I was looking at Nir Shaviv’s site Science Bits and came across an interesting paragraph.

According to the IPCC (AR4), the solar irradiance is responsible for a net radiative forcing increase between the Maunder Minimum and today of 0.12 W/m2 (0.06 to 0.60 at 90% confidence). We know however that the Maunder minimum was about 1°C colder (e.g., from direct temperature measurements of boreholes – e.g., this summary). This requires a global sensitivity of 1.0/0.12°C/(W/m2). Since doubling the CO2 is thought to induce a 3.8 W/m2 change in the radiative forcing, irradiance/climate correlations require a CO2 doubling temperature of ΔTx2 ~ 31°C !! Besides being at odds with other observations, any sensitivity larger than ΔTx2 ~ 10°C would cause the climate to be unconditionally unstable (see box here).
Clearly, the IPCC scientists don’t comprehend that their numbers add up to a totally inconsistent picture. Of course, the real story is that solar forcing, even just the irradiance change, is larger than the IPCC values.

Make sure that you look at the box at Ice Core Truth where he shows that any value for climate sensitivity above 10°C for a doubling of CO2 produces an unconditionally unstable climate. That is the crux of the matter.
Now for a bit of speculation. Why aren’t the alarmist numbers internally consistent? I suspect they are cooking the books.
And speaking of books. There is one on that very subject:
Red Hot Lies: How Global Warming Alarmists Use Threats, Fraud, and Deception to Keep You Misinformed
Cross Posted at Power and Control