Humankind is introducing entirely new kinds of existential risk. Consideration of specific existential risk scenarios supports the notion that the great bulk of threats in the foreseeable future will consist of anthropogenic existential risks especially in light of the international nature of many of the required countermeasures and the shared stakeholdership of all future generations. There will be some types of putative existential risks for which the main uncertainty is normative and others where the main uncertainty is descriptive, but in either case, if something is not known to be objectively safe, it should be assumed to be risky, at least in the subjective sense relevant to decision-making. A reactive approach to existential risk to observe what happens, limit damages, and then implement improved mechanisms to reduce the probability of a repeat occurrence does not work when there is no opportunity to learn from failure. The existing paradigm of social and economic development based on economic growth has remained largely oblivious to the risk of human-induced environmental disasters at planetary and continental scales.