Idle chatter about global warming

The global warming debate rages on. Many folks think that it's a tempest in a teacup, others think that it's a real sign of human impact on the environment. I don't have much of an opinion on the subject, as I'm woefully ignorant of the science. However, the occaisonal debate catches my interest.

A recent discussion on a different message board included this statement:

Water vapor has always been the primary green house gas.

Let's assume this statement is true. The implication from this statement is that if water vapor is the primary greenhouse gas, that all of the CO2 we could possibly produce would never amount to much. Therefore, global warming is not due to human activity, and attempts to limit it are futile.

Maybe. It depends on what we mean when we say "primary". By primary green house gas do we mean that the primary gas has the largest amount in the atmosphere or the largest contribution to the warming effect?

The difference is essential.

A hypothetical example: assume water is 90% of the greenhouse gas by amount. CO2 is 5%, and all of the other gases are 5%. What if CO2 has 100 times the Green House Gas Activity than water?

Relative activity of water = 1 x 90 = 90

Relative activity of CO2 = 100 x 5 = 500

That 5% by volume would have over five times the net impact of water despite being 1/18th the relative amount.