Worst case runaway global warming, leading to a major disruption of the world climate - e.g. like the one in "The Day After Tomorrow". Non-stop wars for resources, with the use of chemical, biological and nuclear weapons. Complete global collapse of society and civilization.
One big problem with collapse are the 400+ commercial nuclear reactors in the north hemisphere. Without connection to a working electric grid and regular diesel deliveries those will go boom and will spread enough radioactive material to make most of the land on Earth deadly for humans.
Seriously, where do people get these ideas from ? This unfounded fear of nuclear fission is a huge reason why we're in this mess to begin with.