Key in the anthropogenic (human-caused) global warming theory is the premise that world increases in greenhouse emissions will lead to higher global temperatures.

The AGW theory in this respect generally goes as follows:

Greenhouse gases are responsible for much of global temperature, with more greenhouse gases in the atmosphere leading to more warming.

Though water vapor constitutes more of the atmosphere's greenhouse gases than carbon dioxide (co2), the warming that co2 (still being a substantial portion of greenhouse gases) sets off a feedback loop. As it goes, more warming leads to melting glaciers and so on, ultimately increasing the amount of water vapor released into the atmosphere. So a feedback loop would compound exponentially ballooning, constantly expanding, world outputs of greenhouse emissions.