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Existantial Risks
Other Pages
Natural Disasters
Asteroid impact
Once a disaster scenario gets the cheesy Hollywood treatment,
it's hard to take it seriously. But there is no question that
a cosmic interloper will hit Earth, and we won't have to wait
millions of years for it to happen.
In 1908 a 200-foot-wide comet
fragment slammed into the atmosphere and exploded over the Tunguska
region in Siberia, Russia, with nearly 1,000 times the energy
of the atomic bomb dropped on Hiroshima.
Astronomers estimate
similar-sized events occur every one to three centuries. Benny
Peiser, an anthropologist-cum-pessimist at Liverpool John Moores
University in England, claims that impacts have repeatedly disrupted
human civilization. As an example, he says one killed 10,000 people
in the Chinese city of Chi'ing-yang in 1490. Many scientists question
his interpretations: Impacts are most likely to occur over the
ocean, and small ones that happen over land are most likely to
affect unpopulated areas. But with big asteroids, it doesn't matter
much where they land. Objects more than a half-mile wide- which
strike Earth every 250,000 years or so- would touch off firestorms
followed by global cooling from dust kicked up by the impact.
Humans would likely survive, but civilization might not. An asteroid
five miles wide would cause major extinctions, like the one that
may have marked the end of the age of dinosaurs.
For a real chill,
look to the Kuiper belt, a zone just beyond Neptune that contains
roughly 100,000 ice-balls more than 50 miles in diameter. The
Kuiper belt sends a steady rain of small comets earthward. If
one of the big ones headed right for us, that would be it for
pretty much all higher forms of life, even cockroaches.
Gamma-ray burst
If you could watch the sky with gamma-ray vision, you might
think you were being stalked by cosmic paparazzi. Once a day or
so, you would see a bright flash appear, briefly outshine everything
else, then vanish. These gamma-ray bursts, astrophysicists recently
learned, originate in distant galaxies and are unfathomably powerful-
as much as 10 quadrillion (a one followed by 16 zeros) times as
energetic as the sun. The bursts probably result from the merging
of two collapsed stars. Before the cataclysmal event, such a double
star might be almost completely undetectable, so we'd likely have
no advance notice if one is lurking nearby. Once the burst begins,
however, there would be no missing its fury.
At a distance of
1,000 light-years- farther than most of the stars you can see
on a clear night- it would appear about as bright as the sun.
Earth's atmosphere would initially protect us from most of the
burst's deadly X rays and gamma rays, but at a cost. The potent
radiation would cook the atmosphere, creating nitrogen oxides
that would destroy the ozone layer. Without the ozone layer, ultraviolet
rays from the sun would reach the surface at nearly full force,
causing skin cancer and, more seriously, killing off the tiny
photosynthetic plankton in the ocean that provide oxygen to the
atmosphere and bolster the bottom of the food chain. All the gamma-ray
bursts observed so far have been extremely distant, which implies
the events are rare. Scientists understand so little about these
explosions, however, that it's difficult to estimate the likelihood
of one detonating in our galactic neighborhood.
Collapse of the vacuum
In the book Cat's Cradle, Kurt Vonnegut popularized the idea
of "ice-nine," a form of water that is far more stable
than the ordinary kind, so it is solid at room temperature. Unleash
a bit of it, and suddenly all water on Earth transforms to ice-nine
and freezes solid. Ice-nine was a satirical invention, but an
abrupt, disastrous phase transition is a possibility. Very early
in the history of the universe, according to a leading cosmological
model, empty space was full of energy.
This state of affairs,
called a false vacuum, was highly precarious. A new, more stable
kind of vacuum appeared and, like ice-nine, it quickly took over.
This transition unleashed a tremendous amount of energy and caused
a brief runaway expansion of the cosmos. It is possible that another,
even more stable kind of vacuum exists, however. As the universe
expands and cools, tiny bubbles of this new kind of vacuum might
appear and spread at nearly the speed of light. The laws of physics
would change in their wake, and a blast of energy would dash everything
to bits. "It makes for a beautiful story, but it's not very
likely," says Piet Hut of the Institute for Advanced Studies
in Princeton, New Jersey. He says he worries more about threats
that scientists are more certain of- such as rogue black holes.
Rogue black holes
Our galaxy is full of black holes, collapsed stellar corpses
just a dozen miles wide. How full? Tough question. After all,
they're called black holes for a reason. Their gravity is so strong
they swallow everything, even the light that might betray their
presence. David Bennett of Notre Dame University in Indiana managed
to spot two black holes recently by the way they distorted and
amplified the light of ordinary, more distant stars. Based on
such observations, and even more on theoretical arguments, researchers
guesstimate there are about 10 million black holes in the Milky
Way.
These objects orbit just like other stars, meaning that it
is not terribly likely that one is headed our way. But if a normal
star were moving toward us, we'd know it. With a black hole there
is little warning. A few decades before a close encounter, at
most, astronomers would observe a strange perturbation in the
orbits of the outer planets. As the effect grew larger, it would
be possible to make increasingly precise estimates of the location
and mass of the interloper.
