In this case, we are told something like, 'The largest greenhouse gas out there isn't CO2, it is water vapour."
Though fundamentally true, this is disingenuous, because it is designed to miselad the reader or listener. The implication is that our activity is making a trivial contribution, and that climatologists' fixation on CO2 is misplaced.
First of all, the true bit: water vapour is the largest contributor to the greenhous effect (1). Bear in mind, however, that no credible scientist has tried to deny this is the case. It is a straw man argument. The mention it is a signal that something murky is afoot. Several further points need to kept in mind, which the denier will neglect to mention:
- First of all, the presence of water vapour in the atmosphere is natural (2). It is water evaporating from oceans, sea, rivers. Human activity does not significantly contribute to it. It is a natural process, with water evaporating, hanging about in the troposphere for a while, and then returning to Earth. If we didn't have this process, we wouldn't have rain, or rivers, and all the water in the world would have flowed to to the lowest point it could have reached. Where it would have frozen, because ...
- The presence of water vapour in the atmosphere is essential. As the predominant greenhouse gas, it is responsible for life as we know it. Without the water vapour, and the other naturally occurring greenhouse gases, the planet would be too cold for us to live on. We're not worried about the natural process that keeps us warm, we're worried about about the change being wrought on top of that, which may make the planet less pleasant to live on.
- Third, water vapour has a very short life span (3), returning to earth through condensation and rain. CO2, N2O and NF3 and other greenhouse gases resulting from human activity have a very long atmospheric lifespan (4) - hundreds to thousands of years - so continuing to produce them results in a build up that simply can't happen with water vapour.
- Fourth, the amount of water vapour in the atmosphere may increase as a result of global warming (5), creating a positive feedback loop. If we warm the planet, it will increase the amount of water vapour in the atmosphere, adding to the greenhouse effect. Which may kick off a feedback loop which leads to more water vapour, and so on. There is a possibility it will cool the planet, through increasing cloud cover, but we aren't sure which way it would go.
1 - 'Greenhouse effect: greenhouse gases,' wikipedia article, viewed 25th of April, 2009. (http://en.wikipedia.org/wiki/Greenhouse_effect#Greenhouse_gases)
2 - 'Greenhouse gas: role of water vapour,' viewed 25th of April, 2009. (http://en.wikipedia.org/wiki/Greenhouse_gas#Role_of_water_vapor)
3 - 'Water vapour: water vapour in Earth's atmosphere,' wikipedia article, viewed 25th of April, 2009. (http://en.wikipedia.org/wiki/Water_vapor#Water_vapor_in_Earth.27s_atmosphere)
4 - 'Greenhouse gas: global warming potential,' wikipedia article, viewed 25th of April, 2009. (http://en.wikipedia.org/wiki/Greenhouse_gas#Global_warming_potential)
5 - 'Water vapour: water vapour in Earth's atmosphere,' wikipedia article, viewed 25th of April, 2009. (http://en.wikipedia.org/wiki/Water_vapor#Water_vapor_in_Earth.27s_atmosphere)