Friday, July 06, 2012

David Evans' battered case for low climate sensitivity

I have been pushing the idea that we should confront the climate change sceptics' position on the point of climate sensitivity (CS), since they are wedded to a figure of 0.5*C increase for a doubling of CO2, while mainstream climate science points strongly to a value in the region of 3*C.

Chuck Long points me to a page on Watts' skeptic blog where David Evans puts the skeptic case systematically, beginning with CS.

Evans begins by accepting that the CO2 we have put in the atmosphere will eventually raise the planet's temperature by 1.1*C, which puts him in the Scientific Approach part of the sceptic assembly, as opposed to the Anything Goes part.

He accepts that the mainstream scientists say that this increase will be amplified by positive feedbacks by a factor of around 3.
As a sceptic, he believes that negative feed-backs will reduce the eventual temperature increase to a trifling 0.5*C.

He mentions only water vapour as a positive feedback, leaving out the following:

  • changes in albedo (reflectivity) 
  • methane from thawing permafrost &c 
  • CO2 releases from warming soil 
  • CO2 from increased forest fires
Oddly, later on he says there are "literally thousands" of feedbacks.
I would like to see his list of thousands, as a matter of curiosity.

He backs his claim for negative feed-backs by referring only to clouds as a strong negative. This is based mainly on Lindzen and Spencer, dealt with on this blog here and here, and their work is countered by Dessler, who finds clouds have a weak positive feedback effect.
In any case, in a world that is warming for whatever reason, the effect of cloud, whether positive or negative, will become less.

He makes no attempt to look at all the other lines of evidence that point to a higher climate sensitivity, work based on other changes to Earth's temperature, all of which point to the same ballpark figure. Lindzen does briefly touch on them.

So Evans hardly makes his case for low CS. 

Instead of trying to establish his own hypothesis, he reverts to attacking the climate science.
His targets are Hansen's 1988 projections, Ocean temperatures. the Atmospheric Hot Spot, and model non-concordance with tropical temperatures.

Hansen's 1988 projections
First he has a go at Hansen's 1988 projections, which have been countered many times before. It is  interesting that the first recourse is to attack work that is 24 years old, although since Hansen was making a prediction, it is fair enough to compare his prediction with observations. His model did not factor in reductions in CFC and sulphates, both of which had a cooling effect.

Ocean temperatures
The next attack is more interesting. He displays a graph showing a disparity between a flat period of recent ocean temperature observations, and climate models. John Cook shows a long term graph which shows clearly that ocean temperatures are indeed rising. So Evans is deploying a classic skeptic's selectivity on data, taking what is convenient to his case and hiding the rest.

Here is a direct , authoritative statement that climate models are consistent with ocean temperature readings from Laurence Livermore Lab.

Atmospheric Hotspot
Next, Evans deploys the Atmospheric Hotspot argument. This argument is based on a persistent misunderstanding on the part of the sceptics: they believe that a hot spot at the top of the equatorial troposphere is a "signature" - a diagnostic test - of AGW - i.e. man-made global warming.

In fact, the signature of warming due to GHG increase (as opposed to increased heat directly from the Sun) is a heating of the troposphere with a cooling of the stratosphere.

Here is the excellent Skeptical Science on Evans' Hot Spot.

Outgoing heat
The final point in Evan's case is based on data taken from Lindzen and Choi 2009, a paper which was severely criticised for methodological flaws, even from the sceptics' own camp.  It was partially improved and presented as Lindzen & Choi 2011, but  the problems remain.

wrote about Lindzen and Choi here.

Even if there is a disparity between the models and the observations at this point, the fact is that Lindzen is looking at one very short term feedback mechanism. Looking at the full range of feedbacks, not just clouds, but albedo, methane and secondary CO2 releases, it is very clear that as ever, their narrow partial view simply does not truly reflect the systemic, long term global processes that are taking place. The fact that they cannot see this disparity between the single facet and the whole picture shows how important it is for us to carry the argument about feedbacks strongly into the sceptics own territory.

Here is a concise refutation of Evans arguments.

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