So if geoengineering works, how much does that reduce any claimed externality from carbon emissions?

Reducing the direct sunlight reaching the ground by 20 percent seems like a pretty large effect to me.  Will the claimed necessity of a carbon tax be adjusted for this possibility?  Personally, I think that warmer weather on net produces more good than bad.  From New Scientist:

Blue skies would fade to hazy white if geoengineers inject light-scattering aerosols into the upper atmosphere to offset global warming. Critics have already warned that this might happen, but now the effect has been quantified.
Releasing sulphate aerosols high in the atmosphere should in theory reduce global temperatures by reflecting a small percentage of the incoming sunlight away from the Earth. However, the extra particles would also scatter more of the remaining light into the atmosphere. This would reduce by 20 per cent the amount of sunlight that takes a direct route to the ground, and it would increase levels of softer, diffuse scattered light, says Ben Kravitz of the Carnegie Institution for Science in Stanford, California. . . . .

This article in Climatic Change (2006) indicates that this effect can be achieved at a cost of $25 to $50 billion.  

This can be achieved by a continuous deployment of about 1–2 Tg S per year for a total price of US $25–50 billion . . . . 

Compare that to the carbon taxes being proposed for just the United States.  Here is a proposal by Obama that talked about the tax revenue amounting to $80 billion a year.  I have no estimate of the deadweight loss.  From the WSJ:

The cost of energy for consumers would be driven higher in President Barack Obama's proposed budget by a carbon cap-and-trade system that is projected to raise about $80 billion a year starting in 2012. . . .The budget projects raising $645 billion from the auction of emissions credits between 2012, when the system kicks in, and 2019. . . .

The US would probably find it preferable to put the sulphur in the atmosphere all on its own. 


More information is available here.