We analyze the impact and the determinants of a global carbon tax maximizing social welfare in an imperfectly integrated economy. Using a model of trade and location with two countries with different population size, we first show that agglomeration of firms in the larger country raises total emissions. Nevertheless, the introduction of a global carbon tax induces a partial relocation of firms from the larger to the smaller country. Thus, even though the carbon tax is identical in both countries, environmental taxation is not neutral for the location of economic activity. Finally, this partial relocation of firms in the smaller country improves the ability of the carbon tax to reduce total emissions.