Excessive quantities of greenhouse gases emitted into Earth's near-surface atmosphere causes Global warming. According to scientists there are several human activities that contributes to global warming. Hence, greenhouse gases are both man-made and occur naturally, are directly and indirectly generated by mankind, consists of a number of gases, including carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and water vapor. Such gases accummulate in the atmosphere and trap heat that normally would exit into outer space.

Global Warming Caused by The Greenhouse Effect

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. Greenhouse gases in the atmosphere act like a mirror and reflect back to the Earth a part of the heat radiation, which would otherwise be lost to space. The higher the concentration of green house gases like carbon dioxide in the atmosphere, the more heat energy is being reflected back to the Earth. The reflecting back of heat energy by the atmosphere is called the "greenhouse effect".

The emission of carbon dioxide into the environment mainly from burning of fossil fuels (oil, gas, petrol, kerosene, etc.) has been increased dramatically over the past 50 years, see graph below. The increase of greenhouse gas concentration (mainly carbon dioxide) led to a substantial warming of the earth and the sea, called global warming.

Greenhouse Gases Generated by Mankind:

The scientific community as a whole has concluded that naturally occurring greenhouse gases have remained fairly constant over the past several hundred years. Greenhouse gases directly and indirectly generated by mankind, though, have increased radically for the past 150 years, and especially in the past 60 years.

Major sources of greenhouse gases generated by mankind are:
- Burning of fossil fuels, which includes oil and gas, coal and natural gas.
- Chlorofluorocarbons, commonly used in refrigeration, cooling and manufacturing applications.
- Methane, which is caused by emissions from landfills, livestock, rice farming (which uses methane-emitting bacteria), septic processes, and fertilizers.

Global Warming Caused by Deforestation

Deforestation is also an important, culprit for causing greenhouse gas emissions. An Observation in 2006 by The United Nations Food & Agriculture Organization (FAO) stated that, most people assume that global warming is caused by burning oil and gas. But in fact between 25 and 30 percent of the greenhouse gases released into the atmosphere each year (1.6 billion tonnes) is caused by deforestation.

Trees are 50 percent carbon. When they are felled or burned, the C02 they store escapes back into the air. Deforestation remains high in Africa, Latin America and Southeast Asia. And the situation is worsening, as decreasing forest cover, almost exclusively from deforestation in tropical countries, was responsible for an estimated 1.5 billion tons of emissions to the atmosphere above what was gained through new plantings.

Global Warming Caused by Vehicles Burning Fossil Fuels

The largest single contributer to the rise of man-made greenhouse gases is, of course, the burning of oil and gas to power vehicles, machinery, and produce energy and warmth.

Global Warming Caused by Motor Vehicles

Motor vehicles are responsible for almost a quarter of annual U.S. emissions of carbon dioxide (CO2), the primary global-warming gas. The U.S. transportation sector emits more CO2 than all but three other countries' emissions from all sources combined. And motor vehicle emissions will continue to increase as more vehicles hit America's roads and the number of miles driven grows.

Three factors contribute to CO2 emissions from cars and trucks. They are amount of fuel used, amount of CO2 released when a particular fuel is consumed and number of vehicle miles traveled.

While many greenhouse gases occur naturally and are needed to create the greenhouse effect that keeps the Earth warm enough to support life, human use of fossil fuels is the main source of excess greenhouse gases. By driving cars, using electricity from coal-fired power plants, or heating our homes with oil or natural gas, we release carbon dioxide and other heat-trapping gases into the atmosphere. Deforestation is another significant source of greenhouse gases, because fewer trees means less carbon dioxide conversion to oxygen.
As the concentration of greenhouse gases grows, more heat is trapped in the atmosphere and less escapes back into space. This increase in trapped heat changes the climate and alters weather patterns, which may hasten species extinction, influence the length of seasons, cause coastal flooding, and lead to more frequent and severe storms.

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