Roger Pielke, Jr., has posted an interesting article on the odds, as expressed by the IPCC, that humanity, natural variation, of a combination of the two is responsible for global warming. Needless to say, the IPCC’s position on this is, in his view, confused and contradictory. He writes:
“The IPCC offers a number of statements expressing its confidence in the likelihood of various claims based on very explicit guidance that it prepared for conveying uncertainties to its readers. These statements are the subject of much confusion and debate. This post discusses the IPCC statements on attribution of increasing global temperatures to various causes, as reported in the IPCC Fourth Assessment Report from 2007.
“First, let’s consider three possibilities:
A) Natural forces alone account for the observed warming.
B) Natural and human forces together account for the observed warming.
C) Human forces alone account for the observed warming.
“Not only are these three possibilities mutually exclusive, meaning that only one of them can be true, but logically one of them must be true, since they cover the range of possibilities. Thus if I were to ask you to express your confidence in the likelihood of each statement being correct, to be logically sound, then your expressed probabilities would need to add to 100%. …
“So here are the conclusions that the logical structure of the likelihood findings of the IPCC lead to:
1) The IPCC finds between a 5% and 10% likelihood that the observed warming is due to natural forces alone.
2) The IPCC finds a 5% to 10% chance that greenhouse gases are responsible for a minority of the temperature increase, leaving other human and natural forces to explain the balance.
“Now let me say that these results are surprising to me. Given the venom that characterizes debates over climate change, and the assumption by many that C) above is the only acceptable view, I wonder if the IPCC simply constructed logically unsound statements, or if in fact uncertainties are as large as they imply. None of this changes my views on any of the policy options on mitigation or adaptation that I have advocated, as 90% to 95% likelihoods are plenty strong. But they do raise questions about strong claims to certainty and the demonization of skeptical points of view.”
For the full text, click here.