[PDF Notes] What is greenhouse effect and how does it works?

Most greenhouses look like a small glass house. Greenhouses are used to grow plants, especially in the winter. Greenhouses work by trapping heat from the sun. The glass panels of the greenhouse let in light but keep heat from escaping. This causes the greenhouse to heat up, much like the inside of a car parked in sunlight, and keeps the plants warm enough to live in the winter.

The earth’s atmosphere is all around us. It is the air that we breathe. Greenhouse gases in the atmosphere behave much like the glass panes in a greenhouse. Sunlight enters the earth’s atmosphere, passing through the blanket of greenhouse gases. The greenhouse effect works in the following manner

(i) Energy from the sun passes though the earth’s atmosphere in the form of visible light (sun light).

(ii) Some of the sun’s energy turns to heat and warms the earth’s air and land.

(iii) The rest of the sun’s energy becomes infrared radiation and is re-emitted into the atmosphere.

(iv) Greenhouse gases in the atmosphere trap some of this infrared radiation and bounce it back to the earth again, warming the planet even more.

Greenhouse Gases Carbon dioxide

It is considered the most predominant anthropogenic greenhouse gas, as 60% of the anthropogenic greenhouse effect is attributed to it. Carbon dioxide is dumped into the atmosphere at a much faster rate than it can be absorbed by the oceans or living things in the biosphere. Since the beginning of industrialization, in 1900s the atmospheric concentration of carbon dioxide has increased by almost one-third in 2002 (280 ppm to 370 ppm).

The reasons are found to be primarily the emissions produced by the burning of fossil fuels. The major source of fossil emission is contributed by the industrial sector (40%). The second important fraction is emitted by the housing sector with 31%, followed by the traffic sector with 22% and the agricultural sector with 4%.


Methane is the second most important greenhouse gas and its contribution to the anthropogenic greenhouse effect is in the order of 20%. Methane is produced during anaerobic decomposition of organic matter such as in swamps, garbage dumps, landfills and sewage treatment plants. Methane is also emitted from coal mining, natural gas production and distribution.

Nitrous oxide

The atmospheric concentration of nitrous oxide has increased in recent years due to fertilizer use and chemical production such as manufacture of nylon. Nitrous oxide is also produced during fossil fuel combustion, biomass burning and changing land use. Like methane, nitrous oxide is produced by both natural and anthropogenic sources.

Chlorofluorocarbons (CFCs)

CFCs are man-made molecules that contain chlorine, fluorine and carbons. Chiorofluorocarbons not only destroy ozone in the stratosphere, but are also important, long- lived greenhouse gas. It contributes to 14% of the anthropogenic greenhouse effect.


It is an important natural greenhouse gas in the stratosphere and the troposphere. While the reduction of ozone concentrations in the stratosphere due to CFCs has a cooling effect, the greenhouse effect is considerably intensified by the increase of ozone concentration in the troposphere. Ozone is formed by photochemical reactions. Unlike other greenhouse gases mentioned above, ozone is a short-lived gas and found in regionally varying concentrations.

Water vapour

Another important factor is the water vapour content of the atmosphere. Water vapour represents about 2% of the total atmospheric composition.

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