Lomborg’s thesis is built on a deep misconception of the Earth’s system and the economics when applied to that system...If there is one truth about the Earth system we should all know, it’s that [it] is driven by interlocking, non-linear processes running at different speeds…We have no data on the consequences if Earth were to cross those tipping points. They could be good, or they could be disastrous. Even if we did have data, they would probably be of little value because nature’s processes are irreversible. One implication of the Earth’s system’s deep non-linearities is that estimates of climactic parameters based on observations from the recent past are unreliable for making forecasts about the state of the world at CO2 concentrations of 560ppm or higher.-- from Partha Dasgupta's review (in Nature, Vol 449, 13 Sep) of Cool It by Bjørn Lomborg.
…[but] the integrated assessment models of Earth’s system on which Lomborg builds his case are arbitrarily bounded on either side of his point estimates. It can be shown that if those bounds are removed (as they ought to be), even a small amount of uncertainty – when allied to a moderate aversion to uncertainty – would imply that humanity should spend substantial amounts on insurance, even more than the 1 – 2% that has been advocated. If the uncertainties are not small, standard cost-benefit analysis as applied to the economics of climate change becomes incoherent, even it those uncertainties are judged to be thin-tailed (gaussian, for example)…
Economics helps us realise what we are able to say about matters that will reveal themselves only in the distant future. Simultaneously, it helps us realise the limits of what we are able to say. That, too, is worth knowing, for limits on what are able to say are not a reason for inaction.
Dasgputa writes that Lomborg advocates allowing atmospheric concentrations to rise to 560ppm, on the assumption that this will cause a rise in global average temperature to increase by 4.7 degrees C.
[P.S. 17 Sep: Kevin Watkins, whose review will appear shortly in Prospect, says Lomborg is talking about a global average temperature rise of 4.7 degrees F - i.e. approx 2.16 C - for a doubling of CO2. So it looks as if there was a typo in the Nature review.]