A simple way to talk about complex issues.We start with the definition of the Scientific Method. This particular version belongs to Richard Feynman (1918-1988): "In general we look for a new law by the following process. First we guess it. Then we compute the consequences of the guess to see what would be implied if this law that we guessed is right. Then we compare the result of the computation to nature, with experiment or experience; compare it directly with observation, to see if it works. If it disagrees with experiment it is wrong." An important consequence of this definition is that a single piece of contradicting evidence is sufficient for a hypothesis to be rejected, on the other hand, no amount of non-contradicting evidence may confirm or prove a hypothesis. Indeed, there always may be alternative hypotheses, some of them completely unknown to us, due to limitations of both our current knowledge and our thinking.
Let us now assume, as a hypothesis, calling it Hypothesis A, that the IPCC climate models, which attribute the recent warming of climate to anthropogenic emissions of carbon dioxide, correctly describe the properties of our terrestrial climate system. To reject this hypothesis, we need at least one piece of contradicting experimental evidence. Such evidence had been provided in the paper by R. Lindzen and Y.-S. Choi, "On the determination of climate feedbacks from ERBE data", Geophys. Res. Lett. vol. 36, pp. L16705, 2009.
The above Figure from their paper (their Fig. 2) compares the experimental response of the climate system (the amount of infra-red and visible radiation escaping from Earth to outer space) to the increase in surface temperature, as measured by the ERBE satellite, the upper left plot (marked ERBE), to the responses of the eleven different IPCC climate models (the remaining plots, marked by abbreviations of the respective models). As we see, all of the models respond in the wrong way, opposite to the real climate system. We have to conclude that our Hypothesis A is incorrect; in other words, the IPCC climate models do not describe the climate system in an appropriate way. One piece of evidence we have shown is sufficient for the conclusion we made, the conclusion being necessary and inevitable consequence of this piece of evidence.
Let us now assume a Hypothesis B, namely, that the IPCC models are able to predict future climate. As we already saw, the models that attribute the recent warming to the effect of greenhouse gases, in particular carbon dioxide, do not appropriately describe the properties of the real terrestrial climate system. Therefore, we can't expect any reasonable predictions from such models, and our Hypothesis B has to be rejected, for the same reason that we rejected Hypothesis A. This time, contradictory evidence is provided by the fact that the models don't even describe the currently existing climate, therefore, without a built-in understanding of how the climate works, the models can't be expected to predict how it will evolve in future.
Our Hypothesis C, which we shall also test, is that IPCC during the 25 years of its existence has been doing something useful. The rejection of the above Hypotheses A and B demonstrates that Hypothesis C is wrong as well. Indeed, the IPCC Climate Predictions and Scenarios are based on their models, which are basically worthless. The IPCC Reports, at least in their part that describes future climate, should therefore be classified as pseudoscience, and expressly neglected in any serious discussion of future climate.
As a side note, IPCC is obviously far too expensive for the scientific results it is producing, to say the very least.