2011-07-16 16:55:12Water vapour distribution and GW


Hi very smart people,


I have a discussion with a "skeptic' about this article :



His interpretation is that this is a contradiction with the IPCC statement :

Water vapour is the most abundant and important greenhouse gas in the atmosphere. However, human activities have only a small direct influence on the amount of atmospheric water vapour. Indirectly, humans have the potential to affect water vapour substantially by changing climate. For example, a warmer atmosphere contains more water vapour. Human activities also influence water vapour through CH4 emissions, because CH4 undergoes chemical destruction in the stratosphere, producing a small amount of water vapour.


I don't agree but find it difficult to give a good explication about that. Can somebody give me an idea?





2011-07-16 20:54:06
Dikran Marsupial
Gavin Cawley

sorry misread the post

2011-07-16 21:21:12
Alex C


This paper's abstract says the exact opposite of what the "skeptic" is saying.  Perhaps what he is having trouble with is what "relative humidity" means, as the paper says that relative humidity over land is falling.  That does NOT mean that humidity itself is falling, as relative humidity is merely the ratio of realized vapor pressure to saturated vapor pressure.  If the land warms faster than the ocean, and especially considering that there is no large body of water (like an ocean) to supply constant evaporative flux of water vapor, then the relative humidity of the air over land will fall as the above atmosphere's capacity to hold water vapor increases without a concomitant increase in water vapor itself.

You can make an analogy to digging an underground pool - the deeper (warmer) you make it, the more water (vapor) it can hold, but if you dig at a rate faster than you can fill it, then the ratio of actual depth (partial pressure in air now) to potential depth (saturated vapor pressure @ that temperature) will fall.

2011-07-17 01:13:34
Mark Richardson

What the hell?



The paper confirms that global mean amount of water vapour is modelled to rise with temperature. If this wasn't true, the value would be ≤0 % K-1. The value is positive, therefore it agrees with the IPCC statement.



That's a global picture, reported by the paper. In some areas there is less water vapour. I suspect this is because there are associated changes in circulation and some regions rely on evaporation to provide some atmospheric water vapour. As you warm them, the land dries out sooner in the season and iirc you can see much lower late-summer water vapour (at least, this is seen in the temperature fields, I can only assume it impacts moisture fields too)

2011-07-17 16:50:37


ok thanks for the answers people