We explain the spatial concentration of economic activity, in a model of economic geography, when the cost of environmental policy – which is increasing in the concentration of emissions – and an immobile production factor act as centrifugal forces, while positive knowledge spillovers and iceberg transportation costs act as centripetal forces. We study the agglomeration effects caused by trade-offs between centripetal and centrifugal forces. The above effects govern firms’ location decisions and as a result, they define the distribution of economic activity across space. We derive the rational expectations equilibrium and the social optimum, compare the outcomes and characterize the optimal spatial policies.