Over the last two centuries, the composition of the Earth's atmosphere has been altered substantially by human activities, including fossil fuel burning, agriculture, deforestation and industrial emissions. In addition to regional problems, such as higher levels of tropospheric ozone and other forms of air pollution, there is growing evidence that these changes will have a significant, long-term impact on the planet's climate and biota. Considerable research effort is now underway to unravel the often complex interactions and feedbacks among climate, atmospheric chemistry, biogeochemistry and ecosystem dynamics under past, present and potential future conditions. Accurate projections of future climate change and its impact on society and the natural environment hinge on two key questions: how will radiatively active trace gas and aerosol concentrations in the atmosphere evolve in the future, and how sensitive is the climate system to these changes in forcing? The questions are, of course, not independent because many of the chemical species of interest have natural biological sources and sinks that respond to climate variations. A third issue, the effect of the expected climate perturbations on society and on natural and managed ecosystems, is crucial to the problem but beyond the scope of this review.
When sunlight reaches Earth's surface some is absorbed and warms the earth and most of the rest is radiated back to the atmosphere at a longer wavelength than the sun light. Some of these longer wavelengths are absorbed by greenhouse gases in the atmosphere before they are lost to space. The absorption of this longwave radiant energy warms the atmosphere. These greenhouse gases act like a mirror and reflect back to the Earth some of the heat energy which would otherwise be lost to space. The reflecting back of heat energy by the atmosphere is called the "greenhouse effect".
The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide CO2, which causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%. It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. Other greenhouse gases include, but are not limited to, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons. Some of the causes of greenhouse effect are mentioned below:
Burning of Fossil Fuels
Fossil fuels like coal, oil and natural gas have become an integral part of our life. They are used on large basis to produce electricity and for transportation. Pollution which gets emit from vehicles contains carbon dioixde, which is one the main gas responsible for increase in greenhouse effect. Apart, from that, the production of electricity consume too much of coal which releases large amount of these gases into the atmosphere.
Forests holds a major green area on this earth. Plants and trees intake carbon dioxide and release oxygen, through the process of photosynthesis, for the survival of human beings. Large scale development have forced people to look for alternate places for living which has resulted in cutting down of trees. The wood that is produced, when burnt releases more carbon dioxide into the atmosphere.
Increase in Population
Over the last few decades, there have been huge increase in the population. Now, this has resulted in increased demand for food, cloth and shelter. New manufacturing industries have come up release some harmful gases into the atmosphere which increases the greenhouse effect. Also, more people means more usage of fossil fuels which in turn has aggravated the problem.
Nitrous oxide is one the greenhouse gas that is used in fertilizer and contributes to greenhouse effect which in turn leads to global warming.
Industrial waste and landfills
Industries which are involved in cement production, fertilizers, coal mining activities, oil extraction produce harmful greenhouse gases. Also, landfills filled with garbage produce carbon dioxide and methane gas contributing significantly to greenhouse effect.