Repeating the title of this post:
Carbon dioxide is NOT the primary greenhouse gas.
If you watched Al Gore's movie (I did) and read the popular press and even a lot of hard-core scientific commentary, you would come away with the very clear and "indisputable" view that carbon dioxide is the primary greenhouse gas contributing to the greenhouse effect. But, it simply isn't true.
I myself thought it was true. I had no reason not to assume that it was true.
But then two weeks ago I was reading up of all this "global warming" and "climate change" stuff on the NOAA web site (National Oceanic and Atmospheric Administration, where a number of the top experts on all of this stuff work) and there it was. I was reading their Climate of 2006 - Annual Report which has a link to their web page for Global Warming Frequently Asked Questions which has the following section:
What is the greenhouse effect, and is it affecting our climate?
The greenhouse effect is unquestionably real and helps to regulate the temperature of our planet. It is essential for life on Earth and is one of Earth's natural processes. It is the result of heat absorption by certain gases in the atmosphere (called greenhouse gases because they effectively 'trap' heat in the lower atmosphere) and re-radiation downward of some of that heat. Water vapor is the most abundant greenhouse gas, followed by carbon dioxide and other trace gases. Without a natural greenhouse effect, the temperature of the Earth would be about zero degrees F (-18°C) instead of its present 57°F (14°C). So, the concern is not with the fact that we have a greenhouse effect, but whether human activities are leading to an enhancement of the greenhouse effect.
Right in the middle of that paragraph, there you see it:
Water vapor is the most abundant greenhouse gas...
Yes, it also says that carbon dioxide is number two:
... followed by carbon dioxide ...
But the key fact there is that carbon dioxide is not number one.
Putting it very clearly, in my own words:
Water vapor is the primary, most abundant, greenhouse gas.
Now, NOAA is a government agency, and under the control of the evil Bush administration, so one might treat any "data" from NOAA with skepticism, but they really are the best of the best when it comes to understanding weather, the atmosphere, and climate.
Nevertheless, I decided to "check" this NOAA research by consulting the Wikipedia. Known for its articles that are created and edited by real people (including professionals and scientists) rather than big business and "the government", here is what Wikipedia has to say about greenhouse gas (you can also read their articles on the greenhouse effect, global warming, and climate change):
Greenhouse gases are components of the atmosphere that contribute to the greenhouse effect. Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring gases. 
There it is, water vapor first in the list.
But wait, there is much more... The article has a section entitled The role of water vapor which states:
Water vapor is a naturally occurring greenhouse gas and accounts for the largest percentage of the greenhouse effect.
There it is:
Water vapor ... accounts for the largest percentage of the greenhouse effect.
The Wikipedia article even gives us some numbers:
The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%. It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.) 
Even the upper end of the carbon dioxide range is well below the lower end of the water vapor range (26% vs. 36%). The midpoint of the water vapor range (53%) is three times greater than the midpoint of the carbon dioxide range (17.5%).
There is certainly a lot more to the topics of greenhouse gas, greenhouse effect, global warming, and climate change, but the relative effect of the various greenhouse gases would seem to be a very relevant starting point, but you wouldn't know that from listening to Al Gore or the popular media or even the expert scientific commentators.
BTW, there may in fact be an upwards trend in water vapor in the atmosphere, at least according to this chart from the Wikipedia. That chart is only for one locality, but a key point is that we aren't reading about global water vapor trends in the press or hearing about it from Al Gore, et al.
Burning of fossil fuels is the heart and soul of the global warming/climate change "movement", with the idea that burning fossil fuels puts an excess of carbon dioxide into the atmosphere. Guess what... burning fossil fuels also puts water vapor into the atmosphere. Don't believe me? See that little cloud coming out of a car's tailpipe on a chilly day? That is simply the exhaust water vapor condensing due to the low temperature. On a warmer day the water vapor goes right out the tailpipe and into the atmosphere. Granted, there are some differences in how water vapor and carbon dioxide persist in the atmosphere. So, I'd like to see some real science that can justify why the focus is on carbon dioxide rather than water vapor. Not some hand-waving brush-off, but some real science.
Hmmm... Al Gore pointed to Hurricane Katrina as an effect of global warming, but what would a hurricane be without water vapor? Couldn't an excess of water vapor account for bigger and fiercer hurricanes? Show me some real science that can justify an excess of carbon dioxide as being a bigger effect on hurricanes than an increase in water vapor.
A key question is that if excess water vapor is a primary cause of the greenhouse effect and even a bigger cause than carbon dioxide, then "carbon segregation" might be a waste of time and energy and resources if the goal is to moderate the greenhouse effect.
Some things to think about.
BTW, I would like to see efficiency-based improvements in energy consumption, and those alone might yield a much larger reduction in the human impact on the environment than all manner of hare-brained schemes for directly manipulating the environment. I would also like to see attention to reducing deforestation and would like to see significant efforts at reforestation (where it makes economic and environmental sense). Trees are great, natural machines for "carbon segregation" (absorbing carbon dioxide and converting it to plant cell matter), and I would love to see some real science that looked at the impact on greenhouse gases of the loss or gain of even one tree. Why are so many people so convinced that burning fossil fuels is a bigger net addition than the net loss of carbon segregation caused by dramatic deforestation over the past hundred (or two hundred) years? Not to mention that paving the world with asphalt roads and parking lots and building roofs has been reducing the ability of the Earth's surface to reflect solar radiation.
I found one very curious statement in the Wikipedia article on greenhouse gas:
Intergovernmental Panel on Climate Change (IPCC) IPCC Third Assessment Report chapter lead author Michael Mann considers citing "the role of water vapor as a greenhouse gas" to be "extremely misleading" as water vapor can not be controlled by humans ; see also  and . The IPCC report discusses the water vapor feedback in more detail .
What on earth does not being "controlled by humans" have to do with it?!?! We want to understand greenhouse gases, the greenhouse effect, global warming, and climate change regardless of the sources of the effects! If some are due to human activity, fine, so be it. But if some are due to natural processes, that should be fine as well. Science should be blind to the source of an effect. The statement highlights the fact that there are significant numbers of people who have an agenda or even multiple agendas that have little to do with a truly scientific understanding of atmospheric and climatic processes.
A lot of my questions and concerns may in fact be addressed somewhere in the vast scientific literature, but my first complaint is that the media and the "promoters" of the global warming/climate change agenda are not giving us a clear and scientifically honest story.
I would like to clearly state that I do recognize that the fact that carbon dioxide is not the primary greenhouse gas does not necessarily tell us that global warming could not be "caused" by excess levels of carbon dioxide, but it does tell us that the conclusion about carbon dioxide being the dominant cause is nowhere near as clear and "indisputable" as a lot of people are claiming.
-- Jack Krupansky