Fertilization of the oceans to promote the growth of CO₂-binding algae has so far been the only geo-engineering measure which is subject to internationally binding governance. According to the precautionary principle, however, all geo-engineering activities which bear transnational risks should be banned worldwide. This includes, for example, the spread of sulphur compounds in the stratosphere to block out sunlight. Exceptions should only be made for the purposes of research which is coordinated at international level. The Convention on Biodiversity could serve as the framework for the exchange of information and the development of guidelines.