which is a measure of the degree that the global climate changes in response to any change in its energy balance. I have mentioned Roy Spencer already.
The scientific evidence points to a figure of around 3*C - that is, when the CO2 in the atmosphere is doubled, the world will heat up by 3*C, plus or minus 1.5*C. Maybe more.
Against the consensus are a handful of sceptic scientists, who claim that sensitivity is low. Chief among these is Roy Spencer, who recently published a paper claiming that climate models overestimate sensitivity.
His paper is naturally seized on by the denial blogosphere trumpeting it as the "Death Blow to Global Warming", (e.g. Forbes) and links to various re-pastings of the Forbes article were treated to a large number of retweets on Twitter last week.
In fact, the Forbes headline distorts Spencer's claim in his paper. He simply concludes "atmospheric feedback diagnosis of the climate system remains an unsolved problem, due primarily to the inability to distinguish between radiative forcing and radiative feedback in satellite radiative budget observations”
The graph shows Spencer's claim. The green line shows the satellite observations of heat losses at the top of the atmosphere, and the red and blue line shows the predictions of the climate models. A significant disparity is shown. Spencer claims that more heat is being lost than the models assume.
Death blow, yes?
First and foremost, Spencer opposes the general view that changes in ocean temperatures cause changes in the clouds. He believes that the clouds cause the ocean temperature changes. However, he has no evidence for his view. It is just his assumption: he believes clouds vary randomly. Maybe it is a direct intervention by God in planetary affairs, because Spencer is a creationist. Given that science is all about investigating causes, and there is an established causal chain between warmer oceans and more clouds, his assumption is very weak.
Second, there is the matter of the model that Spencer uses. Yes, the climate sceptic is using a model, despite the intense criticism that the sceptics direct at modelling. But his model "has no realistic ocean, no El Niño, (ocean current cycle) and no hydrological cycle, and it was tuned to give the result it gave." (Trenberth) In using simple models, Spencer & Braswell is using the central sceptic technique of cherrypicking - taking a partial, not complete, view of the data.
Trenberth and Fasallo have a critique of Spencer's paper on Real Climate.
First, Spencer and Braswell (SB11 from now on) do not supply error margins on their figures. This is an astonishing fundamental error of method.
When the data is re-worked using more relevant timescales and error margins, there is a better fit between the observations and the models, particularly the model that factors in the ocean current changes.
The conclusion is that Spencer has not blown a hole in AGW. If anything he has blown another hole in his own credibility. Climate sensitivity to a doubling of CO2 remains in the 1.5-4.5*C range, which means that we have to decarbonise the global economy.
If you don't believe, or think I am glossing over the details, read this:
Barry Bickmore on Spencer's simple models.
[Update 3Sept] the Editor of Remote Sensing has resigned, because he failed to pick up that the 3 reviewers (climate skeptics) ignored the fact that SB11 arguments had already been refuted, and that SB11 did not address this argument in their paper. In short, it was a defective paper.
As usual, there was a big splash in the MSM and blogsphere claiming that Spencer had sunk AGW, but no comparable splash for the retraction.