Climate Models Allow "the Modeler to Obtain Almost Any Desired Result"
Integrated assessment models (IAMs) are the commonly-used models that attempt to integrate climate science models with economic effect models. In the passage quoted below, "SCC" stands for "social cost of carbon."
(p. 870) I have argued that IAMs are of little or no value for evaluating alternative climate change policies and estimating the SCC. On the contrary, an IAM-based analysis suggests a level of knowledge and precision that is nonexistent, and allows the modeler to obtain almost any desired result because key inputs can be chosen arbitrarily.
As I have explained, the physical mechanisms that determine climate sensitivity involve crucial feedback loops, and the parameter values that determine the strength of those feedback loops are largely unknown. When it comes to the impact of climate change, we know even less. IAM damage functions are completely made up, with no theoretical or empirical foundation. They simply reflect common beliefs (which might be wrong) regarding the impact of 2º C or 3º C of warming, and can tell us nothing about what might happen if the temperature increases by 5º C or more. And yet those damage functions are taken seriously when IAMs are used to analyze climate policy. Finally, IAMs tell us nothing about the likelihood and nature of catastrophic outcomes, but it is just such outcomes that matter most for climate change policy. Probably the best we can do at this point is come up with plausible estimates for probabilities and possible impacts of catastrophic outcomes. Doing otherwise is to delude ourselves.
For the full article, see:
Pindyck, Robert S. "Climate Change Policy: What Do the Models Tell Us?" Journal of Economic Literature 51, no. 3 (Sept. 2013): 860-72.