It is unlikely that even geological boundaries will apply to humans in the future.

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NINE

SCENARIOS
OF HUMAN
EXTINCTION

Will There be an "After Man"?

On May 23, 1995, academician Boris Pevnitsky, Deputy Director of the Russian Federal Nuclear Center, strode up to a small podium in a cramped, steamy room in Livermore, California. His audience consisted of an attentive, cosmopolitan group of middle-aged men: some in sleek Nordstrom�s suits, some in the more dingy variety available at GUM in Moscow, others in United States Air Force uniforms. Dr. Pevnitsky�s message was simple: he proposed that the United States and Russia jointly build the

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largest hydrogen bomb ever conceived, a bomb so great in energy yield that its explosion anywhere on Earth would blow away a significant portion of the atmosphere. But that was Dr. Pevnitsky�s point. He didn�t intend to explode his �classical super� anywhere near the Earth. He intended to use it to destroy an asteroid in space. In the back of the room, a small, shriveled man with graying but still fierce eyebrows looked on, surely with some satisfaction. Dr. Edward Teller, inventor of the H-bomb, must have been pleased to hear that his great dream � the �peaceful� use of thermonuclear weapons � was at last being discussed by an international assemblage of American, Russian, and Chinese nuclear specialists, astronomers, and geologists, 150 strong, in an open forum. All had gathered in the slightly run-down conference center of the Lawrence Livermore Laboratory to discuss one topic: the defense of the Earth from comet or asteroid bombardment.
The Lawrence Livermore meeting was the second such meeting held in a one-month span in 1995. The first, held at the United Nations in New York in late April of that year, had roughly the same theme, but was far more theoretical, dealing with the rate of asteroidal collisions, rather than how to defend against them. The megatonnage necessary to deflect or destroy an asteroid was not overtly discussed, but the message was the same: our planet, and with it every species, every individual, every great and tawdry work of humanity, is endangered by celestial happenstance. The magnitude of this risk remains a topic of utmost importance to our species, for many scientists believe that the greatest threat facing us is not some Hot Zone virus from Africa, but the billion or more comets and asteroids that have Earth-crossing or potentially Earth-crossing orbits. Should Halley�s comet, or some other messenger from the heavens of equal size, hit the Earth, it would bring about the complete destruction of all life on this planet.
Asteroid impact is not the only threat facing our species, only the single most dangerous one. Other threats to the viability of the human species exist as well, including other astronomical disasters (a gamma ray burst, a nearby supernova) as well as Earth-borne causes such as thermonuclear war, biological warfare, disease, and sudden climate change. While any of these disasters would severely reduce human populations, probably none by itself, other than the astronomical causes, is capable of driving all of humanity to extinction. However, several of these effects acting in tandem could do the job.

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The Sky Is Falling! The Sky Is Falling!

Despite its scientifically ludicrous depictions, Hollywood has nevertheless served to educate people about the dangers of Earth-crossing asteroids and comets. The overall message of the late-twentieth-century movies Armageddon and Deep Impact was grounded in reality. Asteroid impact can be viewed as the most dangerous single threat to our species� existence.
The rate of collision of celestial objects with the Earth has been well established, and the destruction wreaked by such impacts is also well understood. Such events have occurred countless times throughout the history of our planet � and they will inevitably recur in the future. For example, it was a great planetary collision nearly 4 billion years ago that created the Earth-moon system, and in so doing may have made our planet unique as a womb for the gestation and diversification of life. A second great collision some 65 million years ago slayed the planet�s dragons and set the stage for mammalian, and ultimately human, evolution. But of greatest relevance to our species are the future collisions, great and small, that will inevitably occur. For alone among the countless species that have populated this Earth, we have it in our power to defend the planet from these strikes.
Just how likely is it that a comet or asteroid collision might destroy our civilization? Is it more dangerous to arm ourselves with nuclear weapons larger and more destructive than any yet developed, ostensibly for planetary defense, than it is simply to pray?
While it is impossible to assign a precise number to the chance of an asteroid impact, we do know that significant hits have occurred as recently as 100 years ago. On June 30, 1908, a relatively small meteor exploded in the lower atmosphere over remote Siberia, releasing the force of a hundred Hiroshima-sized atomic bombs. The blast flattened miles of forest, and 50 miles away, reindeer herders and their stock were blown into the air. Had that same explosion happened over a heavily populated area, it would have produced one of the greatest episodes of human carnage in recorded history. The small fragment that produced this explosion was only about 50 meters across. Just two years ago, an asteroid a hundred times as large barely missed the Earth, and was seen only after it went by. Today, several hundred Earth-crossing asteroids of various sizes have been detected, and that seems to be only a small sampling of the thousands that orbit in the vicinity of Earth, not to

