The threats below are ranked by likelihood that they could happen versus consequences if they do. Keep in mind, we're ranking "apocalyptic" disasters, ones that have the potential to take out civilization or the human race.
Explanation of the Results
To qualify, the threat had to be potentially apocalyptic with strong evidence that it could actually happen (nuclear war, epidemics, impact craters, etc.). While invasion by angry aliens isn't impossible ... calculating the odds, is.
To rank the threats, we used Probabilistic Risk Analysis. This methodology is used by everyone from insurance companies to nuclear reactor designers. To determine consequences and probabilities, we used sources such as Scientific American and peer reviewed science papers. Estimating global disaster, however, is often educated guesswork and this is a very dynamic field. Changes in nuclear weapons proliferation, advances in astrophysics, computer science or medicine, can impact the results. We welcome comments, updates and corrections to our estimates. Download the Threat Calculation Spreadsheet.
- Large Asteroid/Comet Impact in the number one spot? It's just a silly movie plot, isn't it? The Russian meteor and several close calls with small asteroids, have provided a wake up call. Over the last two decades, we've found about 10,000 asteroids that cross earth's orbit - watch the video. Impressive until you realize that represents 1% of the possible earth-threatening asteroids. Additionally, we've only found a few thousand comets out of a population of cometary debris that measures in the billions. Since 1994, we've witnessed several asteroid and comet impacts on Jupiter, any of which would have been cataclysmic for life on earth. Recent studies (below) suggest that the probability of a comet impact may be hundreds of times higher than previously thought. Although the likelihood is still low (about the same as dying in a storm), even a small one, a kilometer across could kill hundreds of millions, carrying more destructive power than all the worlds nuclear weapons combined. A twelve-kilometer asteroid or comet (the size of Halley's) would have the same energy as a full-scale global nuclear war, REPEATED every day for fifty years. It would annihilate mankind and almost all life on earth. Despite its low probability, its apocalyptic consequences put it at the top of the chart. Predicting Space Impacts on Earth and Their Frequency; Extreme Albedo Comets and the Impact Hazard; Jupiter Impacts; Earth in the Cosmic Shooting Gallery.
- Artificial Intelligence is another surprise. This one has the greatest uncertainty and many will argue that there isn't enough information to rank it. We do know, however, that computer-processing power is doubling every eighteen month to two years. Extrapolating that growth, a computer will have more processing power than the human brain within the next 20 years or so, and may exceed the processing power of all of humanity by 2050. But there's a wildcard. We will soon have billions of smartphones. Although each only has the IQ of a lobster, they are all linked and becoming more linked, like the neurons in a human brain. Combined, their IQ would already far surpass ours. Will consciousness arise spontaneously? If it does, will it be benign or malevolent? We don't know. A "Terminator" future is unlikely, but what will happen when computers can out think us? NY Times: The Coming Superbrain; The Singularity Institute; Scientific American, Jun 2011: A Test for Consciousness.
- Global Nuclear or Biological War: As more countries develop a nuclear capability, the chance of a country or terrorist group using a nuclear weapon increases. A Scientific American article (below) estimates that a single detonation or small-scale nuclear exchange is a fifty-fifty probability over the next fifteen years. A full-scale global nuclear war, however, is much less likely since the end of the Cold War, with only a one-in-thirty chance over the next ten years. Despite arms reductions, there are enough warheads left to kill a half billion people, followed by another billion deaths from radiation, disease and starvation. Although the climactic impact would be global, isolated countries, like Australia, might be less vulnerable, allowing for survival of civilization. Biological warfare is much harder to define but would probably have similar probability and consequences to a pandemic (below). Scientific American, Sep 2010: Laying Odds on the Apocalypse; The Effects of Global Thermonuclear War.
A Pandemic is probably the least surprising on the list. With the rise of deadly viruses such as HIV and Ebola, and antibiotic resistant bacteria, such as MRSA, the likelihood of a global epidemic is very high (estimated at fifty/fifty over the next thirty years). Add the possibility of an accidental release of an engineered bug and the threat increases. Unlike in the movies, however, experts believe that a pandemic is unlikely to wipeout mankind or civilization, but the death toll could be horrific. Superbug Gene; Scientific American, Sep 2010: Laying Odds on the Apocalypse.
Solar Super Storms are a real threat. If the solar storm of 1859 struck today, it could take out the world's power grid for weeks or even months. Imagine what happens to our infrastructure with no power for a month. We can't prevent solar storms but we can protect our infrastructure from them - Racing For The Solar Superstorm.
Super Volcanoes are extremely dangerous but the probability is low over the next 100 years, and, currently, there's nothing we can do about them. Why didn't Global Warming make the top of the list? If all the world's ice sheets melt, it will be a twelve-meter sea level rise. Killer weather, such as increased hurricanes and droughts, along with rising sea level, would be catastrophic for coastal cities where a large percentage of the world's population lives. However, for the next 100 years, the International Arctic Monitoring and Assessment Program's worst-case sea level rise prediction, published May 2011, is 1.6 meters by 2100. This would be a disaster but as dire as the consequences might be, we have decades to do something about it or plan for the results, unlike comets, global nuclear war or artificial intelligence. Finally, a Nearby Gamma Ray Burst is deadly but extremely unlikely and there's nothing we can do. Sorry Zombie fans, maybe next year.