There are number of gases present in the atmosphere which are capable of absorbing effectively heat waves and infrared rays while being transparent, to radiation of lower wave lengths. Trace gases such as water vapor, carbon dioxide, methane, chlorofluorocarbons, ozone, and nitrous oxide are some of the gases which constitute troposphere.
Amongst these five gases having rising concentration’s of which has been implicated in causing noticeable rise in the mean global temperature. These gases are carbon dioxide, methane, chlorofluorocarbons, nitrous oxide and water vapors, other gases such as sculpture dioxide, ozone are not able to contribute much as they are quickly cleared from atmosphere.
An additional 1.1 to 2.2 billion tones (1-2 billion metric tonnes) of carbon (4-8 billion tonnes [3.6-7.3 billion metric tonnes] of CO2) are emitted into the atmosphere each year due to deforestation. Like the fossil fuels, which were once living organisms, the extant forests hold vast stores of carbon. When the trees and other plants die and are either burned or allowed to decay, this carbon is converted into CO2.
Furthermore, trees serve the vital function of removing CO2 from the atmosphere as they grow, unless deforested areas are quickly replanted, not only is CO2 emitted directly into the atmosphere, but a vital mechanism for removing excess CO2 from the atmosphere is also destroyed.
Although not yet considered a serious threat, there is another aspect of excess CO2 production: the oxygen portion of CO2 comes from the atmosphere, and we (along with all living aerobic creatures) need this oxygen to breathe. For every tonne of carbon burned, 2.66 tonnes of oxygen are taken from the atmosphere. Since the atmosphere is composed of about 20.9% oxygen, the supply may seem virtually unlimited.
Yet measurements indicate that the oxygen content of the atmosphere is currently decreasing at a rate of about 13 pm annually. Oxygen is being lost from the atmosphere, and as more plants are destroyed, it is not being replaced as fast as it once was.
The most serious immediate concern is that falling oxygen levels may adversely affect oxygen concentrations in the oceans and other standing bodies of water. Slight decreases in oxygen concentrations could have severely disruptive effects on many marine organisms.
Although carbon dioxide is blamed for 50 to 70% of the current abnormal global warming, (depending on the authority consulted and how “global warming” is calculated), it is not the only major greenhouse gas. The other major culprits are chlorofluorocarbon (CFCs), methane (natural gas, CH4), troposphere ozone, and nitrogen oxide (NOx).
The chlorofluorocarbons that promote global warming are the same CFCs that are destroying stratospheric ozone. Indeed, they are up to thousands of times more efficient at absorbing heat and promoting global warming than CO2. At present, CFCs account for 15 to 25% of the human contribution to global warming. This number would have been even higher if steps had not been taken to reduce the CFCs released into the atmosphere.
Thus, there are two good reasons to reduce our reliance on CFCs: to save the ozone layer and to reduce global warming. Methane accounts for an estimated 15 to 20% of current global warming.
It has been estimated that carbon dioxide will account for about half of the temperature increase while methane, nitrous oxide and chlorofluorocarbons will be responsible for the rest.
Green house gases are provided both by industrial and biological processes but the industrial emission are fairly well documented but the knowledge of the production rates of green house gases through biological processes and the factors regulating emissions are still in adequate.
Ranking and percentage share of Global Emission (Green House Index)-2004
In 1950s, almost nobody was worried about the greenhouse effect. John Tyndall, a British physicist, analyzed the gases of the atmosphere one by one to see which of them have the most powerful greenhouse effect. Tyndall discovered that nitrogen and oxygen do not have any greenhouse effect. That means 99% of the atmosphere has almost no greenhouse effect at all.
There gases that do not have what it takes are (i) water vapor, (ii) carbon dioxide and (iii) ozone. There is a lot of water on the surface of our planet so that there is always, lot of water in the lower atmosphere. In fact, water is the single most common greenhouse gas on the earth.
