CO2, CH4, other greenhouse gases, and their contribution to the Greenhouse Effect in the atmosphere

 
 

H2O
 

CO2
 

CH4
 

N2O
 

O3
 

O2
 

N2
 % concentration in atmosphere
0,3
0,036
1,8 10-4
0,3 10-4
0,03 10-4
21
78
 Contribution to GHG effect in %
55
39
2
2
2
-
-
 Relative efficiency of contribution
0,15
1
10
60
600
 
 
Table 1

Concentrations and contributions of the main greenhouse gases in the atmosphere. Relative molecular efficiency is also given, relatively to that of CO2.

Relative molecular efficiency of different GHG given in Table 1 should be interpreted with great caution. They ar emean values at today's concentrations. They do not reflect only the absorption capacity at molecular level. In fact, when the concentration of a molecule increases, the absorption of the atmosphere satures progressively, leading to a decrease of the mean absorption per molecule (a photon absorbed by one molecule cannot be absorbed by another). One can therefore expect that an additional molecule of carbon dioxyde has a relative efficiency less then that of an ozone molecule.

To compare the effect of different GHG, we define the Global Heating Power (GHP) of each gas measured in kg equivalent carbon. The GHP of a kg of carbon is, by definition,, the same as that of 3,66 kg of CO2 containing 1 kg of carbon.

The different gases have variable lifespans, therefore their GHP depends on hw much time later the effect is observed.

Table 2 gives examples of GHP for 1 kilogram of gas emitted.

By definition the GHP of carbon dioxyde is independent of time. Depending on whether a gas remains in the atmosphere for a longer or shorter time than CO2, its GHP increases or decreases as time passes.

Gas
 GHP
20 years
 GHP
100 years
 GHP
500 years
 CO2
0,273
0,273
0,273
 CH4
16,9
6,3
1.9
 CH4/CO2
62
23
7
 N2O
75,1
81
42,6
 HFC
12-2550
3-3300
1-2700
 CFC
1350-2800
1250-3800
437-4450
Table 2

Global Heating Power (GHP) for the emission of one kilo kilogram of different greenhouse gases measured in kg equivalent carbon. HFC's (Hydrofluorocarbures) and CFC's (Chlorofluorocarbures) corresponding to a family of molecules, a fork of GHP's is indicated.

Usually, it is the GHP at 100 years which is takebn in consideration in the discussions on GHG. This could be questioned. One may also be interested by what happens if the concentration in CH4 and CO2 are increased by the same proportion D (case of a GHG source after the new equilibirum is reached). Then it is the GHP at t=0 which should be taken in consideration.