FIGURE 5.10 Efficient abatement policies as a function of threshold specific damages (Keller, 2000). The efficient carbon abatement policies are derived in an “optimal economic growth model.” The figure shows abatement rate contours (reduction rate as a percent of baseline) as a function of time on the horizontal axis and damages from a shutdown of the thermohaline circulation as a percent of world output on the vertical axis. The economic model is based on Nordhaus (1994), modified to allow for an ocean thermohaline circulation collapse. Ocean modeling studies suggest that the thermohaline circulation may shut down when the equivalent CO2 concentration rises above a critical value (Stocker and Schmittner, 1997). The model assumes a CO2-temperature sensitivity of 3.6°C for CO2 doubling and neglects parameter uncertainty. (See Keller, 2000 and Keller et al., 2001 for details.)

curred, then abatement only occurs from the non-THCC part of climate damages.

These first efforts to apply integrated-assessment economic models to abrupt climate change are useful in suggesting how abrupt climate change might influence our thinking about the nature of policies to slow climate change. At the same time, attention needs to be paid to other shocks. In one sense, the THCC is an extremely nasty climatic outcome but, as modeled to



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