- Some results give a value of less than unity. For example, Schwartz gives a CS range of 0.6 - 1.6*C. In the lower section of his probabilities (0.6 - 0.99) this means that he can envisage that heating the planet by 1.2*C in any way will actually cool it. That needs some explanation and thought. Does cooling the planet likewise result in warming? How does that work? If CS is in the region of unity, there would be very little natural variation in climate, which contradicts the contrarians' constant assertion that recent observed climate change is not due to AGW, but to natural variation.
- The low results given by empirical observations depend very much on the time period under consideration. Most current observational studies are necessarily time limited, and therefore will not pick up on slow feedbacks. This might explain why palaeoclimate studies tend to return higher values, since they cover much longer time periods. Incidentally, Bishop Hill tells me that he discounts palaeolimatology because they tend to use computer models.
The excellent Tamsin Edwards (@flimsin) has a paper on this topic here.
- The blanket rejection of computer modelling by contrarians is irrational. Models deserve to be treated with caution, because they are only approximations to reality, but they do have a use. The global climate is a system, and systems need computers to represent their processes. The models are in essence composed of millions of calculations, which used, in the infancy of climatology to be carried out by hand. Would contrarians be happy if climatologists went back to doing each calculation by hand? Yes or no? If no, they must accept models. If yes, they are simply trying to put an end to climatology, in a very similar way to the Jesuit who tried to put an end to astronomy by refusing to look through Galileo's telescope.
- The final and most important point is to look at the actual physical reactions in the climate system, which I have set out here. I identify 8 positive feedbacks, and 3 negatives. Simple arithmetical accounting of their values points to a CS in the region of 3.
In conclusion, the contrarians' hypothesis, which is that the impact of man-made CO2 is small, is not borne out by observations.