Asteroid Impact
Space objects strike the earth all the time, but extinction-level impacts occur only once every 100 million years. After the spectacular collision of the Showmaker-levy 9 comet with Jupiter (and a host of asteroid-disaster flicks) in the 1990s, NASA set out to map all large near-earth objects. But it appears that there are far fewer potential catastrophes in earths neighbourhood than once thought. "A civilisation-killing asteroid would have to be a mile across," says Spahr of the Minor Planet Center. (The space rock that ended the dinosaur era is estimated to have been six times that size.) "There just aren't any asteroids that size out there," he says. There is, however, a large population of as-yet-undiscovered objects several hundred yards across. One that we do know about, a 300-yard-wide asteroid called 99942 Apophis, will pass within the orbits of earth satellites in 2029 and could one day strike the planet. "Worst-case scenario?" Spahr says."You hit Los Angeles, kill millions of people, and shut down the entire West Coast."
Pandemic
For a disease to be globally destructive, it must undergo a flare-up of contagiousness and lethality like the 1918 influenza pandemic, which in the course of two and half years killed 50 to 80 million people. If the next influenza pandemic is as bad as 1918s, the equivalent toll would be 210 million."Knocking off that many people at once would disrupt civilisation,' the CDC's Khan says. He adds, however, that in the past century medical science has developed powerful weapons against disease."we're an intelligent species,"he says"We can fight back." But what if that intelligence were turned against us? Thanks to advances in biotechnology, it will become increasingly possible to custom-tailor a pathogens lethality. "we're on the cusp of what could be a frightening time," says Charles P. Blair, director of the Terrorism Analysis Project at the Federation of American Scientists."I think in the very near future you're talking about a potential extinction event.
Machines Take Over
Moore's law-the observation that computer chips get twice as powerful every two years-implies that, eventually, artificial brains will eclipse the human brain. The big question is, what will the artificial super-intelligence of the future choose to do with it's gifts? "The risk is not so much a Terminator scenario, where you get a super computer that dislikes humans,"says Anders Sandberg, a researcher and futurist at the Oxford Martin Schools Future of Humanity Institute in England."A malign neglect would be a bigger problem. You get something that's very intelligent but has motivations that are completely non-human. [The computer] might not really care about anything that we care about, but since its smarter, it's going to get what it wants."
Gamma-Ray Bursts
When large stars die, they go out in spectacular fashion. Having used up their nuclear fuel, their cores collapse inward into a black hole, which then devours the star inside out. Out of this paroxysm of destruction, powerful beams of energy burst from both poles, shooting gamma rays and charged particles that for a second outshine the rest of the stars in the universe combined. That's great for astronomers, who can observe the gamma-ray bursts, or GRBs, from across the universe, but not so good for any planet that happens to be located in the path of the beams. In a one-two punch, a bath of charged particles would quickly kill everything on one side of the planet while intense gamma rays would ionize the atmosphere and cause years of acid rain. "As a rule of thumb, the danger zone extends to anything within 3000 light-years," says Penn State astronomer Derek Fox, who specializes in gamma-ray bursts. But for us, he says,"it's not a likely threat." The average galaxy experiences a GRB only every 10 million years or so, and the danger zone is a small percentage of that galaxy.
Snowball Earth
Right now people worry about global warming, but fallout from a nuclear war or a super volcano could put enough sunlight-blocking dust in the air to cause the opposite problem: a deep plunge in surface temperatures. If the earth stayed cool long enough, a worse catastrophe could ensue. Back in the 60s, climate modelers realized that if the earth were covered in enough ice, most of the incoming solar radiation would be reflected back into space and the planet would settle into a stable state at about minus 50 degrees F. Then, in 1992, Cal-Tech Geobiologist Joseph Kirschvink proposed that the earth had once spent long stretches of time almost entirely frozen over, leaving evidence of glacial deposits in the tropics. Life clung on in a few sanctuaries heated by volcanic springs. Could it happen again?"Its not something you would need to worry about in 2012, or the next hundred years,"Kirschvink says."Even if the climate became very cold, it would take a long time for glaciers to build up."