The next is carbon dioxide which in turn is more common than ozone. The three most common greenhouse gases are triplets: water is H-H-O (H2O); Ozone is O-O-O (O3), an aberrant and highly unstable form of oxygen. Carbon dioxide of course is O-C-O (CO2).
The actual arrangement in space is a flying bird. Having three atoms, Tyndall called them ‘little triads’. Like nitrogen and oxygen, they are almost perfectly transparent to the sunlight that streams to Earth from the Sun. Unlike these other gases, however, they are partially opaque to heat radiation that rises from the sun-baked ground this heat radiation is infrared (literally, below red).
It is below red in the rainbow spectrum, placing it just outside the range of colors to which him eyes are tuned. A few animals like ticks and pit vipers so sense their target in infrared. We can feel it on our hand and face near a campfire or a hot stove. The planet shines mildly in the infrared by day and night.
When infrared radiation strikes molecules with three atoms, it sets them in shaking and trembling. Carbon dioxide molecules flap and shake; they give off energy in the form of more dark rays, more infrared. Every carbon dioxide molecule in the atmosphere is like a dark star, shining in all directions, up, down and sideways.
In this way, invisible rays of energy get passed back and forth many times between atmosphere and the spheres below-the lithosphere, biosphere, hydrosphere, and cry sphere-before the energy finally migrates to the top of the atmosphere and escapes to the relatively calm and vacuum outer space. This is the greenhouse effect.
The dark rays bounce around inside the atmosphere many times before they finally manage to leak into space. Water vapor, C02, 03 turn the world’s air into a giant heat trap. For billions of years, life on the Earth has depended on this peculiar property of these three gases to keep the planet habitable.
Tyndall was quick to realize that plants are constantly breathing CO2, in and out. The gas fluctuates for several other reasons as well. If the amount of CO2 in the air ever dropped even a little bit, the change would chill the planet. Tyndall even suggested that this might be the explanation for ice ages.
In 1970s, while climate experts were worrying about C02, a few scientists took a sharp look at other gases in the atmosphere. They realized that even rarer gases, measured in some cases in parts per trillion, can have powerful greenhouse effects. Now these gases, too, are monitored by stations of the CO2, networks.
The most notorious of these trace gases are the chlorofluorocarbons (CFCs). Unlike carbon dioxide, these are artificial compounds, which chemists make by linking chlorine and fluorine atoms together with atoms of carbon. The two most important chlorofluorocarbons are CFC-11 (trichlorofluromethane) and CFC-12 (dichlorodifluoromethane).
One of the world’s biggest manufacturers, the E.I. du Pont de Nemours & Company, markets them under the name of Ferns. They make extremely effective refrigerants, spray-can propellants, and foam- blowing agents. They also linger in the air a long time, for about 75 and 110 years, respectively.
The CFC industry just kept growing at about 20% a year. Except ultraviolet radiation nothings breaks the CFC down and there is not much ultraviolet radiation down here on the ground.
Over the decades these ‘immortal’ gases drift from the troposphere and accumulate in the stratosphere. There UV-radiation from the Sun does strike them and breaks them apart into fragments. In fact, the CFC molecules are inert but when fragmented they are highly reactive. In a series of chemical reactions the fragments attack and deplete the ozone in the stratosphere.
The thinner the ozone layer, the more ultraviolet radiation reaches the ground. An ozone hole of the size of a continent appeared in the stratosphere above the South Pole. The inventory of ozone around the whole globe grew measurably thinner. Because of certain symptoms in exposed human skin, comparatively thinner ozone is also suspected over parts of Western Australia and over desert region of Rajasthan.
The ozone hole and greenhouse effects are two faces of a single crisis. The CFC concentration of less than a part per billion may capture significant amount of heat and hold it in.
Because of their quirk nature, the CFCs have extraordinary power as greenhouse gases. The first atmospheric chemist to realize this fact was Verrabhadran Raman than of the University of Chicago. The infrared rays that rise from the sun-baked ground come in a range of wavelengths, and C02 molecules absorb most, if not all, of these wavelengths. There is a small window, as Raman than calls it through which infrared rays can still escape.
