50 to 200 years.
Possible knock-out punches:
1) VIRUS
A virus or super-resistant bacteria will take out just over half our population. That's enough to make our heavy infrastructure collapse. Chaos followed by disease and starvation.
Genetic engineering will be plenty advanced in a few decades, such that any graduate student can design whatever virus they want and produce it at home, straight from their computerized blueprint. No missile-defense shield, smart bomb, or robotic tank can fight a war against that.
Because our industries are so inter-dependent on each other, it's like depending on a mono-culture: one thing fails and the whole house of cards gets wiped out. 6-10 billion people cannot be supported without heavily automated industy.
2) GLOBAL WINTER
A few nukes go off (Pakistan? N.Korea? China?) It wouldn't take much.
Heck, if the super-volcano under Yellowstone goes off, that'd be enough.
If somebody put a nuke down inside Old Faithful, what would it do?
3) ENVIRONMENTAL COLLAPSE.
Thousands of interdependent species suddenly go extinct because key resources
have gradually been lost. As a result, oxygen and CO2 levels get wacked, weather systems change,
and entire ecosystems start failing. Humans can still farm and produce
what they need, but it's like AIDS in the ecosystem.
With natural balances lost, it's easy for blight and plague to run rampant
and wipe out the few food sources that humans depend on.
http://www.courier-journal.com/foryourinfo/051903/051903.html
Quote:According to NOAA, nearly 27 percent of coral reefs are gone, and two-thirds more will be gone within 30 years if things don't change. Some marine biologists fear coral will become extinct in a few decades.
4) WEIRD SCIENCE
Some place like the Relativistic Heavy Ion Collider (RHIC), now operational at Long Island, New York, creates a black hole, strangelets, or a Quantum Vacuum Collapse.
http://armageddononline.tripod.com/science.htm