Population growth has two potentially counteracting effects on pollution emissions: (i) more people imply more production and thereby more emissions, and (ii) more people imply a larger research capacity which might reduce the emission intensity of production, depending on the direction of research. This study investigates how to achieve a given climate goal in the presence of these two effects. A growth model featuring both directed technical change and population growth is developed. The model allows for simultaneous research in polluting and non-polluting technologies. Both analytical and numerical results indicate that population growth is a burden on the environment, even when all research efforts are directed toward non-polluting technologies. Thus, research subsidies alone cannot ensure environmental sustainability. Instead, the analysis highlights the importance of carbon taxes for climate change mitigation.