More mathematically inclined readers will be interested in a discussion paper by Jonty Rougier, a colleague of Tamsin Edwards' at Bristol. I met Jonty while I was at the Met Office earlier in the year and found him very engaging as well as having a very sharp mind.
The paper is rather mathematical for me. It discusses the difficulties of getting a calibration from systems that are sensitive to initial conditions and where there is an attractor, describing the difficulties as almost intractable. It then goes on to list the further complications that are found in environmental systems and in particular in paleoclimate, and concludes:
When these additional complications are added to the intractability of palaeoclimate reconstruction (climate de finitely has sensitive dependence on initial conditions and an attractor), that enterprise must be seen as heroic in the extreme, and we must expect the uncertainties to be very large indeed. But, somewhat surprisingly, they are not; e.g., as shown in the celebrated hockey stick, which was used so much in the Third Assessment Report of the IPCC (Houghton et al., 2001); Montford (2010) provides a readable if slightly hair-raising account.