The article focuses on the long-run relationship between public debt and environmental debt. The latter is defined as the difference between the "virgin state" which is the maximum stock of environmental quality that can be kept intact with natural regenerations and the current quality of the environment. A theoretical model of endogenous growth is built. We show that there is a unique well determined balanced-growth path along which the public debt-to-output ratio increases the environmental debt-to-output ratio. The basic mechanism is that unproductive public expenditures (debt burden) crowd out pollution abatement expenditures. Public debt and environmental debt are therefore complementary. This hypothesis is tested on a sample of 22 countries for the period 1990-2011. The environmental debt is measured by the cumulative CO2 emissions per capita. We use panel time-series estimators which allow for heterogeneity in the slope coefficients between countries. It appears mainly that, in the long term, an increase of 100% in public debt ratio leads to an increase of 74% in cumulative CO2 per capita. In addition, this positive long-run relationship is still present at the country and the sub-sample level, despite some differences in the short-term dynamics.