Dr Happs notes two people who acted as reviewers for the IPCC reports, but who were apparently "ignored" by the IPCC.
Richard S Courtney could not be said to be exactly independent, as shown by Sourcewatch. His qualifications (if any) are very unclear. He apparently has also never published any research on climate change. This may explain why his views were given little, if any, weight by the IPCC. It appears Dr Happs believes all views should be considered equal, or even that contrarian views should automatically be favoured solely on the basis that they are contrarian.
Dr Willem de Lange describes himself as a climate realist (on a blog called Climate Realists).
So, I am a climate realist because the available evidence indicates that climate change is predominantly, if not entirely, natural. It occurs mostly in response to variations in solar heating of the oceans, and the consequences this has for the rest of the Earth’s climate system. There is no evidence to support the hypothesis runaway catastrophic climate change due to human activities.
Dr Happs' evidence is not directly addressable, since it appears to relate to a personal conversation with Professor de Lange. The few points de Lange brings up in his article are comprehensively dealt with at Hot Topic.
The "Sun did it" excuse is also a favourite of Monckton, and is equally wrong.
Dr Happs then falls back on the "there is no consensus" argument which he had initially raised:
(Statistician Dr John McLean) makes the comment "How many times have you heard or read words to the effect that 4000 scientists from the IPCC support the claims about a significant human influence on climate? It's utterly wrong". In fact "Fifty-three authors and five reviewers are all that can be said to explicitly support the claim of a significant human influence on climate. The figure of 4000 is a myth". Indeed, against these few authors and reviewers are the tens of thousands of informed contrary views mentioned earlier. In other words, contrary to what Kevin Rudd implies, the consensus of informed scientists is against the IPCC.
Does anyone else notice the gentle irony that the immediately previous point was that 3000 scientists were ignoring Professor de Lange? This appears to be the case of the appearing and disappearing scientists! Look! They're there when you want them! Look! They've disappeared when you want to ignore them! EoR is deeply impressed.
The staffing of the IPCC may be less than 4000, but that's ignoring the point that they're assessing the work of thousands of scientists. More cherry picking of evidence by Dr Happs to reach a predefined conclusion, it seems.
In the last point EoR wants to consider today, Dr Happs again quotes Dr John McLean, accusing the IPCC of making a "U-turn" for changing early statements that the causes of global warming were indeterminate, to later statements that "the balance of evidence suggests a discernible human influence on global climate". This is hardly a U-turn, but just normal scientific refining of data. EoR would hate to be driving with Dr McLean as a navigator. Taking a fork in the road would be descibed as a "U-turn"!
Uncertainty is part of science. Deniers, on the other hand, are apparently immovably convinced of the absolute certainty of their (recycled) arguments.
Scientists like to say that all knowledge is provisional, tentative and subject to revision. This is true, but non-scientists tend to overread this and believe it means all knowledge is ultimately just opinion and is unreliable.