This paper investigates the relationship between public debt and environmental debt—reflecting CO carbon concentration. First, using an endogenous growth model in which pollution abatement spending can be financed by public debt, we show that public debt and environmental debt are complementary in the long run and usually substitutes in the short run. Second, these predictions are empirically confirmed: in particular, a 1% increase in the public debt ratio leads to an increase of 0.74% in cumulative CO per capita in the long run. Our findings emphasize the difficulty of defining policies that jointly serve both the economic (fiscal) and the environmental goals, due to the short- and long-run conflicting environmental effects of policies that either reduce or do not constrain public debt.