Eric L. Harry Guest Post–“How Does the World End? Let Me Count the Ways”
Before we get started, let’s define what we mean by “the end of the world.”
Extinguishing all life on earth would be difficult. Every day, 800 million viruses and tens of millions of bacteria rain down from the troposphere onto every square meter of the planet’s surface. Extremophile microorganisms flourish at unimaginable pressures in the depths of the Marianas Trench. Russian cosmonauts claim to have found plankton surviving the vacuum and radiation of space outside the International Space Station. Most disasters short of being devoured by a black hole would allow for survival of some remnant, self-replicating, DNA-based organism.
But the extinction just of our species would be easier. That would occur when the number of humans fell below the minimum viable population estimated at between 500 and 1,000 survivors (although we would need 5,000 to avoid inbreeding). For the purposes of this article, therefore, the end of the world means reducing the human population to 500 or fewer people.
With that rosy scenario as our predicate, what should we worry about and what should we not?
Pandemic (Worry)
Nature is a highly capable killer, so I chose a pandemic as the villain in my Pandora series of novels. (See also The Last Man, Earth Abides, and The Stand.) Combine environmental disruption releasing a previously dormant microbe, airborne transmission via mere breathing, pre-symptomatic infectivity turning fleeing refugees into unwitting carriers, and an extremely interconnected world spreading the pathogen at the speed of jet travel, and all but successfully isolated humans could be infected within a year.
How best to break down the cordons sanitaire to get at those last isolated communities? Raging mobs and cold-blooded sociopaths. In my Pandora series, I chose a neuro-invasive virus like rabies that alters behavior by causing brain damage, making victims aggressive, risk-taking, pain insensitive, obsessive, paranoid, and devoid of empathy. Technically, infecting 100% of humans is not extinction because there are survivors. But read any book or watch any movie about zombies (World War Z, 28 Days Later, The Walking Dead) and it’s clear that people so radically changed are no longer really humans, but Homo insapiens.
Nuclear War (Not To Worry)
Although many authors and filmmakers have tried to kill us off via nuclear war (On the Beach, The Day After, Alas Babylon, A Canticle For Leibowitz), those weapons simply are not destructive enough to do the job. Unrestricted nuclear war would kill sizeable percentages of the warring countries’ populations and reduce the survivors to a radiation-shortened life of foraging in a charred hellscape. But the good news is that our wide dispersion about the globe means the vast majority of humanity would survive, so no extinction here.
Astronomical Phenomena (Worry) 
Extinction by planetary collision is a low-probability/high-impact event (pardon the double entendre). Every day, 60 tons of meteoric material falls to the earth, with thousands of objects between the size of a grain of sand and a pebble leaving bright trails across the sky. Only about 500 meteorites per year impact the surface. Once a year on average, a 4-meter wide rock disintegrates in air in a 3-kiloton thermal explosion. Once a decade, a 10-meter-wide rock explodes at energies equivalent to five 15-kiloton Hiroshima bombs. The meteor that struck the atmosphere over Chelyabinsk, Russia, in 2013 was 20 meters wide and released 400-500 kilotons of energy (or 26-33 Hiroshima bombs). Once every 5,000 years, 100-meter wide objects strike the earth. That may have occurred over the Tunguska River in Siberia in 1908, when an object between 50 and 190 meters in diameter released between 10 and 15 megatons of energy (i.e., up to 1,000 Hiroshima bombs). And once every 440,000 years, kilometer-wide objects hit the earth. (Read Lucifer’s Hammer for how to survive that.)
The asteroid or comet that struck the Yucatan Peninsula 66 million years ago was between 11 and 81 kilometers in diameter, killed 75% of all species (including the dinosaurs), and released the energy equivalent of between 21 and 921 billion Hiroshima bombs. Even so, modern humans would probably avoid extinction from a repeat of that event. But 4.5 billion years ago, during the Late Heavy Bombardment, the Theia Impact by a Mars-sized object is theorized to have coughed up our moon in the Big Splash and would have extinguished all human life. (The aptly named 2011 movie Melancholia by Lars Van Trier, whose inspiration came to him, unsurprisingly, during a depressive episode, describes just such an extinction.)
Near-earth supernovae are another cause for concern. In the last 11 million years, there have been 20 within 1,000 light years from earth, each temporarily raising global temperatures an average of 5-7° F. Currently, there are six supernova candidates within that distance. To threaten species extinction, however, supernovae need to be close and/or large. The closest of the six—IK Pegasi—is a relatively safe 150 light years away and would be a smaller Type Ia supernova (up to 1.4 solar masses). The other five would all be larger Type II supernovae (between 8 and 50 solar masses), but range from 261 to 861 light years away. We are probably, therefore, safe from supernovae.
The last serious threat from above is a gamma ray burst—the most powerful force known to man—which is caused either by the collapse of a supermassive star or the collision of two neutron stars. It gives off the same amount of energy in milliseconds to hours as our sun emits over its entire 10-billion year life. Worse yet (or better, depending on which way it is oriented), that energy is believed to be beamed in two focused and opposite directions. Getting hit directly by such an intense jet of energy from anywhere in our galaxy would dangerously degrade our ozone layer and possibly trigger an ice age by enshrouding the planet in photochemical smog. Closer still and it could strip our planet of its atmosphere, killing all human life. We observe about one such burst per day somewhere in the universe, but so far none in our own galaxy. They should occur close enough to have some observable impact on the earth once every five million years or so. But on the bright side, that means life on this planet has dodged about a thousand such bullets to date.
