The greenhouse effect, definition and main responsible gases

Definition and actors of the greenhouse effect

Keywords: definition, warming, climate, climatic, albedo, GWP, carbon equivalent, Earth, ecosystem, global ...

Definition: what is the greenhouse effect?

The greenhouse effect is a natural process of climate warming which intervenes in the radiative and thermal balance of the Earth. It is due to greenhouse gases (GHGs) contained in the atmosphere, namely mainly water vapor, carbon dioxide CO2 and methane CH4.

This effect was named thus by analogy with the practice in culture and garden center of building greenhouses letting the heat of the sun pass and retaining it trapped inside in order to allow the plants to benefit from an artificial micro-climate.

The "functioning" of the greenhouse effect and albedo

When the sun's rays reach the Earth's atmosphere, part (about 30%) is directly reflected by the air, clouds up to 20% and the Earth's surface up to 10% (in particular oceans and icy regions like the Arctic and Antarctic), it is albedo.
Incident rays which have not been reflected back to space are absorbed by the heat capacity of the atmosphere by greenhouse gases (20%) and at the earth's surface (50%).

greenhouse effect (diagram)
Diagram of the greenhouse effect

This part of the radiation absorbed by the Earth gives it heat, which it in turn releases towards the atmosphere in the form of infrared rays (black body radiation).

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This radiation is then absorbed in part by greenhouse gases. And thirdly, this heat is re-emitted in all directions, including toward Earth.

It is this radiation which returns to the Earth which is the “greenhouse effect”, it is at the origin of an additional supply of heat to the Earth's surface. Without this phenomenon, the average temperature on Earth would drop to -18 ° C.

It must be understood that the energy of space received by the earth and the energy of the earth emitted to space are equal on average, otherwise, the temperature of the earth would change in one direction permanently, towards always colder or worms always hotter. If the average exchange of energy with space is not zero, this leads to storage or de-storage of energy from the earth. This change can result in a change in the temperature of the atmosphere.

The Greenhouse Gas (GHG)

Greenhouse gases are gaseous components of the atmosphere that contribute to the greenhouse effect.

The main greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (or nitrous oxide, with the formula N2O) and ozone (O3) .

Industrial greenhouse gases include heavy halocarbons (chlorinated fluorocarbons including CFCs, HCFC-22 molecules such as freon and perfluoromethane) and sulfur hexafluoride (SF6).

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Approximate contributions to greenhouse gas main:

  • Water vapor (H2O): 60%

  • carbon dioxide (CO2): 34%

  • Ozone (O3): 2%

  • Methane (CH4): 2%

  • nitrous oxide (NOx): 2%

Global warming potential (GWP) of greenhouse gases (GHG)

Gases do not all have the same absorption capacity of terrestrial infrared radiation and they do not all have the same lifetime.

To compare their impact on global warming, the IPCC (Intergovernmental Panel on Climate Change) offers PRG Index (Global Warming Potential).

The PRG is an index allowing to evaluate the contribution relative to global warming of the emission of 1kg of greenhouse gases by comparison with the emission of 1kg of CO2 during a determined period which is generally 100 years . By definition, the PRG at 100 years of CO2 is set at 1.

Most common GHG PRGs:

  • carbon dioxide (CO2): 1

  • Water vapor (H2O): 8

  • Methane (CH4): 23

  • Nitrous oxide (N2O): 296

  • Chlorofluorocarbons (CFCs or CnFmClp): 4600 14000 to

  • Hydrofluorocarbons (HFCs or CnHmFp): 12 12000 to

  • Perfluorocarbons (PFCs or CnF2n 2 +): 5700 11900 to

  • sulfur hexafluoride (SF6): 22200

Example: PRG 100 years of nitrous oxide which is 296 means that the impact of 1 kg N2O is equivalent to the impact of 296 kg CO2 after a century.

The carbon equivalent

Another unit is sometimes used: "carbon equivalent" which is obtained by multiplying the GWP by the ratio between the mass of a carbon atom (C = 12g.mol-1) and that of a molecule dioxide carbon (CO2 = 44g.mol-1).

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So we have: Carbon equivalent = PRG x 12/44

For fossil fuels that produce CO2, this unit represents precisely their carbon mass. It is also used for all other gas even for those that do not contain carbon.

So here are the carbon equivalent of the most common GHGs:

  • carbon dioxide (CO2): 0,273

  • Water vapor (H2O): 2,2

  • Methane (CH4): 6,27

  • Nitrous oxide (N2O): 81

  • Chlorofluorocarbons (CFCs or CnFmClp): 1256 3818 to

  • Hydrofluorocarbons (HFCs or CnHmFp): 3,3 3273 to

  • Perfluorocarbons (PFCs or CnF2n 2 +): 1555 3245 to

  • sulfur hexafluoride (SF6): 6055

Example: the equivalent carbon 1 ton of CO2 is 12 / 44 TEC (tons carbon equivalent) or 0,273 TEC.

Read more: the probable consequences of the greenhouse effect

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