What is the radiative forcing of water vapor as a global warming gas?

Answer 1

The radiative forcing of water vapor is the amount of extra energy retained by the earth due to the water vapor int he atmosphere.

There are two slightly different definitions of 'radiative forcing' that can cause some confusion: the most common definition of forcing is the change in energy due to changes in the atmosphere's makeup (e.g., if you double the carbon dioxide, how much extra heat is retained). But, sometimes, people will use absolute values of radiative forcing, which counts all the heat absorbed by the chemical (e.g., if there was no water in the air, how much cooler would we be).

By the first definition, the radiative forcing of water is more-or-less zero. Human activity hasn't really changed the amount of water in the atmosphere by much, and the earth 'resets' its water levels fairly quickly through rain, evaporation, etc. Since we're not changing the amount of water in the air (at least directly), we're not forcing any temperature changes.

By the second definition, the radiative forcing of water is (very approx) 56 watts/m^2...or, even more approximately, if the earth had no water, it would be around 15C cooler.

Answer 2

I am not sure what you mean by "the radiative forcing", but water vapor is a more powerful greenhouse gas than carbon dioxide. The big question is will the small amount of warming caused by carbon dioxide increase or decrease the level of water vapor in the air. If it increases it, then it magnifies the warming; that is what the doom and gloom people are predicting. If it reduces it, then the air may warm very little or even not at all; this is what the oil companies are saying. The inconvenient truth (pun intended) is that nobody knows for sure which way it will go.