The alarmist media bought it at the time, which may well have been the idea.
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A new paper from the Global Warming Policy Foundation reveals that the IPCC’s 2013 report contained a remarkable logical fallacy, says Climate Change Dispatch.
The author, Professor Norman Fenton, shows that the authors of the Summary for Policymakers claimed, with 95% certainty, that more than half of the warming observed since 1950 had been caused by man.
But as Professor Fenton explains, their logic in reaching this conclusion was fatally flawed.
“Given the observed temperature increase, and the output from their computer simulations of the climate system, the IPCC rejected the idea that less than half the warming was man-made. They said there was less than a 5% chance that this was true.”
“But they then turned this around and concluded that there was a 95% chance that more than half of observed warming was man-made.”
This is an example of what is known as the Prosecutor’s Fallacy, in which the probability of a hypothesis given certain evidence is mistakenly taken to be the same as the probability of the evidence given the hypothesis.
As Professor Fenton explains:
If an animal is a cat, there is a very high probability that it has four legs. However, if an animal has four legs, we cannot conclude that it is a cat. It’s a classic error, and is precisely what the IPCC has done.”
Source here.
Tallbloke's Talkshop 
The idiots at the IPCC need to learn that computer games (simulations) are meaningless when applied to a chaotic system because it can’t be defined mathematically. All their games show is the bias of those writing them, but then the object isn’t to show truth but to scare the people into giving them more money.
How politics (read vast committees–a creature of innumerable legs, vastly long body, and no brain) has always worked.
What gets me is the number of people in the world who think that gov’t is the natural and only means to fix social “problems.”
The rise in temperature is man-made because our models show a rise in temperature if we assume man-made CO2 raises temperature.
They keep proving their hypothesis by assuming their hypothesis is proven by their hypothesis,
Phoenix says…
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In short, confirmation bias.
The most telling point in Fenton’s paper is this one:
“The illusion of confidence in the coin example comes from ignoring (the ‘prior probability’) of how rare the double-headed coins are. Similarly, in the case of climate change there is no allowance made for the prior probability of man-made climate change, i.e. how likely it is that humans rather than other factors such as solar activity cause most of the warming. After all, previous periods of warming certainly could not have been caused by increased greenhouse gases from humans, so it seems reasonable to assume – before we have considered any of the evidence – that the probability humans caused most of the recent increase in temperature to be very low.
Only the assumptions of the simulation models are allowed,
and other explanations are absent.
In both of these circumstances, classical statistics can then be used to deceive you into presenting an illusion of confidence when it is not justified.
Some additional commentary about why this is important. In a more general sense, IPCC was locked into Prosecutor’s Fallacy from the beginning. In a trial the Prosecutor’s hypothesis is that the accused is in fact the perpetrator of the crime. All evidence is interpreted to support that claim. And of course, the defense makes its case presenting evidence why the defendant is not the perpetrator.
IPCC was formed on the hypothesis that humans are making the earth warmer, and dangerously so. All the reports, especially the summaries by policy makers interpret evidence to support that claim. However, science proceeds by skepticism, ie.examining the evidence to determine the probability of the null hypothesis, in this case that humans are not to blame. As Fenton points out in his paper, the only evidence for human causation was that climate models only replicate the past temperature record when CO2 warming is included. They said, there is only a 5% chance humans didn’t cause the observed warming.
But all the other perpetrators have been ignored, explained away rather than examined. The climate models only tell what they have been told, and CO2 is the required driver of warming, excluding other factors. And anyone raising questions about other factors is labeled a “denier.” This was the genius move by climatists: to make their claim the established belief, rather than the null hypothesis that it had always been. Well, since we stopped making human and animal sacrifices to appease the gods.