Why have just a few gases made it to the GHG list?

When one kilogramme of gas is evaluated against the same amount of carbon dioxide in a stipulated time, we can find a measure of the relative global warming contribution, technically known as Global Warming Potential (GWP).The time span is significant to maintain uniformity in the GWP, as each Greenhouse Gas (GHG) has a different life span in the atmosphere before it breaks down or is removed. Normally, for official reports, a time span of 100 years is usually used. If a gas absorbs radiations of a wavelength which is already being absorbed by the atmosphere, then the GWP is low, on the other hand, if it absorbs light at a wavelength that normally passes through the atmosphere (without being absorbed) then the GWP is high.

Gases	Formula	Lifespan	GWP
			20	100		500
Carbon dioxide	CO2	Variable	1	1	1
Methane	CH4	12±3	72	25	7.6
Nitrous oxide	N2O	114	289	298	153
Sulphur hexafluoride	SF6	3200	16300	22800	32600
Chlorofluorocarbons	CFC12	45	1100	10,900	5200

Since the industrial revolution, concentrations of greenhouse gases have increased linearly. The GHGs are found to have a tendency to absorb and emit infrared (thermal) radiations. These gas molecules have two or more component atoms, which are loosely bound, due to which they vibrate on absorption of heat. When the infrared radiations strike a molecule such as carbon dioxide and causes its bonds to bend and vibrate (this is called the absorption of IR energy), the molecule gains extra kinetic energy that may either be transmitted to other molecules through molecular collision or be re-radiated back to the space, earth causing a general heating of the earth and near surface atmosphere. The major components of the atmosphere (N2 and O2) are two-atom molecules are tightly bound, and so they cannot vibrate, as a consequence they neither absorb heat nor do they contribute to the greenhouse effect. On the contrary it is believed that these gases allow the radiations to pass through the (near surface) atmosphere.

                                 Eg of effect of infrared radiations on CO2 gas. 

The dips in the lines are caused by the absorption of energy; hence only 10% of the energy is transmitted.

Due to accelerated anthropogenic activities (combustion of fossil fuels, deforestation etc) and the long life span (variable), the concentrations of CO2 are relatively high amongst all the other gases making it the dominant GHG. Although water vapor is the real strongest GHG in the atmosphere its concentration is kept in control due to precipitation and so they are not as harmful as CO2.

I hope this blog helped in understanding why the GHG’s cause global warming.

Thank you for reading, stay tuned there is a lot more coming up.