One of the false arguments global warming deniers make is that CO2 measurements are unimportant because water vapor is much more effective greenhouse gas. This is one of those statements that has just enough truth behind it to obscure the lie.Yes, water vapor is a very potent greenhouse gas, much more so than CO2, but what the deniers don't want to admit is that the reason there is water vapor in the atmosphere is because something else warmed it up in the first place.
This process makes water vapor a positive feedback agent with the potential to approximately double the amount of warming due to other sources. As the atmosphere gets warmer the amount of water vapor in the atmosphere also increases. The increased level of water vapor will then trap more heat, resulting in warmer temperatures and even more water vapor. But, the water vapor cannot do it alone. There must be some agent to start the process and that agent in our current situation is CO2. By dumping billions of tons of CO2 in our atmosphere every year, we trap heat in the atmosphere that would otherwise radiate into space and this leads to warmer atmosphere. This warmer atmosphere then leads to more water vapor.
Since water vapor is so important in the process we need to know just how much effect it has on the climate. A team of scientists used measurements from instruments onboard the Aqua spacecraft to make direct measurements of this effect. According to their work, water vapor amplifies global warming by 2.2 watts per square meter per degree Celsius (plus or minus .4 watts per square meter per degree Celsius). In comparison, the solar index (the amount of energy reaching Earth from the Sun) is about 1360 watts per square meter. For every one degree Celsius change in temperature, water vapor increases the amount of energy stored in the atmosphere by about .16%. It may not sound like much, but you keep doing that every day for a long period of time and it will add up to a very large amount of energy stored in the atmosphere that we would not otherwise have.
The scientists point out that this is only a short-term measurement because the amount of data is small. This figure is subject to short-term changes in the weather and climate fluctuations. They used these figures in models to try and determine a long-term value and the models suggested it is between 1.9 and 2.8 watts per square meter per degree Celsius. As more data is collected this figure will be refined to a more accurate value.