Explain the mechanism behind global warming ?
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Answer:
Modern global warming is the result of an increase in magnitude of the so-called greenhouse effect, a warming of Earth's surface and lower atmosphere caused by the presence of water vapour, carbon dioxide, methane, nitrous oxides, and other greenhouse
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Global Warming : Mechanism, Effects of Global Warming are as follows:
The greenhouse effect is a process by which radioactive energy leaving a planetary surface is absorbed by some atmospheric gases, called greenhouse gases. They transfer this energy to other components of the atmosphere, and it is re-radiated in all directions, including back down towards the surface. This transfers energy to the surface and lower atmosphere, so the temperature there is higher than it would be if direct heating by solar radiation were the only warming mechanism.
The primary greenhouse gases in the Earth’s atmosphere are water vapour, carbon dioxide, methane, nitrous oxide, and ozone. the most abundant greenhouse gases in Earth’s atmosphere are:
i. Water vapour.
ii. Carbon dioxide, methane.
iii. Nitrous oxide.
iv. Ozone, chlorofluorocarbons.
The greenhouse gases present in the troposphere and resulting in an increase in the temperature of air and the earth are discussed here:
CO2:
It contributes about 55% to global warming from greenhouse gases produced by human activity. Industrial countries account for about 76% of annual emissions. The main sources are fossil fuel burning (67%) and deforestation, other forms of land clearing and burning (33%).
CFC:
These are believed to be responsible for 24% of the human contribution to greenhouse gases. They also deplete ozone in the stratosphere. The main sources of CFCs include leaking air conditioners and refrigerators, evaporation of industrial solvents, production of plastic foams, aerosols, propellants etc.
Methane:
It accounts for 18% of the increased greenhouse gases. Methane is produced when bacteria break down dead organic matter in moist places that lack oxygen such as swamps, natural wetlands, paddy fields, landfills and digestive tracts of cattle, sheep and termites.
N2O:
It is responsible for 6% of the human input of greenhouse gases. Besides trapping heat in the troposphere it also depletes ozone in the stratosphere. It is released from nylon products, from burning of biomass and nitrogen rich fuels (especially coal) and from the breakdown of nitrogen fertilizers in soil, livestock wastes and nitrate contaminated groundwater.
1. Mechanism of Global Warming:
i. The incoming radiation from the Sun is mostly in the form of visible light and nearby wavelengths, largely in the range 0.2 – 4 1m, corresponding to the Sun’s radiative temperature of 6,000 K. Almost half the radiation is in the form of “visible” light, which our eyes are adapted to use.
ii. About 50% of the Sun’s energy is absorbed at the earth’s surface and the rest is reflected or absorbed by the atmosphere. The reflection of light back into space – largely by clouds – does not much affect the basic mechanism; this light, effectively, is lost to the system.
iii. The absorbed energy warms the surface. Simple presentations of the greenhouse effect, such as the idealized greenhouse model, show this heat being lost as thermal radiation. The reality is more complex: the atmosphere near the surface is largely opaque to thermal radiation (with important exceptions for “window” bands), and most heat loss from the surface is by sensible heat and latent heat transport.
Radiative energy losses become increasingly important higher in the atmosphere largely because of the decreasing concentration of water vapour, an important greenhouse gas. It is more realistic to think of the greenhouse effect as applying to a “surface” in the mid-troposphere, which is effectively coupled to the surface by a lapse rate.
2. Effects of Global Warming:
(i) Global Temperature Increase:
It is estimated that the earth’s mean temperature will rise between 1.5 to 5.5°C by 2050 if input of greenhouse gases continues to rise at the present rate. Even at the lower value, earth would be warmer than it has been for 10,000 years.
(ii) Rise in Sea Level:
With the increase in global temperature sea water will expand. Heating will melt the polar ice sheets and glaciers resulting in further rise in sea level. Current models indicate that an increase in the average atmospheric temperature of 3°C would raise the average global sea level by 0.2-1.5 meters over the next 50-100 years.
(iii) Effects on Human Health:
The global warming will lead to changes in the rainfall pattern in many areas, thereby affecting the distribution of vector-borne diseases like malaria, filariasis, elephantiasis etc. Areas which are presently free from diseases like malaria; schistosomiasis etc. may become the breeding grounds for the vectors of such diseases.
Warmer temperature and more water stagnation would favour the breeding of mosquitoes, snails and some insects, which are the vectors of such diseases. Higher temperature and humidity will increase/aggravate respiratory and skin diseases.
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