The significantly divergent trajectories of terrorism and climate change as security issues for Western states in the early years of the 21st century represent a puzzle. While sharing some attributes – uncertainty and the primacy of risk-management responses – climate change clearly represents a more fundamental threat to life than terrorism. Despite this, terrorism has occupied a prominent place on states’ security agendas, while climate change has been decidedly marginal. This paper explores this divergence. Employing the securitisation framework, the paper maps the approach to terrorism and climate change as ‘security’ issues among key proponents of the ‘war on terror’, before exploring why these two issues were treated in such different ways. This analysis suggests a clear inclination to define and approach terrorism as an urgent security threat necessitating emergency measures: a willingness not evident in the case of climate change. While noting elements of the latter that militated against its securitisation, the paper points to the role of ideology – the beliefs and commitments of political leaders in particular – in driving choices around the construction of the security agenda. It concludes by suggesting that unlike the response to terrorism, impediments to enacting emergency measures to address the climate crisis remain.