Surprised you asked me this dopey question, since it was addressed in one of my earlier posts...
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From JunkScience.com:
So, greenhouse [effect] is all about carbon dioxide, right?
Wrong. The most important players on the greenhouse stage are water vapor and clouds [clouds of course aren't gas, but high level ones do act to trap heat from escaping, while low-lying cumulus clouds tend to reflect sunlight and thereby help cool the planet -etl]. Carbon dioxide has been increased to about 0.038% of the atmosphere (possibly from about 0.028% pre-Industrial Revolution) while water in its various forms ranges from 0% to 4% of the atmosphere and its properties vary by what form it is in and even at what altitude it is found in the atmosphere.
In simple terms the bulk of Earth's greenhouse effect is due to water vapor by virtue of its abundance. Water accounts for about 90% of the Earth's greenhouse effect -- perhaps 70% is due to water vapor and about 20% due to clouds (mostly water droplets), some estimates put water as high as 95% of Earth's total tropospheric greenhouse effect (e.g., Freidenreich and Ramaswamy, 'Solar Radiation Absorption by Carbon Dioxide, Overlap with Water, and a Parameterization for General Circulation Models,' Journal of Geophysical Research 98 (1993):7255-7264).
The remaining portion comes from carbon dioxide, nitrous oxide, methane, ozone and miscellaneous other 'minor greenhouse gases.' As an example of the relative importance of water it should be noted that changes in the relative humidity on the order of 1.3-4% are equivalent to the effect of doubling CO2.
http://www.junkscience.com/Greenhouse/
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Water Vapor Rules the Greenhouse System
Water vapor constitutes Earth's most significant greenhouse gas, accounting for about 95% of Earth's greenhouse effect (4). Interestingly, many 'facts and figures' regarding global warming completely ignore the powerful effects of water vapor in the greenhouse system, carelessly (perhaps, deliberately) overstating human impacts as much as 20-fold.
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 activities 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.
Good article on this this morning....
http://www.americanthinker.com/2009/01/co2_fairytales_in_global_warmi.html
Incidentally, we also know that man-made CO2 pollution being responsible for global warming is not supported by historical fact. The period known as the Holocene Maximum (approximately 7500 to 4000 years ago) is a good example — so-named because it was the hottest period in human history, long before humans invented industrial pollution. Furthermore, earth’s temperature and CO2 levels today have reached levels similar to a previous interglacial cycle known as the Eemian Interglacial Period of 120,000 - 140,000 years ago, a cycle lasting some 20,000 years before the earth returned to a full-fledged ice age immediately afterward.[i]
But still the AGW crowd clings to their mantra. I can only surmise that AGW advocates are experiencing cognitive dissonance and splitting, consciously choosing to be unaware of earth’s geophysical and climatic history.
http://www.americanthinker.com/2009/01/co2_fairytales_in_global_warmi.html