The black hole wouldn't have to come
all that close to Earth to bring ruin; just passing through the
solar system would distort all of the planets' orbits. Earth might
get drawn into an elliptical path that would cause extreme climate
swings, or it might be ejected from the solar system and go hurtling
to a frigid fate in deep space.
Giant solar flares
Solar flares- more properly known as coronal mass ejections-
are enormous magnetic outbursts on the sun that bombard Earth
with a torrent of high-speed subatomic particles. Earth's atmosphere
and magnetic field negate the potentially lethal effects of ordinary
flares. But while looking through old astronomical records, Bradley
Schaefer of Yale University found evidence that some perfectly
normal-looking, sunlike stars can brighten briefly by up to a
factor of 20. Schaefer believes these stellar flickers are caused
by superflares, millions of times more powerful than their common
cousins. Within a few hours, a superflare on the sun could fry
Earth and begin disintegrating the ozone layer (see #2).
Although
there is persuasive evidence that our sun doesn't engage in such
excess, scientists don't know why superflares happen at all, or
whether our sun could exhibit milder but still disruptive behavior.
And while too much solar activity could be deadly, too little
of it is problematic as well. Sallie Baliunas at the Harvard-Smithsonian
Center for Astrophysics says many solar-type stars pass through
extended quiescent periods, during which they become nearly 1
percent dimmer. That might not sound like much, but a similar
downturn in the sun could send us into another ice age. Baliunas
cites evidence that decreased solar activity contributed to 17
of the 19 major cold episodes on Earth in the last 10,000 years.
Reversal of Earth's magnetic field
Every few hundred thousand years Earth's magnetic field dwindles
almost to nothing for perhaps a century, then gradually reappears
with the north and south poles flipped. The last such reversal
was 780,000 years ago, so we may be overdue. Worse, the strength
of our magnetic field has decreased about 5 percent in the past
century. Why worry in an age when GPS has made compasses obsolete?
Well, the magnetic field deflects particle storms and cosmic rays
from the sun, as well as even more energetic subatomic particles
from deep space. Without magnetic protection, these particles
would strike Earth's atmosphere, eroding the already beleaguered
ozone layer (see #5). Also, many creatures navigate by magnetic
reckoning. A magnetic reversal might cause serious ecological
mischief. One big caveat: "There are no identifiable fossil
effects from previous flips," says Sten Odenwald of the NASA
Goddard Space Flight Center. "This is most curious."
Still, a disaster that kills a quarter of the population, like
the Black Plague in Europe, would hardly register as a blip in
fossil records.
Flood-basalt volcanism
In 1783, the Laki volcano in Iceland erupted, spitting out three cubic miles of lava. Floods, ash, and fumes wiped out 9,000 people and 80 percent of the livestock. The ensuing starvation killed a quarter of Iceland's population. Atmospheric dust caused winter temperatures to plunge by 9 degrees in the newly independent United States. And that was just a baby's burp compared with what the Earth can do. Sixty-five million years ago, a plume of hot rock from the mantle burst through the crust in what is now India. Eruptions raged century after century, ultimately unleashing a quarter-million cubic miles of lava- the Laki eruption 100,000 times over. Some scientists still blame the Indian outburst, not an asteroid, for the death of the dinosaurs. An earlier, even larger event in Siberia occurred just about the time of the Permian-Triassic extinction, the most thorough extermination known to paleontology. At that time 95 percent of all species were wiped out.
Sulfurous volcanic gases produce acid rains. Chlorine-bearing compounds present yet another threat to the fragile ozone layer- a noxious brew all around. While they are causing short-term destruction, volcanoes also release carbon dioxide that yields long-term greenhouse-effect warming.The last big pulse of flood-basalt volcanism built the Columbia River plateau about 17 million years ago. We're ripe for another.
Global epidemics
If Earth doesn't do us in, our fellow organisms might be up
to the task. Germs and people have always coexisted, but occasionally
the balance gets out of whack. The Black Plague killed one European
in four during the 14th century; influenza took at least 20 million
lives between 1918 and 1919; the AIDS epidemic has produced a
similar death toll and is still going strong. From 1980 to 1992,
reports the Centers for Disease Control and Prevention, mortality
from infectious disease in the United States rose 58 percent.
Old diseases such as cholera and measles have developed new resistance
to antibiotics. Intensive agriculture and land development is
bringing humans closer to animal pathogens.
International travelmeans diseases can spread faster than ever. Michael Osterholm,
an infectious disease expert who recently left the Minnesota Department
of Health, described the situation as "like trying to swim
against the current of a raging river." The grimmest possibility
would be the emergence of a strain that spreads so fast we are
caught off guard or that resists all chemical means of control,
perhaps as a result of our stirring of the ecological pot. About
12,000 years ago, a sudden wave of mammal extinctions swept through
the Americas. Ross MacPhee of the American Museum of Natural History
argues the culprit was extremely virulent disease, which humans
helped transport as they migrated into the New World.
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