How is Registration like Global Warming?

Registration for fall courses is upon us, and as always some students are dragging their feet meeting with their advisers and signing up for classes. Beyond getting a seat in each of the classes you want, there is a larger reason to take this process seriously: the Dean often cancels smaller classes on the basis of those registration numbers. This means that the aggregate consequences of students' individual choices adversely affect the curriculum, at everyone's expense. If, say, 20% don't bother to register because they feel sure they'll get into the classes they want, the result is that some of those classes may not be there in September, which serves no-one's interest.