Climate change moved rapidly up the international political agenda between 1979 and 1988. What explains this shift? Existing explanations focus on how an international epistemic community built a scientific consensus that informed state interests by reducing uncertainty. However, in 1988 scientists actually heightened uncertainty about the future consequences of climate change by depicting it as a security threat “second only to a global nuclear war.” To account for this, I integrate insights from science and technology studies and securitization theory. In doing so, I theorize how scientists speak the grammar of security and construct existential threats. I argue that scientists catalyzed political action in the climate case by drawing on ideas about time, technology, and humanity's place in the universe. I conduct a discourse analysis of key scientific texts in the 1980s to uncover the frames and discourses scientists used to place climate change on the international political agenda.