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mention the estimated billion comets hovering far out in space. If an object larger than a mile in diameter were to strike the Earth, not only would our civilization be threatened, but so too would our species. And as we saw with the impact of Comet Shoemaker-Levy 9 on Jupiter, the time between detection and impact for even a giant comet � a species-killing comet, had it struck Earth � can be less than a year.
The reality is that this planet will continue to be hit by debris from space. Just as with earthquakes, the question is not if, but when � and how big. Very large impacts � such as those that might bring about mass extinctions � seem to occur at intervals of tens to hundreds of millions of years. But the frequencies and ages of craters found on the surface of the Earth demonstrate that asteroids of up to a kilometer in diameter seem to hit the Earth at million-year frequencies. Such collisions could be expected to disrupt human agriculture on a global scale for many years following the impact, and would certainly lead to a great slaughter of humanity. The complete extinction of humanity might occur through the impact of a comet or asteroid greater than about 15 kilometers in diameter � half again as large as the one that ended the Mesozoic era (and killed off the dinosaurs in the process) some 65 million years ago.
Although the impact of a very large asteroid or comet could completely eliminate the human race, a more likely scenario is that some significant proportion of our population would be removed by the direct effects of such an event, and the rest of the job would be done by the aftereffects. Just how easily that could occur was shown by astronomer John Lewis of the University of Arizona in his 1999 book, Comet and Asteroid Impact Hazards on a Populated Earth. Lewis not only wrote about the dangers of such events, but included with his published book a software program that allows the reader to simulate such impacts.
Lewis�s simulation program uses a statistical analysis to calculate the human deaths resulting from an impact. Michael Paine, a scientist at the Jet Propulsion Laboratory at Pasadena, ran the program to simulate the effects of likely asteroid impacts on human populations over the next million years. His analysis led to sobering results. Assuming a constant population of 5 billion people on the Earth at any time, the total death toll was 7.5 billion people over a million-year time span � or 7,500 fatalities per year. But the impacts were not evenly distributed. Paine�s simulation yielded ten occurrences in which the impacting body was from one thousand yards to a mile in diameter, five in which it was a mile to 1.3 miles in diameter, and one in which the impacting body was greater than 1.3 miles in diameter. The latter event resulted in 2.5 billion deaths when a 1.3-mile comet hit the

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American Midwest, releasing the energy of 60,000 H-bombs. Seven million people were killed instantly, and the rest died of starvation as sunlight was blocked and crops failed.
If our species survives for a long period of time � as I believe it will � then we will become used to occasional devastating global events that � at a minimum � knock humanity back into the Stone Age for long periods of time. One such event in conjunction with another human-killing factor could indeed cause our extinction. Of course, the impact of an even larger asteroid or comet could do the job all by itself.

