Water vapor is a much larger contributor to the greenhouse effect than CO2. There is a lot more of it in the atmosphere.
The reason water vapor cannot be a forcing (i.e. root cause) of global warming is because of how short the water cycle is. If the amount of water vapor in the atmosphere increases, it is rather quickly balanced back by fixing some of the water vapor into liquid water. We call this process "precipitation."
I've seen estimates that the length of the water cycle is on the order of a week or two. So if human activity puts a lot of extra water vapor into the air--say, with a nuclear power cooling tower--then the excess water vapor will precipitate back into the liquid water within a week or two.
Human society has adapted very well to this cycle. We have built our societies to manage rain and snow [1].
The carbon cycle is much longer--I've seen estimates on the order of several decades at least. Thus, if human activity puts an excess of CO2 into the atmosphere, that will be there for several decades or longer. Thus the effect has quite a bit longer to compound, producing changes that seem long-term to human societies.
We have not built our societies to adapt to the effects of these types of trends. Think of how many human structures within just a few meters elevation above mean sea level, for example. Like: most of Manhattan.
Water vapor is a feedback. If the average atmospheric temperature goes up (for any reason), it can hold more water vapor, which then further increases the greenhouse effect, which raises the temperature more, etc. One reason scientists build complex computer models is to figure out how a little bump in temperature (from CO2 for example) can be magnified by feedbacks like water vapor.
[1] Well, at least in the volumes that have been common over the past few hundred years.
The reason water vapor cannot be a forcing (i.e. root cause) of global warming is because of how short the water cycle is. If the amount of water vapor in the atmosphere increases, it is rather quickly balanced back by fixing some of the water vapor into liquid water. We call this process "precipitation."
I've seen estimates that the length of the water cycle is on the order of a week or two. So if human activity puts a lot of extra water vapor into the air--say, with a nuclear power cooling tower--then the excess water vapor will precipitate back into the liquid water within a week or two.
Human society has adapted very well to this cycle. We have built our societies to manage rain and snow [1].
The carbon cycle is much longer--I've seen estimates on the order of several decades at least. Thus, if human activity puts an excess of CO2 into the atmosphere, that will be there for several decades or longer. Thus the effect has quite a bit longer to compound, producing changes that seem long-term to human societies.
We have not built our societies to adapt to the effects of these types of trends. Think of how many human structures within just a few meters elevation above mean sea level, for example. Like: most of Manhattan.
Water vapor is a feedback. If the average atmospheric temperature goes up (for any reason), it can hold more water vapor, which then further increases the greenhouse effect, which raises the temperature more, etc. One reason scientists build complex computer models is to figure out how a little bump in temperature (from CO2 for example) can be magnified by feedbacks like water vapor.
[1] Well, at least in the volumes that have been common over the past few hundred years.