Current Events — HabitableZone

Current Events » in reply to Think about this.

re: Think about this.
gbaikie — (archived)

"Water vapor is a far more effective green house gas than CO2. Water vapor is over 25 times more abundant in the atmosphere than CO2. If that's the case, then how could an increase in CO2 levels bring the world to a meltdown, but all that water vapor isn't?"

The rational is that increasing CO2 levels will "force" an increase in water vapor.
Example:
"Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century."
http://en.wikipedia.org/wiki/Global_warming

The large uncertainty of 1.1 to 6.4 is largely due to the uncertainty of how much increase in water vapor will occur from rising CO2 levels.

The PROBLEM is that there is no evidence that the rising CO2 that has already occurred, has had any affect upon the amount water vapor in the Earth atmosphere.

If increasing CO2 level doesn't cause a significant increase in global vapor, then a doubling of CO2 would be predicted to cause about a 1 C rise in CO2.

Of course we don't actually know that even a doubling of CO2 would cause a 1 C rise in global temperature. And it should be noted that doubling of CO2 is the terminology normally used rather than some specific numbers. This politically useful because they could mean doubling from pre-industrial levels [and they sometime say this]. Or one can assume it a doubling some more current number, like 380 ppm.
It's unlikely we will have 680 ppm before the end of the Century, but doubling of some pre-industrial level may be possible- assuming the trend holds. Which with the last 10 years of non-warming and the very recent cooling casts a fair amount of doubt on any predictions they are making.

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