This article is a non-technical review of the economics of global policy on reducing greenhouse gas emissions. Quite a lot is known about the likely physical consequences of anthropogenic climate change, but much uncertainty remains. In particular, account needs to be taken of possible catastrophes such as ice sheet melting. How are we to balance the known costs of taking action to reduce greenhouse gas emissions in the present against the uncertain benefits of such action for future generations? How convincing is the case for substantial measures to be undertaken now? If the case for such action is accepted, should emissions be controlled via Kyoto-style national emissions targets or by the imposition of carbon taxes? How can the challenges of burden sharing between developed and developing countries be addressed?