We analyze the effect of various combinations of state and national emissions policies on national emissions of a global pollutant, specifically, greenhouse gas emissions. We highlight the effect of unintended increases in out-of-state emissions on the efficacy of overlapping state policies. We show that emission taxes do not necessarily prevent a completely offsetting increase in out-of-state emissions when states add a state-level emissions tax to the national emissions tax. In particular, states small relative to their market will be unable to reduce national emissions with a state-level CO2 tax or a system of tradable permits. However, under a national cap-and-trade regime that allows states to be carved out, a state of any size can reduce national emissions by setting a tighter state cap. This combination yields a lower total cost than the equivalent combination of national and state CO2 taxes (if one exists) but increases the cost to consumers outside the market.