Question

Given the IR absorption spectra of `CO_2` and `H_2O`, why is it that we are concerned about `CO_2`?

Answer 1

Consider the sublimation point of carbon dioxide, versus the boiling point of water.

Explanation:

The sublimation point of carbon dioxide (at 1 atm) is `-78` `""^@C`. The normal boiling point of water is `100` `""^@C`. All of the carbon dioxide emitted worldwide ends up in the atmosphere. As to the greenhouse effect of water vapour, it is routinely incorporated in climate change models as a feedback phenomenon rather than as the forcing phenomenon expressed by carbon dioxide. In other words water vapour magnifies the effect of the climate forcing.

In any event, there is nothing much we can do practically to reduce the concentration of water vapour in the atmosphere. Why not? Because 2/3 of the Earth's surface is covered by water, and its vapour pressure will always be expressed as a function of temperature.