We are very familiar with the term ‘Global warming’. What does this mean? What is it’s consequence on life on earth? Which materials are responsible for this? All these will be discussed here.
The historical record of global temperature shows that our planet has maintained its average temperature within a limited range, but within those bounds it is changing continuously, although at a very slow rate.
From the zero dimensional (although the temperature variation is a function of latitude, longitude, altitude and time, single average global temperature which is not a function of location or time is considered) energy balance model which equates the solar energy absorbed by the earth and the energy the earth radiates back to space, the average surface temperature of the earth would be 254K (- 19°C).
The actual value of average surface temperature however, is about 288K (15°C). Thus, the model differs from reality in a wide range.
This is so because the interaction between the atmosphere and the radiation that is emitted from the earth’s surface was not considered. This means the atmosphere has a big role for maintaining the right kind of temperature for sustaining life on the earth.
It is seen that the solar energy just outside the earth’s atmosphere has wavelength less than 3 pm whereas the energy radiated from the surface of the earth has a wavelength greater than 3 pm.
The overlap between the two spectrums is negligible, indicating that while solar energy being short wavelength radiation, energy radiated from earth’s surface is long wavelength or thermal radiation. Thus, only some of the solar radiations and maximum of the outgoing radiations are IR (IR ranges from 0.7 pm to 100pm).
The radiant energy from the earth’s surface, in its attempts to pass through the atmosphere, had to interact with various gases and aerosols. Three things might occur-(1) radiant energy gets scattered (2) absorbed (3) passes through unaffected.
Most of the long-wavelength energy (14 pm) radiated by the earth is absorbed by gases (gases in which the vibration creates a net dipole moment and become infrared active) such as water vapour (H70), carbon dioxide (C02), methane (CH4), nitrous oxide (N20), molecular oxygen (02), ozone (03) and chlorofluorocarbons (CFCs) which do not occur naturally and are entirely created by human beings are known as greenhouse gases.
It is to be noted that between 7 pm and 12 pm there is a relatively clear sky for outgoing thermal radiation and is known as atmospheric radioactive window.
The overall effect is thus-some of the thermal radiation from earth’s surface escapes to space, through atmospheric window, maximum of the radiation absorbed by the above mentioned gases, heats the atmosphere, radiate some energy back to earth as well as little energy back to the space.
Had there been no such gases, all energy would have been radiated from the earth to the atmosphere and the earth would have been cold.
The greenhouse gases therefore act as. a thermal blanket around the globe, raising the earth’s surface temperature and this phenomenon is known as greenhouse effect.
The term greenhouse effect is based on the concept of a conventional greenhouse, where the glass enclosing the green plants is acting as blanket.
Glass allows short wave length solar energy into the interior, but the longer wavelength radiation which is radiated by the greenhouse is absorbed by the glass window and hence, the inside is warmer than the outside.
It is however to be mentioned that the much effect or elevated temperature inside is due to the reduction in convective cooling of the interior due to the glass wall.
In cooler climates, tropical plants are grown in glass houses. They allow short wave solar radiations to enter inside, but do not allow long wave terrestrial radiation to return to the atmosphere.
They trap heat and make the houses warmer than the surrounding environment and help the growth of plants.
Water vapour (H20):
Water vapour is the most important greenhouse gas since about 2/3 of the earth’s surface is covered with water and thus huge amounts of water vapours evaporate into the atmosphere (IR bands less than 8 pm and greater than 18 pm). Water vapours retain heat and contribute significantly to global warming.
Carbon dioxide (C02):
In the atmosphere it is used by plants for the preparation of their food, the process being known as photosynthesis. The released oxygen is used by animals for respiration and ecological balance exists.
But increased population has led to rapid industrialization, rapid deforestation and has created enormous increment of C02 in the atmosphere. This gas absorbs heat (IR absorption bands centered at 15 pm as well as at 2.7 pm and 4.3 pm) and keeps the atmosphere warm.
With rapid increment of C02 concentration in the atmosphere, the average temperature will increase and may be more than 4°C in the next 100 years.
This in turn will affect plant and animal life. The snow cap will melt and there will be an increment of sea level and there will also arise, the possibility of sinking of many cities in the coastal area.
It is a naturally occurring, colourless, odourless gas. It is produced by bacterial fermentation under anaerobic conditions as in rice paddies, swamps, marshes, etc. Natural source of methane is wetlands, termites and oceans.
Anthropogenic emissions of methane are the result of food production and fossil fuel use. Since food and energy production will continue to rise to meet the increasing demands of a growing population, methane emission will also continue to rise. Methane is removed from the atmosphere primarily through reactions with hydroxyl radicals.
As concentration of OH’ decreases due to reactions with methane, rate of removal of CH4 will decrease, it will be retained for a longer duration in the atmosphere, absorbing IR (IR absorption bands centred at 3 pm and 8 pm) for a much longer time, increasing global temperature to a great extent.
Further the water vapour and ozone thus produced are also greenhouse gas. Global warming further leads to freeing of large amounts of methane currently frozen in the permafrost in the far northern regions of the world, leading to further increment of methane that in return would again add to global warming on a larger scale.
It is a colourless gas with a faint pleasant smell. This naturally occurring greenhouse gas is increasing in concentration due to human activities. The anthropogenic N20 emissions are due to tropical agriculture.
Other sources are production of nylon, combustion of fuels containing nitrogen. This gas decomposes slowly in the stratosphere by photolysis and thus has a longer duration and long lasting repercussions on global warming and sustenance of life is endangered.
The stretching IR absorption bands of N20 lie at 7.8 pm and that of bending bands at 8.6 pm. The band at 7.8 pm is on the shoulder of the atmospheric window and 8.6 pm is right on the window, making it a potent greenhouse gas.
Chlorofluorocarbons (CFC) comprise of chlorine, bromine and fluorine. Such compounds are created through anthropogenic activities. Due to its inertness and water insolubility it cannot be destroyed by chemical reactions or get removed by rain.
These molecules get absorbed nearly at 9 pm with some bending and stretching bands in the vicinity of atmospheric window and therefore, are potent greenhouse gases, contributing nearly about 17 per cent of the effect.
Ozone has a strong absorption band at 9 pm, right in the middle of atmospheric window, thus, is a greenhouse gas of importance. The concentration of ozone varies from place to place and season to season.