Assumptions, uncertainty, and catastrophic/existential risk: National risk assessments need improved methods and stakeholder engagement.

Two key shortcomings of national risk assessments (NRAs) are: (1) lack of justification and transparency around important foundational assumptions of the process, (2) omission of almost all the largest scale risks. Using a demonstration set of risks, we illustrate how NRA process assumptions around time horizon, discount rate, scenario choice, and decision rule impact on risk characterization and therefore any subsequent ranking. We then identify a neglected set of large-scale risks that are seldom included in NRAs, namely global catastrophic risks and existential threats to humanity. Under a highly conservative approach that considers only simple probability and impact metrics, the use of significant discount rates, and harms only to those currently alive at the time, we find these risks have likely salience far greater than their omission from national risk registers might suggest. We highlight the substantial uncertainty inherent in NRAs and argue that this is reason for more engagement with stakeholders and experts. Widespread engagement with an informed public and experts would legitimize key assumptions, encourage critique of knowledge, and ease shortcomings of NRAs. We advocate for a deliberative public tool that can support informed two-way communication between stakeholders and governments. We outline the first component of such a tool for communication and exploration of risks and assumptions. The most important factors for an "all hazards" approach to NRA are ensuring license for key assumptions and that all the salient risks are included before proceeding to ranking of risks and considering resource allocation and value.