Matt Ridley: The Perils Of Confirmation Bias. Read it at GWPF [updated at 12.25pm to prevent confusion]
The modus operandi of the Intergovernmental Panel on Climate Change (IPPC) has been to accumulate evidence to champion rather than challenge a hypothesis, namely that rising carbon dioxide levels will in future cause dangerous climate change.
A good example is the IPCC’s claim that only models that incorporate high-sensitivity carbon dioxide-induced warming countered by aerosol induced cooling can match (or “hindcast”) the recent upward progress of global average temperatures. The problem with this is that different models use different values of assumed cooling from aerosols. That is to say, the cooling effect of aerosols has been picked so that it fills the gap between observed and expected warming. The modellers are therefore in effect saying: we observe warming of X, we predicted warming of X+5, so there must have been cooling of 5, therefore our prediction is correct.
Read more here
http://www.thegwpf.org/matt-ridley-the-perils-of-confirmation-bias/