The most prevalent of our greenhouse gases is water vapour. It's responsible for about two-thirds of the natural greenhouse effect. Carbon dioxide is a much more powerful absorber, however, and so a small increase in C02 can have a wider impact.The important thing about water vapour is that it's an agent of positive feedback. As global temperatures rise, the oceans warm up and release extra water vapour. This water vapour can then absorb energy and, in turn, radiate some of it toward the ground, thus helping the global temperature to rise even further-which causes still more water vapour to be released. Eventually, it's believed, the extra moisture and heat would generate enough clouds to keep this process from becoming a runaway greenhouse effect, but how this safety valve would operate on a global level is unclear.