War and Thermonuclear War

Although it is doubtful that thermonuclear war alone could wipe out humanity, one scenario for human-induced extinction is a massive thermonuclear exchange, perhaps aided and abetted by chemical and biological warfare at the same time. While the causes of such warfare could come from many sources, the twenty-first century and beyond will probably see a variety of smaller wars fought over food, land, and water. Unto themselves these wars certainly pose no threat to the entire species � unless, of course, they escalate into a full-blown nuclear-chemical-biological exchange.
Human beings have proved to be a cantankerous lot, and unfortunately the efficiency of anti-human weaponry has markedly increased since our stone-throwing days. Prior to the twentieth century, war was an exercise between combatant armies. In that century, however, it spread from armies to noncombatant populations. William Eckhardt, in his �War Related Deaths Since 300 B.C.�, has estimated that the number of war deaths per 1,000 people in the total world population rose from 9.7 in 1700-1799 to 16 in 1800-1899 and to 44.4 during the twentieth century. This change was promoted by the twentieth century�s propensity for attacking an enemy nation�s economy, infrastructure, and civilian population. Total war deaths for the twentieth century totaled nearly 110 million humans, at least half of whom were civilians. Yet alarming as this rising death toll is, our extinction would require far more than 45 deaths per thousand.
The reasons for future conflicts are easy to discern. First, the need to supply food to a growing human population has caused a great increase in the need for arable land and for water to irrigate crops. New high-yield strains of rice and other grains, for example, require more water than less productive crops. Often, aquifers that have taken centuries or millennia to fill take only years or decades to discharge. One solution to water problems has been to build dams, but damming rivers that travel

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through more than a single country is a sure means of creating conflict. From 2000 to 2020 the worldwide demand for water is expected to double due to the need to irrigate new agricultural lands in dry areas, as well as the need to provide water for the growing human population and for industrial uses. There will also be renewed demand in dry countries such as Jordan, Israel, and Syria, where aquifers have been nearly emptied. Not surprisingly, in the late twentieth century, the World Bank prophesied that most twenty-first-century wars would be fought over water supplies.
Whatever its cause, war has been a plague on humanity as long as there has been human memory and legend. Prior to the latter half of the twentieth century, however, none of the myriad means of human warfare posed a threat to the species. But the unleashing of the atom and effusion reactions has changed all that. Today, humanity holds the seeds of its own destruction in its hands.
Following the explosion of the first atomic bomb in 1945, and the first thermonuclear (or hydrogen) bomb in the early 1950s, the stockpile of such weapons has grown alarmingly. The National Research Council estimated that the five major nuclear powers (the United States, Russia, Britain, France, and China) possessed nearly 70,000 nuclear weapons in the mid-1980s. This number had diminished by the latter part of the 1990s to a total of about 35,000 weapons, with about 23,000 of these deployed at about 90 sites in Russia. While a smattering of other weapons exist, notably in Israel, India, and Pakistan, it is the two superpowers that maintain the majority of the arsenal�s numbers. The total is expected to decline further, to about 6,500 active weapons for the United States and Russia combined by 2003 � roughly the same number present in 1969.
While the reduction of weapons systems has somewhat diminished the overall danger, the truth of the matter is that there are far more nuclear weapons still on line than would be necessary to exterminate humanity from the Earth � one instance in which the word �overkill� is not hyperbole. Scenarios of nuclear exchanges include vast devastation and radiation poisoning, with long-term aftereffects due to the lengthy half-lives of the radioactive materials released into the environment. In an article published in 1982, Paul Crutzen and John Birks pointed out a further peril, later dubbed �nuclear winter�. They suggested that multiple nuclear explosions could blacken the atmosphere with enough soot and dust to reduce sunlight by 99% for a period of 3 to 12 months, depending on the number, yield, and type of target of the exploding warheads. Such a cloud cover could reduce average global temperatures to well below freezing in the interiors of North America and Asia. Such temperature changes, while not nec-

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essarily dooming humanity, would certainly reduce agricultural crop yield to a trickle.
The concept of nuclear winter gained credence from studies of the terminal Cretaceous asteroid collision and from additional work by Carl Sagan and his colleagues. They showed that a full-scale nuclear exchange could indeed bring on a �nuclear winter�, which in turn could quite easily lead to the extinction of the human species.

