Roy Spencer is their main man. Unusually for an AGW sceptic, he is an actual climate scientist, specialising in satellite measurements. He sets out his position here.
I am indebted to Barry Bickmore for his critique of Spencer, which I have condensed here.
Spencer believes that CS is low because clouds are the cause, not the effect, of raised temperature. When an unspecified Something causes cloud cover to increase, the atmosphere cools (due to more albedo), and when the Something causes cloud cover to decrease, the atmosphere heats up. (He is alleging that the El Nino/La Nina phenomena are due to random variations in cloud cover, a momentous and counterintuitive assertion for which he has zero evidence).
Spencer and Braswell 2008 claim to find evidence that clouds give negative feedback. When temperature increases, water vapour goes up, cloud cover increases, and the earth cools down. Cloud will stop the planet overheating. We have nothing to worry about. CS is low. (In fact the opposite occurs, as shown by the correlation of cloud cover and temperature here).
Spencer tries to back up his theory by looking using a simple "zero-dimensional" climate model.
There is an irony here. Sceptics damn climate modelling as useless, but at the same time they are prepared to use them. However, the models they use are simple, and do not attempt to factor in all of the variables that go to make up global climate. In using these simple models, they are using the same techniques that their followers use on their blogs - they just use a selected part of the system, and avoid looking at the whole system. (More on Spencer's model here).
Anyway, Spencers simple little model was applied to look at sea surface and satellite data, and they found that the feedbacks were strongly negative. In the short term. Spencer admits that in the long term, cloud feedback is weakly positive, which is what the mainstream modellers say. Strongly negative feedback would mean that we need not worry about global warming.
Since Spencer and Bradwell published in 2008, two other important papers covering cloud feedback have come out.
Dessler made an estimate of cloud feedback from top of the atmosphere heat radiation and came up with a result of -0.2 to 1.28 Watts per square metre for cloud feedback - that is, a weak positive feedback, with a small chance of negative feedback. Dessler's results are consistent with the mainstream assumptions.
Second, Murphy and Forster 2010 reviewed Spencer's work, and showed that he had underestimated the positive feedback by a factor of 10.
So, to summarise:
- The evidence, from a variety of sources, by modelling, by palaeontological evidence, and by observation, points to a CS of 3.
- The main sceptic who tries to put forward evidence of low CS is refuted by direct observational evidence and by a review of his methodology.
- Spencer brings nothing to the table capable of overthrowing the total evidence for CS of 3.
- Therefore CS of 3 still stands.
Unless Lindzen can produce anything? That is the next post.