Global catastrophic and existential risks hold the potential to threaten human civilization. Addressing these risks is crucial for ensuring humans' long-term survival and flourishing. Motivated by the gravity of these risks, Congress passed the Global Catastrophic Risk Management Act in 2022, which requires that the Secretary of Homeland Security and the administrator of the Federal Emergency Management Agency coordinate an assessment of global catastrophic risk related to a set of threats and hazards. The U.S. Department of Homeland Security Science and Technology Directorate and the Federal Emergency Management Agency requested the Homeland Security Operational Analysis Center's support in meeting this requirement. The authors of this report document findings from the resulting analysis. This report summarizes what is known about the risks associated with six threats and hazards: artificial intelligence; asteroid and comet impacts; sudden and severe changes to Earth's climate; nuclear war; severe pandemics, whether resulting from naturally occurring events or from synthetic biology; and supervolcanoes. The risk summaries cover the following aspects: where feasible, estimates of the likelihood and potential consequences of each risk; factors causing the risk and associated uncertainties; and whether the risk is likely to change in the next decade. Because the broader goal of the Global Catastrophic Risk Management Act is to reduce risk to human civilization, the authors also identified known and potential mitigation strategies for the six threats and hazards and drew insights from the assessment relevant to managing the risks they pose to society.