Solar Storm
Late last year a major solar storm launched a wave of charged particles through the solar system at 4 million mph, setting the stage for a display of northern lights that could be seen as far as Arkansas. But while delightful to the eye, such a storm could someday herald a disaster. The earths magnetic field prevents the suns deadly particles from striking the surface. The motion of those particles, however, can induce strong currents on the ground. During the worst solar storm ever recorded, in 1859, the currents were so intense that telegraph lines burst into flames."If we had a storm like that today, it would be possibly quite catastrophic,"says Jeffrey Love, a geomagnetic researcher with the U.S Geological Survey. "Months without electricity could cause losses of trillions of dollars and basically wreck the economy."
Super Volcano
Two million years ago, a massive volcanic eruption near what is today Yellowstone National Park shot 600 cubic miles of dust and ash into the atmosphere, 2400 times more than Mount St. Helen's did in 1980. If such an eruption happened today, "it would greatly interrupt business as usual around the planet." Since that ancient blast, massive eruptions have been taking place every 600,000 years or so, and the last one was 640,000 years ago. On the bright side, the intervals between the Yellowstone volcano eruptions are extremely erratic. Statistically speaking, it's very unlikely to blow in 2012, or even within the next millenium.
Geomagnetic Reversal
Right now, the magnetic north pole is up near the rotational north pole, but this hasn't always been the case. Throughout the earths history, the north and south magnetic poles have swapped places, a phenomenon known as geomagnetic reversal. It happens irregularly, every 100,000 to 1 million years, and the last time they flipped was 780,000 years ago. So maybe we're due. Geophysicist J. Marvin Herndon has suggested that the reversal could cause the geomagnetic field to temporarily collapse, disrupting everything from power grids to gas pipelines to communication satellites. But there's also no need for immediate panic. While a flip would occur quickly on a geological time scale, it is far longer in human terms, between 1000 and 10,000 years. "Whether it's going to do us harm is an academic question," says Jeffrey Love of the U.S. Geological Survey, "because it's not going to happen tomorrow, and it's not going to happen in our lifetime.
Nuclear War
On Sept.26, 1983, a satellite-monitoring unit at a secret facility near Moscow received a warning: Five nuclear missiles had launched from a base in the U.S. Luckily, the unit's officer, Stanislav Yevgrafovich Petrov, was sceptical about the reliability of newly installed equipment, and he chose to wait rather than immediately pass along an alarm that might trigger a nuclear war. His judgement may have saved millions of lives. Nuclear tensions have subsided since the end of the Cold War. But the threat remains. More countries than ever have the bomb, and terrorist groups and rogue states remain a worry. A study published in 2008 by the journal Physics Today suggests that a regional war involving as few as 100 bombs could cause a nuclear winter, resulting in the lowest temperatures in 1000 years, while an exchange involving thousands of weapons would, the study concluded, "likely eliminate the majority of the human population". "Nuclear war is the near-term risk that people tend to forget about," says Sandberg of Oxford Martin. "If you think historically, we've probably been very lucky."
Artificial Black Hole
In 1945, a physicist working on the first atomic bomb raised a disturbing possibility: What if the energy released by the fissioning nuclei ignited the atmosphere and wiped out life on earth? Obviously, that didn't happen, and mankind survived its entry into the nuclear age. But the notion that physicists could unwittingly trigger a world-ending catastrophe has not gone away. In 1999, as the Brookhaven National Laboratory prepared to fire up its Relativistic Heavy Ion Collider (RHIC), a Hawaiian man named Walter Wagner filed a lawsuit to have the facility shutdown. He claimed that the collision of high-energy subatomic particles could spawn tiny black holes that could subsequently grow until they swallowed the earth. In more than a decade of operation the RHIC has not produced a black hole, but Wagner is currently warning of the same danger for Europe's Large Hadron Collider, which is generating yet higher energies. Mainstream physicists dismiss the threat. "This danger simply does not exist," says Brookhaven Lab physicist Dmitri Kharzeev. "These energies are high in human terms, but the cosmic rays that naturally occur in space are much more energetic. If high-energy particle collision could produce black holes, one would have swallowed us a long time ago."