Definition and actors of the greenhouse effect
Keywords: definition, warming, climate, climate, albedo, GWP, carbon equivalent, Earth, ecosystem, global…
Definition: What is the greenhouse effect?
The greenhouse effect is a natural process of global warming that takes place in the Earth's radiative and thermal balance. It is due to greenhouse gases (GHG) contained in the atmosphere, namely mainly water vapor, carbon dioxide CO2 and methane CH4.
This effect was so named by analogy with the practice in cultivation and gardening of building greenhouses letting the heat of the sun pass and keeping it trapped inside to allow 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 (around 30%) is directly reflected by the air, clouds at 20% and the surface of the Earth at 10% (especially the oceans and icy regions like the Arctic and Antarctica) is the albedo.
Incident rays that have not been reflected back into space are absorbed by the heat capacity of the atmosphere by greenhouse gases (20%) and on the earth's surface (50%).
This part of the radiation absorbed by the Earth gives it heat, which it in turn returns towards the atmosphere in the form of infrared rays (black body radiation).
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 towards the Earth which is "the greenhouse effect", it is at the origin of an additional contribution of heat to the terrestrial surface. Without this phenomenon, the average temperature on Earth would drop to -18 ° C.
It should be understood that the energy of space received by the earth and the energy of the earth emitted towards space are equal on average, otherwise, the temperature of the earth would change in one direction permanently, towards always colder or always warmer. If the average exchange of energy with space is not zero, this results in a storage or destocking of energy from the earth. This change can result in a change in 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, of 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).
Approximate contributions to the greenhouse effect of the main gases:
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).
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.