Water vapor is 99.999% of natural origin. Other atmospheric greenhouse gases, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and miscellaneous other gases (CFC's, etc.), are also mostly of natural origin (except for the latter, which is mostly anthropogenic).
Human activites contribute slightly to greenhouse gas concentrations through farming, manufacturing, power generation, and transportation. However, these emissions are so dwarfed in comparison to emissions from natural sources we can do nothing about, that even the most costly efforts to limit human emissions would have a very small-- perhaps undetectable-- effect on global climate.
Water vapor (clouds) not only absorbs infrared radiation from the Earth's surface, but it also reflects incoming radiation from the Sun. This phenomenon caused mainly by clouds and ice is called albedo. The average total albedo of the Earth is 30%.
Clouds cover about 60% of the surface of Earth and even a small change makes a big difference. IPCC models include changes in sunlight, but only due to increased cloud formation caused by global warming. They do not include any external factors leading to the amount of sunlight heating the surface of Earth.
"Externally-driven albedo (EDA) is albedo that is not attributable to feedbacks in response to surface warming. Possibly due to non-terrestrial influences, EDA includes any changes in albedo due to modulation by the Sun. For instance, cosmic rays are widely suspected of encouraging cloud formation, or any number of solar effects might change the proportions of ozone and the relative heights of the tropopause at the poles and equator, which affects jetstreams and the amount of clouds. EDA might also involve terrestrial agents, such as cloud changes owing to aerosols released by plankton, or volcanic eruptions could change the lower stratospheric temperature which in turn changes albedo."REFERENCES & LINKS: