Some green house gases occur naturally, while others result from human activity. The various natural greenhouse gases are: water vapor, carbon dioxide, nitrous oxide, ozone and methane. Carbon dioxide, methane and nitrous oxide, levels in the atmosphere are added by human activities of industry, transport, agriculture, organic and solid waste combustion. Very powerful green house gases that are not naturally occurring include hydroflurocarbons (HFCs) perflurocarbons (PFCs) and sulphur hexafluroide (SF6), which are generated in a variety of industrial processes.
Greenhouse effect around the Earth
Green house gases in the atmosphere behave much like the glass panes in a green house. Green houses are like small glass houses whose transparent glass roof and walls allow sunrays to pass through. It keeps the heat from escaping, thus allowing vegetables and flowers to grow even in cold weather.
An effect similar to the one in glass chamber is responsible for keeping the Earth's surface warmer than it would otherwise be. Green house gases in the atmosphere around the Earth act like a glass of the greenhouse chamber.Earth receives a large amount of energy from the Sun, which emits U.V. radiations, visible light and infra-red (IR) radiations. Some of the solar radiation is reflected away by the atmosphere and Earth, while some of the infrared radiation is absorbed and re-emitted in all directions by the green house gas molecules. The effect of this is to warm the Earth's surface and lower the atmosphere.
A large part of the Sun's radiation pass through the Earth's atmosphere and Earth absorbs these IR radiations of short wavelength. This warms the Earth's surface and increases its temperature. It starts emitting infra-red radiations of longer wavelengths. The partially radiated infra-red radiations from the Earth are absorbed by CO2 and green house gases. This restricts the outward flow of infrared radiation and effectively stores some of the heat in the atmosphere, producing a net warming of the surface. The heating of the atmosphere due to absorption of infra-red radiations by carbon dioxide and other gases is called greenhouse effect.As more and more infra-red radiations are trapped, the atmosphere becomes hotter and therefore, temperature rises.
Note: Each green house gas differs in its ability to absorb heat in the atmosphere. HFC's and PFC's are the maximum heat absorbent's. Methane traps over 21 times more heat per molecule than carbon dioxide. Nitrous oxide absorbs 270 times more heat per molecule than carbon dioxide.
Further only long-lived gases have the potential for affecting the global environment. This effect arises principally because the concentrations of long-lived gases are the result of many years of accumulated emissions. The mean atmospheric life time of CH4 is about 8 years. The mean lifetime of N2O in the atmosphere is about 150 years. CO2 is chemically inert and is not destroyed by photochemical or chemical processes in the atmosphere. It is either lost by transfer into the ocean or biosphere, or it builds up in the atmosphere.Advantages of Greenhouse Effect
The presence of carbon dioxide and other gases in the atmosphere produces the greenhouse effect, which keeps the atmosphere warm. The warm atmosphere is very essential for the survival of life on Earth in the following ways:
- Precipitation of water, formation of clouds, rainfall etc. life in the biosphere depend on these resources.
- The warm atmosphere helps in the growth of vegetation and forest etc. These are sources of food, shelter etc.
- This effect helps in rapid bio-degradation of dead plants and animals.
Global warming
Harmful results of Greenhouse Effect is global warming. While the green house gases enable life to exist, problems arise when the atmospheric concentrations of these gases increase. All climate changes up to the industrial revolution occurred naturally as very few gases were released by human activity into the atmosphere. Human activity through rapid industrialization, combustion of fossil fuel, demands of population growth and deforestation have altered the chemical composition of gases in the atmosphere.
In the last century the Earth's surface temperature has risen by over 1 F. Accelerated warming has taken place over the last two decades and the ten hottest years were recorded in the last 15 years of the century, 1998 being the hottest year. Scientists expect the temperature to rise an additional 0.6 to 2.5 C in the next 50 years and 1.4 5.8 C. (with significant regional variation) in the next 100 years. The impact of a change in the Earth's temperature on climate can be gauged by the fact that at the peak of the last ice age (18,000 years ago) the temperature was only 7 F colder than it is today.Since the beginning of the industrial revolution atmospheric concentration of carbon dioxide has increased nearly by 30%. Methane concentration has increased by more than 100% and nitrous oxide by about 15%. These increases have enhanced the heat trapping capability of the Earth's atmosphere.
In this industrial age, excessive burning of fossil fuels have released enormous amount of carbon dioxide in atmosphere, which cannot be removed by plants and ocean waters. Thus, concentration of carbon dioxide in atmosphere is steadily rising and increasing the greenhouse effect or global warming.
Estimating future emissions is difficult, though scientists in western countries have developed several different scenarios based on differing projections of underlying factors. One projection states that in the absence of emission policies, carbon dioxide concentrations are projected to be 30-150 % higher than todays levels in the next 100 years. This in turn will have disastrous consequences. Scientists are estimating the following impacts and consequences:
Rising sea levels
Globally sea levels have risen 4-8 inches over the last century. The high temperatures have already started melting polar ice caps sea icebergs and glaciers. This has released enormous amount of water. Warmer temperatures also heat water and make it expand; thus ocean levels need more space and further rise. The level of sea is likely to rise by several inches to up to 3 feet during this century. This would lead to vast coastal flooding, huge expense by Governments and individuals to safeguard oceanfront property, submerging of low-lying coastal countries e.g., Denmark, Bangladesh. It would also cause the conversion of fresh water habitats and eco-systems into saltwater areas endangering vegetation, crops, animals and water supply along the coast.
Change in climate
Global warming has already occurred and will continue to do so. Evaporation will increase as climate warms and there will be an increase in average global precipitation. Local climate conditions are likely to change depending on the modifications of local rainfall patterns. The quality and quantity of drinking water, water availability for irrigation, industrial use, electricity generation and the health of fish may be significantly affected by changes in precipitation and increased evaporation.
Soil moisture is likely to drop affecting crops and the need for irrigation. The high temperature may reduce crop production, thereby causing famines. Intense rainfall (extremes) is also predicted. Tropical rains and hurricanes will be more frequent and stronger. Increased rainfall may cause more frequent flooding. Climate change would likely add stress to major river basins worldwide.Human health and heat stress
Along with warmer temperatures and higher humidity, the number of very hot days is expected to rise in many areas around the globe. Heat stress occurs in such conditions and the young, old and poor people are more susceptible to it. High temperatures will reduce work efficiency of human beings.
Further air pollution, changes in food and water supplies resulting from global warming, will all affect human health. Changing patterns of precipitation and temperature may produce new breeding sites for pests, shifting the range of infectious diseases.Alteration of habitats and ecosystems
Many of the worlds habitats and ecosystems depend on a delicate balance of rainfall, temperature and soil type. A complex food chain supply exists based on these factors. Higher temperatures and precipitation changes are likely to cause diverse changes such as increased forest susceptibility to fire, disease and insect damage also, deserts may expand into existing grass lands.
Similarly, changes in ocean temperature will adversely affect the marine life. If future changes occur as rapidly as some scientists predict, plants, animals and marine life may not be able to adapt quickly enough to survive.Cooling of stratosphere
Increasing concentration of greenhouse gases are expected to cause cooling of stratosphere. This could happen because most of the thermal IR radiation will be trapped and absorbed at lower altitudes; little will be left to warm the stratosphere. Cooling of stratosphere would mean enhanced greenhouse effect i.e., greater global warming.