Catastrophic Climate Change

Never in the history of life on Earth has there been an organism better adapted for climate change than the human species. The most serious environmental threat to most species is temperature change, but we can deal with that quite easily: if too hot, remove clothes, or install an air conditioner, or move to a cooler climate if necessary. Dealing with cold is even easier: put on warmer clothes, build a fire, install insulation: in short, let technology deal with the problem. No other animal has the ability to control its body temperature so quickly and easily. Thus temperature change is usually not a threat to our species� survival.
Or is it? One of the great scientific discoveries of the latter part of the twentieth century has been a new understanding of the rate of climate change in the past. It had long been assumed that major changes in the Earth�s climate were drawn-out events spanning great intervals of geologic time. Evidence to the contrary began to emerge when deep cores of ice drilled from the ancient ice sheets covering parts of Greenland and Antarctica were analyzed for their isotopic content. To everyone�s surprise, these long records of global climate revealed intervals of extremely rapid temperature change. The newly discovered data painted a much more dramatic picture of climate change: they showed that major changes could overtake the Earth in a decade or less. And the changes would not be limited to the high north or south � their effects would be global. Such rapid changes, if superimposed on the large human population and its present agricultural needs, would be a recipe for chaos and at least partial extinction of our species.
Somewhat paradoxically, it may be that global warming will trigger a rapid change involving a sudden cooling of the Earth. In a thoughtful article published in the Atlantic Monthly, William Calvin of the University of Washington outlined a scenario that, if it did not exactly drive our species into extinction, could certainly set up the social chaos that would lead to global war and the loss of significant portions of the human population in mere decades. Calvin called this scenario the �great climate flip-flop�.

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Calvin argues that catastrophe could come with a sudden cooling of Europe. At the present, Europe is anomalously warm for its latitude. Whereas most of the populous parts of North America lie at latitudes between about 30� and 45� N, most of the population of Europe is about ten degrees farther north: London and Paris are at nearly 50� N, Berlin at 52� N, Copenhagen and Moscow at 56� N, and the cities of Scandinavia at 60� N. Yet in spite of its more northerly location, the European subcontinent is extremely productive. Its agricultural industry supports twice the human population of North America on a much smaller land-mass. Much of its agricultural success comes from a climate warmer than its latitude might otherwise dictate. Europe�s warmth comes from the Gulf Stream, a tropical water current flowing up the Eastern Seaboard of North America and then vaulting across the Atlantic to push masses of warm water against the European landmass. It keeps northern Europe about 10� to 20� warmer than it otherwise would be.
The current bringing heat to Europe is not a single waterway, but is composed of several segments. One branch of this current carries warm water to the vicinity of Iceland and Norway. Eventually this water cools, and when it does, it sinks deeper into the ocean. It then returns to more southerly latitudes, but does so as a cool deepwater current, rather than the warm surface current it begins as. As it moves south it also carries more salt with it, for salt water is heavier than fresh water and tends to sink because of its greater density. Warmer, fresher water thus travels on the surface and returns at depth as cooler, saltier water. Paradoxically, this system would be shut down if more fresh water were added to it on the sea surface. Thus the movement of salt in the current system is integral to maintaining the steady supply of warm water to the coast of northern Europe.
The scenario that could lead to a failure of the warm northern Atlantic current is global warming. If the glacial ice covering Greenland were to melt at a higher rate than it currently does, it would flood the sea surface in the regions concerned with fresh water. The normal circulation pattern would thus be disrupted, causing the northern branch of the current to begin its return prior to reaching Greenland. The warm temperatures that these currents bring would not reach the shores of Europe, and Europe would suddenly cool. Thus a strange paradox: global warming would ultimately cause a sudden cooling of Europe.
The failure of one single current would, at first glance, not seem to be the stuff of sudden global climate change. Yet the world�s oceans are but a single body of water, and heat flow is global. The breakdown of any one current system necessar-

