Lecture, three hours; discussion, one hour. Designed for juniors/seniors. Global catastrophic risks pose challenge to modern civilization because of their superhuman extension in space, time, and knowledge realm. Their reach is global; their impact is long-term; and their comprehension is complex. Human interests and outlooks are local, regional, and national as well as egoistical, particularistic, and tribal. Overlapping generations spell intergenerational conflicts among living--young, middle-aged, and old--and between ancestors and descendants. Deeply and variably specialized experts struggle to communicate across scientific disciplines, across natural and human sciences, and across pure and applied sciences--only to hit brick wall in their communications with lay public, which is variously represented by elected politicians, appointed bureaucrats, organized interest groups, and fluid social movements. Study of ensuing clash of science. P/NP or letter grading.