The timing of taxes on CO2 emissions when technological change is endogenous

How do technology spillovers affect the relationship between emissions taxes and technological change? Without spillovers, a regulator applies Pigovian taxes which lead to a first-best optimum (optimal emissions and optimal technology investment). Given spillovers, Pigovian taxes are likely to be second-best optimal if emissions-saving technology and production technology are equally undersupplied; raising taxes above the Pigovian level boosts emissions-saving investment, but only at the expense of production investment. The technologies are equally undersupplied when there is a degree of symmetry between the sectors, and the economy is on a balanced growth path. On a transition path with rising atmospheric stocks and a high level of investment in emissions-saving technology, a regulator may raise carbon taxes above the Pigovian level in order to encourage investment in emissions-saving technology at the expense of production technology. I show this using both analytical and numerical results.