We propose an emission cap adjustment policy for an emission trading system (ETS) that allows banking of allowances for later use and faces business cycle uncertainty. The forward looking banking decisions cause a commitment problem with respect to regulator’s next period allowance supply decision. We tackle the commitment problem by focusing on the Markov equilibrium of the policy. The equilibrium policy has a surprisingly simple structure, combining features from both price and quantity instruments. The regulator’s ability to choose a new cap for each period does not render banking obsolete, but on the contrary, makes banking an integral part of the equilibrium policy. Using data from the EU ETS to calibrate the model, we solve the Markov equilibrium policy for the case of carbon emissions regulation and compare Markov policies with and without the possibility to bank allowances. A counterfactual analysis of the EU ETS, during the period of 2009–2013, shows that the optimal Markov policy would have led to a steadier allowance price level and a stricter emission regulation than what was actually observed in the EU ETS.