Scott Sumner’s not officially in the Pigou Club – and for the record I’m officially not in the club.  Still, in a just world, Scott’s words of wisdom would provoke Pigovians to beg him to become their King:

If we set up this entirely rational system of pricing the externalities
contributing to global warming, the optimal carbon tax may fall almost
to zero.  The reason in one word: geoengineering.  If geoengineering
schemes costing $250 million can lower global temperatures by one
degree, you’d have companies like Exxon-Mobil building tubes to the
heavens in northern Alberta so fast your head would be spinning. 
Remember the flip side of taxes of $1,000,000,000,000 per degree of
warming is subsidies equally large for technologies that cool the Earth
by one degree.
  The success of geoengineering would dramatically reduce
the benefit from carbon reduction measures.  You say the tubes might
not work?  Maybe, but there are lots of other ideas as well.  Believe
me, with that sort of subsidy these firms would find a way to get the
sulfates up into the stratosphere. [emphasis mine]