This paper examines how offshoring shapes emissions leakage and global emissions in response to unilateral environmental policy. We use a general equilibrium offshoring model with heterogeneous firms, based on standard modeling assumptions, where firms allocate labor between production tasks and emissions abatement. We find that global emissions respond non-monotonically to a unilateral emissions tax increase: for small cross-country tax differentials, emissions fall; but as the difference widens, leakage exceeds 100%, raising global emissions due to a global technique effect. The cleanest domestic firms start offshoring and incumbent offshoring firms become dirtier under declining effective foreign taxes. We isolate the offshoring margin – one underexplored channel of leakage – and contrast our findings with comparable models of trade in final goods: the mode of globalization matters. Complementing the unilateral reform with a border carbon adjustment (BCA) prevents emissions leakage but may raise inequality between countries.