Who has responsibility for addressing the security implications of climate change? States and the United Nations justify their existence on the promise of providing security. Yet, although the national and international security implications of climate change are increasingly acknowledged, incorporation of climate change in national security planning or institutional arrangements is far from universal, while debates in the UN Security Council about its role in addressing climate change have been characterized by contestation. This article examines key debates about the responsibilities these institutions have for providing security in the face of the threats posed by climate change, examining the extent to which these institutions accept responsibility for providing security in these contexts. Drawing on Toni Erskine's notion of institutional moral agency, the article examines a 2017 inquiry into the national security implications of climate change in Australia, and the September 2021 UN Security Council debate on the international security implications of climate change. These two case studies explicitly focus on the question of institutional responsibility – of the Australian Government and the UN Security Council respectively – for addressing the threat of climate change. In both cases these institutions stop short of accepting responsibility for providing security in the face of climate change, with limited policy responses or institutionalization as the result. With the security implications of climate change increasingly apparent, and increasingly recognized by these (and other) actors, the failure to accept responsibility raises potentially significant questions about the legitimacy of these institutions themselves. [ABSTRACT FROM AUTHOR]