“Daly (sub. D107) has pointed to the work of Newell et al. 1989 and Lindzen 1990 for support of the notion that uncertainties in the interpretation of the modern observational record, and uncertainties in the physics of the feedback processes associated with clouds and atmospheric moisture by scientists in the IPCC scientific assessment process, has led to an overestimate of the warming associated with the enhanced greenhouse effect. On the other hand Greenpeace Australia (sub. D117) cites the work of Leggett 19913 for support of the notion that there are possible positive feedbacks, whose effects have not been factored into conventional climate models, but whose presence could mean that the IPCC’s predictions are substantial underestimates of the global warming to be expected from business-as-usual release of greenhouse gases.

Newell et al. 1989 focus on the degree to which the scientists who prepared the IPCC WG I report may have overstated the warming to be deduced from an analysis of the modern, instrumental record of near surface land temperature. Lindzen 1990 advances arguments that the damping effect of clouds on global warming from the enhanced greenhouse effect has been substantially underestimated. In testimony on 7 October 1991 to the US Senate Commerce, Science and Transportation Committee Lindzen somewhat modified his position, while continuing to argue that the current GCM modelling of the behaviour of water vapour in the atmosphere is unsatisfactory.

These failures to estimate the magnitude of the enhanced greenhouse effect would also be expected to manifest themselves as errors in the rate of warming, though the complicating factors associated with the time delays being caused by the response of the oceans, and by natural climate variability, means that it is not a simple process to use currently available climate observations to readily choose between the hypotheses. The implications of overestimation and underestimation by the IPCC would not be symmetrical. If the IPCC range represents an overestimate of warming due to an enhanced greenhouse effect, acting on the basis of it would essentially involve incurring unnecessarily large costs associated with reducing emissions.”

Originally published here by the Australian Industry Commission (IC) now Productivity Commission (PC) on 15 November 1991.