Comments on the model simulations

We have presented highly idealized numerical simulations of extremely simple climate engineering involving spatially uniform reductions of incoming sunlight at the top of the atmosphere over specified northern polar regions and the entire Earth. Commonly discussed schemes, such as the release of sunlight scattering particles into the stratosphere (i.e. above the tropopause, where rain-out does not limit the residence time of such particles) might be expected to produce similar climate responses, but this expectation has not yet been carefully evaluated.

We start by asking what sort of climate one might like to achieve and then investigate what sorts of radiative forcing change might be needed to produce that climate. Thus, our simulations consider idealized patterns of radiative forcing change. This is in contrast to complementary approaches (e.g. Rasch et al. 2008) in which one simulates a release of sunlight scattering particles and then investigates what patterns of radiative forcing and climate are thereby produced.

We have used similarity to the simulated pre-industrial climate as a basic metric of performance of a climate engineering system. The pre-industrial climate may not be 'optimal' (in one or more senses) and one could imagine evaluating performance of climate engineering systems based on a variety of cost functions that do not involve reference to either present or past climates, but instead to some other conception of what would constitute a desirable climate.

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