1. Nuclear War. Post 1980s, the West has experienced a detente of tensions with most nuclear powers. However, the recent grudge match with North Korea has brought the possibility of a genuine nuclear to the forefront. The Doomsday clock stands at two and a half minutes to midnight – not terribly comforting. Physics Today published an article saying as few as 100 nuclear weapons would bring about a nuclear winter and likely end humanity.
2. Asteroid Impact. While objects fall from space to earth every day, most of them burn up once they hit our atmosphere or fall in places nobody notices. But an asteroid big enough to wipe out earth could happen – but it would have to be massive: about a mile across. It would release the energy of several million nuclear weapons, instantly wiping out everyone but Keith Richards and cockroaches. Scientists estimate this only happens once every ten million years or so. But the scary thing? Nobody knows when the last one was, so … yeah, it could be tomorrow.
3. Biological Warfare. Humanity has been fortunate so far. There have been no large-scale attacks with chemical weapons. An attack of anthrax, in particular, would be devastating. Aerosol particles of 1.5 to 5 microns could kill 90 percent of the population. But organic disease is just as dangerous: just as the plague wiped out a huge portion of Europe, an avian flu could kill half the world’s population. If terrorists are ever able to get their hands on weaponized bio-weapons, life on earth will significantly change.
4. Solar Storm. The sun is constantly spewing solar eruptions to earth, like x-rays, charged particles, and magnetised plasma, which usually result in nothing more than radio disruptions and power outages. More severe consequences include an increase in the risk of cancer. But a big enough blast could destroy entire cities, wiping out power grids and communication networks that could cost $20 trillion in damages and leaving us vulnerable to second-wave attacks. Welcome to your instant Blade Runner universe.
5. Methane Burst from the Seabed. Yes, a galactic fart could end us all. It will be the least glamorous way to go, but some scientists believe the most likely. The theory is that methane gas slowly builds over millions of years, causing climate change that destroys life as we know it. Temperatures would rise, melting ice caps, flooding some cities while rendering others desert wasteland. The result is a world where humanity cannot survive.