Jo Nova steps into the Climate Sensitivity debate

Joanne Nova, author of the Skeptics' Handbook, has a new blogpost "Man Made Global Warming Disproved", which must be the millionth time that climate contrarians have made this claim. The Twitter #agw hashtag is making much of it.

It is a compressed summary of the contrarians' key arguments, a litany of claims and statements. Reading it through, I recognise many of the claims as long since rebutted. This is a feature of contrarian claims - the climatologists can dispose of their claims, but this makes no difference. The contrarian process cares not a fig for scientific refutations, because their case is based on publication in newspapers and the internet, where impression takes precedence over accuracy.

I'm not going to address her claims, because they have all been refuted before, mainly on DeSmogblog and if I re-iterated the refutations here, it would make no difference (see above).

Instead, I wish to pick up on two things she says.

1. "The direct effect of CO2 is only 1.2*C"
   This is an important admission, as it separates her off from the cohort of those who deny that CO2 is a greenhouse gas. 
2. "Observations show that net feedback is probably negative, which would instead reduce the direct effect of the extra carbon dioxide". "A multitude of observations are in rough agreement that any increase in global average temperature caused by a doubling of CO2 is more likely to be about half a degree".  Nova provides no methodical evidence to back these claims. They are mere unbacked statements. 

However, they are a start. She has made a statement that can be tested: "Net feedback is probably negative". This statement can be tested. 

1. First, we can look at the aggregated feedbacks, as I have done here. The net result of the feedbacks I have collected is clearly positive, from those that I have been able to quantify. Admittedly, I have not been able to quantify all feedbacks, but on the other hand, Joanne Nova has not tried to quantify any. No doubt they will try to do so soon.
2. Contrarians need to look at all the other evidence, from paleoclimatology and other lines of inquiry, that point to a climate sensitivity greater than that claimed by contrarians, and show how in each and every case, their figure for climate sensitivity can explain the findings. It is remarkable that so many different lines of inquiry should point to the same figure, around 3*C. In any other discipline, this evidence would be enough to be generally accepted, but where the interests of major corporations are affected, the standards of evidence are set higher, and I guess we have to accept that fact and act accordingly. 

Joanne may have set out to disprove climatology, but she has set the scene for her own hypothesis to be wrecked.