This paper studies sustainability from both an economic and environmental perspective. We concentrate in two separate but interlinked environmental issues: the state of the world's forests and the stock of CO₂ in the atmosphere. These two issues are closely related to human economic activity. Our paper provides many insights on the different trade-offs faced to achieve sustainability and draws policy implications as to what is the path leading to sustainability in the long run. We study ustainability by means of a powerful tool: Viability Theory.