We consider the socially optimal use of solar  geoengineering to manage
climate change and its  implications for carbon emissions abatement
policy.  We show that solar geoengineering is a substitute for
emissions abatement; optimal policy includes less abatement, by up to
eight percentage points, and has a lower carbon price, by up to fifteen
percent, than  recommended by models that ignore solar  geoengineering.
However, it is an imperfect  substitute, since it reduces temperature
without  reducing atmospheric or ocean carbon  concentrations. Carbon
concentrations are higher but temperature is lower when allowing for
solar  geoengineering. Ignoring geoengineering in climate  models can
lead to welfare losses of up to 4 percent  of GDP. Uncertainty over
climate sensitivity leads to  more abatement and solar geoengineering,
while  uncertainty over solar geoengineering damages leads to less
geoengineering.

We consider the socially optimal use of solar  geoengineering to manage climate change and its  implications for carbon emissions abatement policy.  We show that solar geoengineering is a substitute for  emissions abatement; optimal policy includes less abatement, by up to eight percentage points, and has a lower carbon price, by up to fifteen percent, than  recommended by models that ignore solar  geoengineering. However, it is an imperfect  substitute, since it reduces temperature without  reducing atmospheric or ocean carbon  concentrations. Carbon concentrations are higher but temperature is lower when allowing for solar  geoengineering. Ignoring geoengineering in climate  models can lead to welfare losses of up to 4 percent  of GDP. Uncertainty over climate sensitivity leads to  more abatement and solar geoengineering, while  uncertainty over solar geoengineering damages leads to less geoengineering.