do not occur naturally, but have been developed by man for industrial purposes. Their share of greenhouse gas emissions from industrialized countries is around 1.5%. But they are extremely powerful-they can trap heat up to 22,000 times more effectively than CO2-and they can stay in the atmosphere for thousands of years.