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ily causes changes in others. If the North Atlantic current failed, the entire world would experience sudden climate change. Europe would go into a deep freeze, and it seems likely that its human population would revert to warfare over the territory necessary to support the suddenly starving millions. It may sound alarmist and overly dramatic to talk about "suddenly starving millions," but it is important to remember that Europe currently has 650 million people and is largely self-sufficient in its food production. Almost simultaneously with the global current change, that ability to be self-sustaining would disappear. Calvin describes this scenario as follows:

Calvin makes the point that without its warming current, Europe would have a climate like that of present-day Canada, and if Europe had weather like Canada�s, it could feed only one of twenty-three of its inhabitants.
What makes a sudden global cooling especially threatening is that it is not a �point source� disaster. The Earth is regularly stricken with calamities such as hurricanes, tornadoes, and catastrophic earthquakes. These disasters cause the loss of much human life, and they are usually followed by rescue and recovery efforts that are often global in scale. But such disasters are always of short duration and of limited geographic extent. Neither hurricanes nor earthquakes perturb any significant percentage of the Earth�s surface for more than a few days. An abrupt cooling, on the other hand, could last decades or centuries. Calvin argues that even a meteor strike killing a majority of the human population in a short period of time would not be as catastrophic as a longer-term disaster that killed just as many � the killing effects of the meteor strike would soon be over, but global cooling would continue to stretch its deadly effects over decades at a minimum, and more likely over centuries.

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Disease

In the last years of the twentieth century great attention was centered on human communicable diseases, sparked by a spate of movies and best-selling books. What are the chances that a new disease could bring about the extinction of humanity? For instance, what if a 100% fatal disease such as HIV were spread as readily as the common cold? And what if such a disease were used as a weapon? Biological warfare, like nuclear warfare, does have the potential for a radical reduction of the human population if world war erupted. Most disturbing may be the stockpiling of diseases for which we no longer are vaccinated (e.g., smallpox), and the genetic engineering of new, virulent strains of disease, the subject of countless movie and book plots.
Two observations argue against the possibility of a species-ending epidemic. First, there is no evidence that any single disease has ever killed off any species. Second, humanity now has a technology that can combat disease with increasing efficiency each year. Nevertheless, disease remains a potent method of reducing human population numbers, and when combined with (or resulting from) other human killers in synergy, it could well be a potent force leading to the extinction of our species, especially if global warming causes tropical diseases to move into previously temperate regions.

Death from the Robots

It is difficult to arrive at any scenario of human extinction (or any scenario of anything, for that matter) that has not already been featured in some Hollywood movie. So too with our next potential villain, artificially constructed machine intelligence. In the famous Terminator and Terminator 2 movies (and to some extent in The Matrix as well), the near-future world is run by malevolent robots that are trying to exterminate the human species, or at least enslave it. Such a scenario seemed highly likely to Ted Kaczynski, the infamous Unabomber, who in his manifesto published by The New York Times and The Washington Post, wrote, �Let us postulate that the computer scientists succeed in developing intelligent machines that can do all things better than human beings can do them. ... If the machines are permitted to make all their own decisions . . . the fate of the human race would be at the mercy of the machines�.

