One common argument against CO2 having any effect on the climate is that it is such a small quantity - only 400 parts per million. It is small compared to the full atmosphere, but how does it compare with the largest green house gas, water. Water in the Atmosphere, the Water Cycle, from USGS Water-Science School If all of the water in the atmosphere rained down at once, it would only cover the globe to a depth of 2.5 centimeters, about 1 inch. If the entire atmosphere were compressed to the density of water, the level would be about 10 meters high. The portion belonging to CO2 would be .04%, or .004 meters. The portion of water vapor is 25 mm or .025 meters The ratio of water vapor to CO2 is 0.25 / 0.004 = 6.25. But since the atomic weight of CO2 is 44 and the average atomic weight of air is lower, 29, conversion of volume to weight would be: 6.25 x 29/44 = 4.1 The Molecular Greenhouse Gas Composition of the Atmosphere Taking into Account Vertical Variation The approximate mass of all water substances in the atmosphere is 12.9×1018 grams. The amount of carbon dioxide is 3×1018 grams. Another way to calculate the ratio of water vapor to CO2 is to use the ratio of those figures. 12.9/3.0 = 4.3. The two different methods agree that the volume of CO2 is about a quarter of the volume of water vapor. In this light an increased amount of CO2 in the atmosphere has a much larger effect than your “gut feel” that the concentration of CO2 is so small. The amount of water isn't that much bigger. Bottom line: This analysis is not about GW or AGW. If you want to argue against AGW you have to use arguments other than your feeling that CO2 is such a small percentage of the atmosphere. Please compare it to the other green house gasses.