The forcing phenomena has been described in the phenomena section.

Using the Myhre equation        (eq. 6), the forcing caused by CO₂ only will be:

• for every doubling of the CO₂ concentration:            F = 5.35· ln(2) = 3.71 W m^-2
• or, since the BIE:                                                   F = 5.35· ln(400/280) = 1.91 W m^-2

Furthermore, Myhre calculated that from the BIE up to 1998 – the date of his publication at which CO₂ concentration was 366 ppm – the combined contribution of all known CHGs had been 2.25 W m^-2, of which 1.94 was attributable to CO₂, CH₄ and N₂O, and the rest stemming from other GHGs (halocarbons and chlorofluorocarbons).

Considering that N₂O is formed during the combustion of fossils and by the reaction of methane with air, and assuming that the other GHGs are not highly varying, the formulas proposed by Myhre can be simplified:

N₂O = 157.57 + 0.390 · CO₂ + 0.00788 · CH₄           (regression with R² = 0.9987)

F_CH4 = 0.0002237 · CH₄ - 0.1645                                   (regression with R² = 0.9997)

F_N2O = 0.002299 · N₂O - 0.624                                      (regression with R² = 0.9999)

F_GHG = F_CO2 + F_CH4 + F_N2O + F_others

With CO₂ at 399 ppm, CH₄ at 1825 ppb, and N2O at 326 ppb, the current (2014) radiative forcing due to GHGs is 2.6 W m^-2, with the following distribution when using Myhre’s formulas:

Contributors to GHG forcing

The primary temperature increase due to this overall forcing can be recalculated:

Surface and top of atmosphere temperature resulting
from the radiative forcing caused by GHGs.

Another very important greenhouse gas is water vapour which is present in the atmosphere at highly variable concentrations, from 0 up to 5 vol % (or 50’000 ppm) at different latitudes, altitudes and seasons.

Vapour acts as a gas absorbing long wave IR radiation as other GHGs. But it also contributes to heat accumulation and transport by its change of state.

The great alembic evaporates water mostly over oceans in tropical regions, condenses it, and let it rain or snow at different places. With its high heat of vaporisation (2444 kJ kg^-1 at 288 K) water is THE climate adjusting variable.

To generate average rainfalls of 1133 mm per year over the whole globe, an energy exchange of 88 W m^-2 must take place twice: to evaporate and then to condense the water. This is

-          considerable in comparison with the average solar input of 341.5 W m^-2,

-          very large in comparison with radiative forcing of 2.59 W m^-2,

-          and 2700 times larger than the total energy supplied by human activity in 2012 (0.033 W m^-2).

Today’s total GHG forcing of 2.59 W m^-2 corresponds to the heat of condensation necessary for a rainfall of less than 3 mm/month, a relatively hard to measure variation (see chapter 1.4.3).

With these characteristics the forcing attributable to water vapour is strong, highly variable in time and space, and undistinguishable from any other cause.