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This view is also expressed by roboticist Hans Moravec, in his book Robot: Mere Machine to Transcendent Mind, and by Bill Joy, the co-founder and chief scientist of Sun Microsystems, in his chilling 2000 article Why the Future Doesn�t Need Us. As Moravec says, �In a completely free marketplace, superior robots would surely affect humans. . . . Unable to afford the necessities of life, biological humans would be squeezed out of existence�. Moravec foresees the fusion of a human being and a robotic body or being to produce a superintelligent hybrid of some sort (as always, Sci-Fi has been there, done that, the Borg from Star Trek being only the most recent entry into this genre). To Moravec, such a being would succeed humanity � and cause our extinction sooner or later.
Why robotics? Part of the promise is a better way of life for the organic makers � us. To do away with the mind-killing labor that bedevils most of humanity would indeed be a social and intellectual breakthrough. But robotics holds an even greater promise � the extension of our individual intellects, if not our bodies. If we can download our consciousness into a machine (and by machine I mean organically produced as well as purely inorganic artificial intelligence), we will indeed be on the verge of some sort of immortality. But at what cost? As Bill Joy notes, �If we are downloaded into our technology, what are the chances that we will thereafter be ourselves or even human? It seems to me far more likely that a robotic existence would not be like a human one in any sense that we understand, and that robots would in no sense be our children, that on this path our humanity may well be lost�.

Gray Goo

Of all threats facing humanity, perhaps none is so dangerous � or so poorly understood � as that posed by nanotechnology. Nano means �small�, and many technologists now see the future of technology as the manipulation and assembly of matter at molecular and even atomic size scales. Such molecular-level assemblers could utterly transform human society by creating extremely inexpensive products, medicines, and even energy through the construction of virtually free solar panels. Because most of the new products would be created from organic rather than metallic and other mineral material, there would be far less pollution and fewer other environmental consequences of manufacturing.
The future in such a world might be Utopian. On the other hand, it might mean the extinction of humankind. It is this vision that is most starkly illuminated in Bill Joy�s cautionary 2000 essay. Joy views nanotechnology as the most dangerous of

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the new trio of technologies, dubbed GNR, for Genetics, Nanotechnology, and Robotics. As he notes, �Molecular electronics � the new subfield of nanotechnology where individual molecules are circuit elements � should mature quickly and become enormously lucrative within this decade, causing a large incremental investment in all nanotechnologies. Unfortunately, as with nuclear technology, it is far easier to create destructive uses for nanotechnology than constructive ones. Nanotechnology has clear military and terrorist uses�.
Joy sees the military applications of nanotechnology as potentially dangerous to the existence of our species. Moreover, just as the nonmilitary use of atomic energy holds undeniable threats to human life from nuclear power plant accidents, so too does the potential exist for industrial accidents in commercial nanotechnology applications. Yet while one cannot imagine any scenario in which the release of radioactivity from an industrial application of nuclear power would threaten the entire human species, a �runaway� nanotechnology could. Such a case is described in Eric Drexler�s book Engines of Creation:

Among the cognoscenti of nanotechnology, this threat has become known as the �gray goo problem�. Though masses of uncontrolled replicators need not be gray or gooey, the term �gray goo� emphasizes that replicators able to obliterate life might be less inspiring than a single species of crabgrass. They might be superior in an evolutionary sense, but this need not make them valuable. The gray goo threat makes one thing perfectly clear: we cannot afford certain kinds of accidents with replicating assemblers.

No End?

Although the litany of dangers facing our species seems daunting, none is an unambiguous death sentence. Each can be dealt with if our species shows foresight. These dangers must confront any race that climbs the evolutionary ladder to intelligence. As Carl Sagan says in his book Pale Blue Dot: �Some planetary

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civilizations see their way through, place limits on what may and what must not be done, and safely pass through the time of perils. Others, not so lucky or so prudent, perish�.
My own view is that we will successfully negotiate the hazards threatening our species. We will not kill ourselves off. We will not die off from disease. We will wax and wane in numbers as the long roll of time still facing this planet buffets our species with all manner of climate changes, asteroid impacts, runaway technology, and evil robots. We will persevere. But the animals and plants along for the ride on this planet that we have so cockily co-opted will not be so fortunate.
Perhaps this view that we are unkillable � at least as a species � is naive. But even if we are to live as long as an average mammalian species � between 1 and 3 million years � we still have huge stretches of time left, for our species is barely a quarter of a million years old. And who says we are average? My bet is that we will stick around until the very end of planetary habitability for this already old Earth.

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CONTENTS