Warning
Over
Dolphin 'Extinction'
Humans
Extinct By 2040
20
Ways
The World Could End
By Corey S. Powell
We've had a good run of it. In the 500,000
years Homo sapiens has roamed the land we've built cities,
created complex languages, and sent robotic scouts to
other planets. It's difficult to imagine it all coming
to an end. Yet 99 percent of all species that ever lived
have gone extinct, including every one of our hominid
ancestors. In 1983, British cosmologist Brandon Carter
framed the "Doomsday argument," a statistical way to judge
when we might join them. If humans were to survive a long
time and spread through the galaxy, then the total number
of people who will ever live might number in the trillions.
By pure odds, it's unlikely that we would be among the
very first hundredth of a percent of all those people.
Or turn the argument around: How likely is it that this
generation will be the one unlucky one? Something like
one fifth of all the people who have ever lived are alive
today. The odds of being one of the people to witness
doomsday are highest when there is the largest number
of witnesses around so now is not such an improbable
time.
Human activity is severely disrupting almost all life
on the planet, which surely doesn't help matters. The
current rate of extinctions is, by some estimates, 10,000
times the average in the fossil record. At present, we
may worry about snail darters and red squirrels in abstract
terms. But the next statistic on the list could be us.
Natural
Disasters
1 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.
2 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.
3 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.
4 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.
5 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.
6 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.
7 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.
8 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 travel means 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.
Continued
in Part 2