Which gases are greenhouse gases, and why?

The most important so-called "long-lived" greenhouse gases are CO2, methane, nitrous oxide and CFC. In total they account for more than 97 per cent of the direct effect of the long-lived gases on how much heat is retained in the atmosphere.

A range of different gases can act as greenhouse gases. The common denominator for them is that they can absorb heat (thermal infrared radiation), which is emitted from the earth, and re-emit it. When the sun shines on the earth, about 70 percent of the energy gets through the atmosphere. The remainder is retained or reflected by, for example, clouds. It is mainly invisible light that gets through.

On the earth’s surface the light is retained and converted into heat, which is sent back into space in the form of thermal infrared radiation – in the same way as a bonfire emits radiant heat. A part of this heat does not escape from the atmosphere, but is retained by greenhouse gases such as CO2 and methane. The thermal radiation causes the gas molecules to vibrate with energy, until the energy is released and re-emitted as thermal radiation. In this way the atmosphere retains for a time a part of the energy the earth receives from the sun. This is the greenhouse effect.

The atmosphere consists mainly of nitrogen and oxygen. The molecules of these gases are simple – they consist of only two atoms. They have a lot of difficulty in capturing thermal radiation and in vibrating. Therefore they are not greenhouse gases. Other molecules have a structure that can capture heat.

The most significant greenhouse gas is water vapour. But water vapour stays in the atmosphere only very briefly, and the amount of water vapour in the atmosphere is affected by other greenhouse gases, because hotter air causes increased evaporation from the oceans in particular.

The discussion about the anthropogenic greenhouse effect is therefore concentrated on the so-called ”long-lived” greenhouse gases. The most important ones are CO2, methane, nitrous oxide and CFC gases. In total they account for more than 97 per cent of the direct effect of the long-lived gases on how much heat is retained in the atmosphere. (Photo: Scanpix/Reuters)

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ROHIT (RIT) VISHWAKARMA
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