States, transnational networks of scientists, corporate actors, and institutions in the climate change regime have known for decades that iron ore, when dumped in the ocean, can stimulate the growth of plankton. Over the past twenty years, normative disagreements about appropriate behavior have shaped international governance of the phenomenon. Prior to 2007, firms lobbied governments to treat the oceans as a carbon sink and to allow corporations that dumped iron to sell carbon credits on the international market. However, after 2007 a transnational coalition of oceanographers and advocates opposed this agenda by linking it to an emergent antigeoengineering discourse. Crucial to their efforts was their interpretation of uncertainty: for opponents, scientific uncertainty implied possibly devastating consequences of iron dumping, which was thus best addressed with extreme caution. This normative approach ultimately shaped governance, since advocates successfully used it to lobby institutions in ocean governance to prevent carbon credits from being issued for ocean fertilization. Since these subjective understandings of certainty influenced global ocean governance, this article explains international behavior as a consequence of changing norms.