What does it take for Fritz Vahrenholt, a professor in the Department of Chemistry at the University of Hamburg, a former German environment minister and one of the fathers of the German environmental movement (and, unlike everyone at Desmogblog.com, who simply write character assassinations the moment people deviate from their worldview, has actually devoted a lot of time and money to replacing fossil fuels) to lose faith in the Intergovernmental Panel on Climate Change (IPCC)?

It took working with the IPCC.  
In February 2010, I was invited as a reviewer for the IPCC report on renewable energy. I realised that the drafting of the report was done in anything but a scientific manner. The report was littered with errors and a member of Greenpeace edited the final version. These developments shocked me. I thought, if such things can happen in this report, then they might happen in other IPCC reports too.
The bit about Greenpeace writing IPCC reports is well known but critics sputter that Vahrenholt was not working on a climate change group and instead a renewable energy one and he's totally a denier now so he doesn't count anyway.  I guess that means everyone knew the renewable energy part of the IPCC was manned by political activists rather than scientists but we should believe the rest of the IPCC is okay. The IPCC working groups on climate change also had no problem using made-up conjecture (melting Himalayan glaciers, farm yields down 50% in Africa by the end of this decade) so he is not completely wrong in worrying - if the IPCC had Exxon writing their reports I would hope the skepticism would be the same from people who care about the environment and science.  And it is fair to say that because the IPCC is hyper-politicized (and now they have completely lost their minds and imposed gender and geographical quotas for its membership - so Africa, with 1/100th the science output of the USA, has 5 members on the IPCC while the US will only have 4) it is beset by similar problems throughout its working groups.  

Even if it is not the case, perception matters in policy debates.  If the IPCC looks as unethical and slanted as fossil fuel companies then the public will feel like they are no longer getting science versus agendas, they are just picking one world view over another.

Writing in The TelegraphVahrenholt makes some fine points about the insecurity of the results and certainly the numerical models, but it is a bit of a red herring.  Any science that uses numerical models has an inaccuracy built in, though the creators of the model never believe that.  Having worked at three companies in the physics analysis business I can't count the numbers of times I met an academic who claimed to have created a better analysis tool - invariably it would turn out to be some kind of assumption-based model with a terrible interface and mesh (or, worse, command line) that only worked accurately on their suite of test problems.  In real applications, it broke down. Corporations and universities instead bought their analysis software from people whose families were fed based on the accuracy of the tool. It's serious stuff and that is what the IPCC lacks.  There is no accountability for being inaccurate.

Dr. Fritz Vahrenholt, once adored by activists, now vilified.  What changed?  He saw how sausage got made in the sausage factory of UN climate studies. He still cares about the environment, he is just not sure the IPCC does. Credit and link: The Telegraph

That said, climate science outside the IPCC has gotten a much better understanding of both statistics and numerical models in the last decade.  Continuing to hammer on flaws in old models is like beating on Darwin for not knowing about modern synthesis or Henry Ford for not building a 2012 car. But Vahrenholt gets a bit of a special dispensation because he is putting his money where his beliefs are.  He is not denying that emissions are a problem but rather saying that we should not be in the panic mode the IPCC wanted everyone in back in 2006.  We have some time to move elegantly.

We clearly have to migrate from fossil fuels but we have time to do it and, he argues, it doesn't make sense to trade a climate catastrophe in the future for the economic one happening right now.

He has a point.  America has spent $72 billion in the last two years on alternative energy schemes - not basic research, that would make some sense, but subsidizing private technology companies so that we can 'beat China' in solar power installations and whatever else.  That's 240,000 science projects that did not get funded to do basic research on new alternative energy ideas. Basically, that is 10 years of the entire National Science Foundation budget squandered on pet projects of the administration.

If you're in America and don't like the economy, keep in mind that we are back at mid-1990s levels of CO2 emissions, just like the IPCC wanted.   And we are an economic ghost town, while India, China and Mexico, all exempt from climate change treaties, are doing quite well.