Global thermonuclear war within 7 years
It will start in the middle east. A rogue splinter cell of fundamentalists will assemble, place, and detonate a small tactical 2-stage tritium-boosted thermonuclear device and detonate it in Tel Aviv. Israeli intelligence will discover the bomb originated from Iran, and this will result in a full scale nuclear assault on Iran. This will immediately escalate and involve conventional, chemical & biological, and nuclear exchanges world-wide as world leaders decide where their alliances lie. 3.5 billion people will be killed outright, a further 1 billion will die within a year from radiation and biological effects. A nuclear winter will block out the sun's light and the temperature of the earth will plunge the world into an ice-zone. Crops and animals will all fail. The very earth will be polluted for 100,000 years.