Earth gets heat energy from sun in the form of sun rays. The sun rays penetrate the atmosphere and heat the surface of the earth. However all the the energy so obtained is not absorbed by the earths surface. A part of it is reflected in the form of Infrared energy and it is absorbed by certain gases like carbon dioxide, ozone and methane present in the atmosphere. These gases are present in very small quantity in the air.
The heat so absorbed by these gases is useful to maintain the temperature of the earth surface warm which is essential for life. This is called green house effect. This gases are therefore called green house gases. However the same is useful only to certain limit.
Greenhouse gases definition
"Gaseous constituents of the atmosphere that absorb and emit infrared radiation responsible for the greenhouse effect. The primary greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide and ozone."
Atmospheric concentrations of carbon dioxide (CO2
) are increasing globally at a rate of about 1% or more annually. Similarly other greenhouse gases such as ozone (O3
), nitrogen oxides (N2
O and NO2
), carbon monoxide (CO) and chlorofluorocarbons are increasing proportionally with increasing industrial emissions, all are linked to population growth. Together these atmospheric gases are impacting forest trees directly by affecting gene expression, physiological processes, growth, productivity and fitness.
In addition increasing greenhouse gases may affect forest ecosystem processes such as nutrient and water cycling. These gases are also trapping considerable radiant energy near the Earth's surface, resulting in the so-called "greenhouse effect"
, that may significantly alter global climate in the next century.
The most important greenhouse gases are carbon dioxide, methane, nitrous oxide, hydrofluorocarbon, perfluorocarbons and sulfur hexafluoride. Chlorofluorocarbon (CFCs) and hydrochlorofluorocarbons (HCFCs)
, which are also powerful greenhouse gases but they are being progressively phased out under the Montreal Protocol, as they also damage the stratospheric ozone layer.
Each greenhouse gas has a different capacity to cause global warming, depending on its properties o radiate, its molecular weight and its lifetime in the atmosphere.
The list of greenhouse gases are given below with their lifetime and GWP
(Global Warming Potential) values.
||100 years Global Warming Potential
||50 - 200
||12 - 23
|Sulfur hexafluoride ||3200 ||23,900 |
|Trifluoroiodomethane ||< 0.005 ||< 1 |
If the amount of these gases in the atmosphere increases, then the amount of heat absorbed trapped by them is also more and hence the effects of the same are also more. This increases the temperature of the earth, called as global warming which is not desirable.
The greenhouse gases responsible for global warming is shown in the diagram.
Humans have invariably disturbed the natural process of the green house effect by excessive pollution. It is because of 4 major factors.
- Industrial pollution: industries produce gases which contribute to global warming.
- Uses of fuels: there has been substantial increase in use of vehicles and related machines like generators etc. These contribute to production of gases which are not desirable. These artificially produced gases remain suspended in the air and absorb and retain more resulting in excessive temperature of the earth.
- Trees are very essential for ecological balance and maintaining temperature balance. The trees absorb carbon dioxide from atmosphere in photosynthesis and thus maintain the balance in green house effect. However when trees are cut indiscriminately, the carbon dioxide is not utilized and hence the excess present in the air causes more global warming.
- Growth of population also contributes to global warming. More lives means more activity, means more fossil consumption and therefore more global warming.
Greenhouse gases released into the atmosphere as a result of human activities are termed anthropogenic (human created) greenhouse gas emissions.
Human release approximately 50 different gases into the atmosphere that can contribute to the greenhouse effect.
Many of these gases are released only in small quantities and at this stage, do not contribute significantly to global warming. Fewer than 10 gases account for nearly all the atmospheric radiative forcing that can be attributed to humans. These gases can be further subdivided into two main categories.
- Those covered under the Kyoto Protocol - carbon dioxide, methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6).
- The ozone depleting substances covered under the Montreal Protocol - cholorofluorocarbons (CFCs), halons and several other gases. Most published national and global greenhouse gas emissions data refer only kyoto gases, eventhough the Montreal Protocal gases and other emissions are also important contributors to global warming.