In September 1987, a historic International Convention was held in Montreal where representatives from most of the world’s industrial nations signed a treaty to slow down global CFC production. However, more compounds are still being made. CFC-11 and CFC-12 were still accumulating at a rate of about 5% per year. CFC-13, which is the best solvent for cleansing computer microchips, was increasing at 11% a year.
They were by far the fastest increasing constituents of greenhouse gases (CO2, increasing at a little less than half a per cent year). Nothing can be done now to remove the millions of tones of CFC already released into the atmosphere since 1930. These chemicals will continue to drift into the stratosphere. They will go on eating ozone and magnifying the greenhouse effect for more than a century.
Another greenhouse gas is methane. It is also helping to dirty the all important window. Adding one molecule of methane increases the greenhouse effect of Earth’s atmosphere as much as adding 20 more molecules of CO2. Methane is sometimes called swamp gas, or marsh gas, because it is a by-product of decay. It is also called natural gas because it seeps from the walls of coal mines, where it can be mined as a fossil fuel in its own right.
Methane (CH4) concentration is now growing at a rate of about 1% per year-twice as fast as CO2. Its concentration is now about 1.7 parts per million, which is already more than double the pre-industrial level. When methane drifts into the stratosphere, it is broken down into carbon and hydrogen. The carbon atoms link up with oxygen to form CO2.
The hydrogen links up with oxygen to form water vapor-which is normally extremely scarce in stratosphere. Now we have two more greenhouse gases. In the coldest part of the stratosphere this water vapor tends to form small ice crystals which drift through stratosphere.
During this drifting these tiny ice crystals collect stray atoms of chlorine from the shattered CFCs. Many chemical reactions are promoted on these solid surfaces, and chlorine atoms on the surface of this ice can eat ozone.
Methane gas seems to be erupting from the very top and very bottom of food chain. It is released by human beings. We release it chiefly by mining natural gas and by burning petroleum.
Also bacterial decay of leaves and other organic wastes in swamps, marshes, and rice paddies also release methane in the atmosphere. Methane rise is so inexplicably rapid (-50 million tonnes a year) that there may be other sources still undiscovered.
Other pollutant gas that is emitted is carbon monoxide (CO), a poisonous gas. More than half of the CO in the air comes from human beings, from exhaust pipes, automobiles, and factory smokestacks, and from smoky fires in the damp forests of the tropics. By putting more and more CO into the air we are over-whelming the atmosphere’s immune system, exhausting the hydroxyl component of the atmosphere.
The hydroxyl (OH) is to the atmosphere what antibodies are to the human immune system. Hydroxyl components are used up in attacking and destroying carbon monoxide. Because there is so much hydroxyl around, methane is free to increase.
The more methane there is in the air, the less hydroxyl. So CO can increase faster. The carbon monoxide (CO) has no direct greenhouse effect, because it has only two atoms, and greenhouse gas needs at least three.
Also, nitrous oxide, popularly known as laughing gas, is accumulating in the atmosphere, Nitrogen molecules make up most of every breath we take and they are highly stable. It takes a lot of energy to break these molecules and use them in living molecules.
Our lungs are not equipped to do so. Only a few specialized soil bacteria are capable of pulling nitrogen out of the air. All plants get their nitrogen from these species of symbiotic bacteria and all living creatures get their nitrogen from the plants. Still other species of bacteria return the borrowed nitrogen to the air.
We must know that the burning of fuels produces not only CO, CO2, but also compounds of nitrogen and oxygen. Nitric oxide (NO) has one atom of nitrogen and one atom of oxygen. Nitrous oxide (N2O) has two atoms of nitrogen and one atom of oxygen. Nitrous oxide (with three atoms) has a greenhouse effect. One molecule of this gas is equal to about 250 molecules of CO2. This gas also lasts and lasts. The average molecule of nitrous oxide lingers in the atmosphere for about 125 years.