We are long overdue for a major asteroid or comet strike. That would pretty much end things right there. They don't call them the dinosaur killers for nothing.
That's why these people who propose experimenting (because that's what it is) with nukes are nuts. Purposefully lowering the earth's temp (if it is even possible) would result in mass extinction. Not just humans. It would kill of plant life, plankton, animals...everything.
Tough arguing with the mentally unstable, though. Not a recommended practice. We used to keep them locked up so they wouldn't hurt themselves and/or others. Today they're actors and actresses.