Geophysical Phenomena (Not To Worry)
Death from below due to geophysical events or from our atmosphere simply cannot cause enough harm to threaten human extinction. Take for instance the threat posed by sea level rise due to global warming. Sea levels have risen 6-8 inches since 1900, and are projected to rise another 10-39 inches by 2100. But sea levels were 5,000 inches lower than today 20,000 years ago during the Last Glacial Maximum, and 5,000 inches higher than today 90 million years ago during the middle Cretaceous period. Global warming and sea level rise will cause widespread human suffering in low-lying areas and serious damage to our coastal infrastructure, but they will not threaten the extinction of humanity.
More threatening, in fact, may be the opposite: an ice age. The first major mass extinction on record (of five) was during the End Ordovician period 444 million years ago, when it is theorized that an upthrusting of the Appalachians exposed silicate rock, which scrubbed carbon from the air and triggered an ice age killing 86% of all species. And one of the worst of the five mass extinctions occurred 250 million years ago at the Permian-Triassic boundary, when 95% of all marine species were killed by global cooling. But how do we go from global warming to an ice age?
A volcanic winter is a real possibility. The Yellowstone Caldera has experienced three supereruptions in the past two million years. The last, 640,000 years ago, spewed 4,000 times as much volcanic ash into the air as Mt. St. Helens in 1980 and covered most of Wyoming, Montana, Idaho, and Colorado in three feet of splintered rock and glass. The gargantuan Lake Toba supereruption in Sumatra, Indonesia, a mere 72,000 years ago, was 2.8 times larger still and may have triggered massive acid rains, a 6-10 year winter (think Game of Thrones, but without the army of white walkers), up to a millennium of global cooling, and the last great planetary glaciation. That almost extinguished humanity, reducing our population to between 15,000 and 40,000 worldwide. But seven billion modern, technologically savvy humans today could survive even glaciation, though significantly reduced in number.
AI, Robotics and Nanotechnology (Not To Worry)
Might humans doom ourselves by inventing our evolutionary successors? (See the Terminator movies or read I, Robot.) Combining artificial intelligence that has acquired a survival instinct with robotics designed to fight our wars is much closer to reality now than in 1967 when Harlan Ellison wrote I Have No Mouth and I Must Scream. Self-replicating nanorobots capable of turning humans into gray goo by attacking us at the cellular level could one day mean it’s game over for humanity.
But unlike pathogens arising from nature or chance collisions with objects from space, we humans can choose not to destroy ourselves with technology. We can become Luddites (like by avoiding networked computers, as in the 2004 remake of the Battlestar Galactica series). We can build in safeguards or pull their plugs. (For how our silicon-based adversaries might retaliate by using nuclear blackmail, see the film Colossus: The Forbin Project.) And we can, of course, fight back. Given all the ways in which we can avoid extinction at the hands (or metal claws) of our currently docile inventions, I would lose no sleep worrying about robot uprisings.
Alien Invasion (Worry)
In 1950, physicist Enrico Fermi raised what has become known as the Fermi Paradox—we should have detected visits by aliens or their probes by now, so why haven’t we? Numerous explanations have been proposed. Life is rare. Or life does not necessarily develop intelligence or technology (think highly evolved dolphins). Or hazards like gamma ray bursters periodically cleanse life from whole regions of space. Or civilizations learn the galaxy is dangerous and hide (or don’t and perish). Or the longevity of advanced civilizations—the variable L in the Drake equation—is short, so no nearby societies survive long enough to overlap with our own. (Hence the opening Star Wars crawl, “A long time ago, in a galaxy far, far away…”)
Another possibility is that we routinely detect alien visits but have collectively chosen to ignore them. Buried amid thousands of UFO hoaxes may be real sightings. That explanation has risen in probability after release of videos of U.S. Navy encounters off San Diego in 2004 and off the east coast in 2015. But why ignore aliens hiding in plain sight? Avoiding the stigma of being branded a tin foil hat wearing nut is one reason. But it may also make practical sense. If aliens can travel here from another solar system, you can forget engagement with them on equal terms. And a hostile engagement would not be a war like in the movie Independence Day; it would be annihilation like in War of the Worlds, but without a human victory at the end. Officials would, therefore, be wise to use dismissive skepticism and willful blindness to avoid the confrontation that might be demanded following public announcement of aliens in our midst.
Why would alien visitors not simply land on the Mall and say, “Take me to your leader”? (See Klaatu in the movie The Day the Earth Stood Still.) Do geophysicists searching for oil in Angola pull their seismic boat to the banks of the Cubal River and try to explain their mission to local red-tailed monkeys? If there are alien UFOs, we should mind our own business and hope they mind theirs (whatever that might be) because, unlike our thermonuclear warheads, their weapons are not likely to be puny.
Eric L. Harry launched his Pandora series of science fiction thrillers with Pandora: Outbreak and continued it with Pandora: Contagion and Pandora: Resistance. Raised in the small town of Laurel, Mississippi, he graduated from the Marine Military Academy in Texas and studied Russian and Economics at Vanderbilt University, where he also earned a JD and MBA. In addition, he studied in Moscow and Leningrad in the USSR, and at the University of Virginia Law School. He began his legal career in private practice in Houston, negotiated complex multinational mergers and acquisitions around the world, and rose to be general counsel of a Fortune 500 company. He left to raise a private equity fund and co-found a successful oil company. His previous thrillers include Arc Light, Society of the Mind, Protect and Defend and Invasion. His books have been published in eight countries. He and his wife have three children and divide their time between Houston and San Diego.
