Prof. Dr. Christoph Böhringer Chair of Economic Policy Department of Economics, University of OldenburgWe show that imposition of a state-level environmental tax in a federation crowds out preexisting federal taxes. We explain how this vertical fiscal externality can lead unilateral state-level environmental policy to generate a welfare gain in the implementing state, at the expense of other states. Using a computable general equilibrium model of the Canadian federation, we show that vertical fiscal externalities can be the major determinant of the welfare change following environmental policy implementation by a state government. Our numerical simulations indicate that – as a consequence of vertical fiscal externalities – state governments can reduce greenhouse gas emissions by over 20 percent without any net cost to themselves.