In recent years, it became widely accepted to depict climate change as a security threat. A link is made between climate change and security in two different ways - in political debate as well as in the academic literature. 1) The causal-military one keeps de facto the traditional concept of security, defined in terms of state and war, but focuses on possible causal chains from climate change to violent conflict. 2) The per se type of linking argues that climate change as such is a security issue. This is in line with widening proper in security studies: more kinds of threats count as security. In this perspective, the combined effects of climate change on livelihood, future economic conditions, and the conflicts in type 1 all sum up to climate change being a threat. Viewed from securitization theory, this leads to three puzzles: 1) Why does this happen now, when previous suggestions for 'environmental security' were ultimately rejected not only by the security experts but more importantly by leading intellectuals among environmentalists. 2) What difference does it make? How will it influence the politics of climate change if the issue has been securitized? 3) How? If climate change is to be securitized, on what terms and in what format would it be most likely to contribute constructively to political management in a form that is both efficient in relation to the climate and tolerable politically and economically? Along the way, the articles also presents the theory of securitization and discusses some problems for the theory that arise in the case of ‘climate security'. The article ends with a discussion of the possibility that securitization becomes part of an important component in a transformation of the international institutional structure for climate policy.