by Robert Ferguson
Andrew Dessler requested a guest posting on the blog of Roger Pielke Sr.
Here follows Christopher Monckton’s short comments:
I’m looking forward to Roger Pielke’s reply.
If the water vapor feedback were anything like as strongly positive as he pretends, then the climate would be far more unstable than it is. Though his post doesn’t explain it, the water vapor feedback is almost 50% offset by the negative lapse-rate feedback: the greater the water vapor feedback, the greater the compensating lapse-rate feedback, which leaves the net feedback at around 1 W/m2/K. Also, though he mentions clouds very briefly, he does not follow through, and I’m sure Roger will do so. As the atmosphere warms, the space it occupies can carry near-exponentially more water vapor, and evaporation from the surface increases. There is, therefore, a negative evaporation feedback that is not mentioned or quantified in the IPCC’s documents, and – more importantly – the more water vapor plus evaporation the more clouds at low level, which are optically dense and reflect more sunlight harmlessly back to space.
We can measure this not only by visual imaging (ISCCP) but also by contrasting the short-wave and long-wave radiation channels as measured on the ERBE and CERES satellites, or even by monitoring changes in Earthshine. Result of all this real-world measurement: in 1983-2001, the only two decades during which we could in theory have caused “global warming”, the CO2 radiative forcing using the UN’s method was 0.8 W/m2 but the positive forcing from an unconnected and natural diminution in cloud cover over the period was 4.5 W/m2, more than five and a half times bigger.
And that’s just the direct forcing. The water vapor feedback, if it had been anything like as big as the UN and Dr. Dessler would have us believe, should have caused a massive increase in global temperature. It didn’t. – Christopher