Robert Pindyck on climate models

Robert Pindyck has a recent NBER working paper that looks at one of the critical tools used in climate policy:
Climate Change Policy: What Do the Models Tell Us?
Robert S. Pindyck
NBER Working Paper No. 19244, July 2013 The abstract answers the question in the paper's title:

Very little. A plethora of integrated assessment models (IAMs) have been constructed and used to estimate the social cost of carbon (SCC) and evaluate alternative abatement policies. These models have crucial flaws that make them close to useless as tools for policy analysis: certain inputs (e.g. the discount rate) are arbitrary, but have huge effects on the SCC estimates the models produce; the models’ descriptions of the impact of climate change are completely ad hoc, with no theoretical or empirical foundation; and the models can tell us nothing about the most important driver of the SCC, the possibility of a catastrophic climate outcome. IAM-based analyses of climate policy create a perception of knowledge and precision, but that perception is illusory and misleading.