The fifteen scientists worked under an umbrella organization, the Non-Governmental International Panel on Climate Change, to produce a document that contrasts with the official (and very political) reports issued by the International Panel on Climate Change. So it's NIPCC vs. IPCC.
The NIPCC report claims that the IPCC have been selective in their use of data and have ignored a most important feature of computer models - feedback. From even a lay point of view it is clear that every action creates a reaction so why did the IPCC ignore feedback? Apparently the politics of climate change forced the IPCC to be selective, to produce an end-result that had been ordained to be necessary. That is simply bad science and history has demonstrated many times over that consensus science is rarely successful.
The NIPCC analyzed how feedback could work within the atmosphere and predicted that water vapor, the largest component of "greenhouse gas" does a remarkably efficient job of stabilizing the climate. The test of how effective this feedback could be is to study global satellite data (the IPCC chooses land based stations, often unsuitably positioned in heat islands like cities). Global satellite data from the last 10 years shows no perceptible change in temperature. Unfortunately 10 years is not long enough for me (the geologist in me speaking!) but even so this information is at direct variance with the oft touted news that we are experiencing uncontrollable anthropogenic global warming during the same time span.
I could go on, but actually it makes a lot more sense for me to recommend reading Booker's article and then downloading the NIPCC report. Fifty pages sounds like a lot but the latest IPCC synthesis of their AR4 report is longer.
Fred Singer makes the point in his preface that the debate on anthropogenic global warming is being suppressed and decisions based on the IPCC report are being blindly accepted by politicians. The debate must continue and where better to conduct it than on the internet?