This follows on to previous discussion of Manzi here. Manzi's original piece is at The New Republic.
First off, what are integrated assessment models?
The DICE model, developed by William Nordhaus, is a dynamic integrated model of climate change in which a single world producer-consumer makes choices between current consumption, investing in productive capital, and reducing emissions to slow climate change.(emphasis added)
... Current carbon emissions add to atmospheric concentrations via a fixed retention ratio, and realized temperature change is modeled by a three-box model representing the atmosphere, mixed-layer upper ocean, and deep ocean. Damage from climate change is a quadratic function of realized temperature change with a 3-degree change calibrated to cause a 1.3 percent world GNP loss.
Begging the question, wouldn't you say?
Jim Manzi bases his approach to the future of the world on the fact that IPCC WG II quotes the results of such models as showing modest impacts of climate change, but those modest impacts are built in. These are not simulations. These are nothing like the physics based models we have in physical climatology. These are guesses.
What does IPCC really have to say about them, other than the graph which Manzi ultimately references?
"It is likely that the globally aggregated figures from integrated assessment models underestimate climate costs because they do not include significant impacts that have not yet been monetised. It is virtually certain that aggregate estimates mask significant differences in impacts across sectors and across regions, countries, and locally. It is virtually certain that the real social cost of carbon and other greenhouse gases will rise over time; it is very likely that the rate of increase will be 2% to 4% per year. By 2080 it is likely that 1.1 to 3.2 billion people will be experiencing water scarcity; 200 to 600 million, hunger; 2 to 7 million more per year, coastal flooding."I am no great enthusiast for the AR4 WG II report, and they are not entirely free from responsibility for Manzi's grossly dangerous conclusion. But a fair reading of the executive summary ought to have been enough to dissuade Manzi from using the figure in question. The text which he advises the world to base its entire future is an egregious cherry pick.
Nobody who calls GCMs into question should pay the least attention to the Nordhaus type model, which is grossly underconstrained by theory or observation, and instead is constrained by guesswork.
Manzi takes a different tack, treating IPCC as authoritative. Thus he accepts or claims to accept the WG I sensitivity spectrum, and uses this as cover for picking a single chart out of WG II as representative of IPCC's impact assessment. But that choice is not representative of IPCC's impact assessment at all, and is explicitly disavowed in the chapter executive summary.
I hope Manzi will acknowledge his error and change his position.