I picked overpopulation, because this is a first cause of many of the rest, such as pollution, global warming, etc.

If you want to worry, you've got to think about disease (not on your list). Particularly an airborne-contageous and deadly virus -- like an air spread HIV. A deadly antibiotic-reisistant bacterial pandemic (think antibotic-resistant plague) is another likely event.

Farmers have known for a long time that large monoculture ecosystems are extremely vulnerable to disease, because there is no biodiversity. We "control" that with nasty chemicals that we spray on crops or feed to livestock.

Nobody seems to discuss that human beings in large cities are essentially the same situation.

Jim