Real Disasters

8/3/2019 Real Disasters

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28 November 2009 sky & telescope

Asteroid impacts, gamma-ray bursts,supervolcanoes, and mega-tsunamis

represent threats to humanitys future,

but dont lose sleep worrying about them.

AS E. C. KRUPP eloquently explains in his ar ticle (page22), Mayan astronomers were notpredicting the world toend in 2012. But you might be asked if there are any loom-ing disasters that portend mass death and destruction.

The short answer is probably not. Scientists arestudying several potential cataclysms that could threaten

humanitys collective survival, or at least disrupt our civi-lization on a massive scale. Fortunately, none are likely tooccur within the lifetime of anyone reading this article.

Jupiter recently experienced the most obvious threat:the impact of an asteroid or comet (page 34). As scientistshave pointed out, the impact of a kilometer-scale object isthe sole natural disaster that could annihilate our species.But its also the only one that we could prevent with near-

term technology. A giant asteroid impact 65 million

ago extinguished all dinosaur lineages except birds. the probability of such a large-scale calamity occurriin the next century is about one in a million. Moreovastronomers are well on their way toward catalogingalmost all of the near-Earth asteroids that are big enoto threaten our collective survival, which will leave erarer comet impacts as the remaining potential threa

A much more likely disaster is a smaller impact oairburst, such as the explosion over Siberia on June 31908. These events occur perhaps once every 100 to 1years, on average. Impactors on the order of 100 metare so diffi cult to detect that we might have no advanwarning. Yet these objects could flatten a city or regi

inundate coastlines if a big chunk splashes into an oand spawns a tsunami. The sudden death of millionpeople would certainly have a profound effect on civition. But such a tragedy would not threaten humanitsurvival unless it triggered an accidental nuclear war

Television programs have touted the danger poseda more distant astronomical phenomenon: gamma-rbursts. A massive star exploding in our galaxy as a G

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SandT.m November 200

and with a jet aimed precisely at Earth, could seriously

damage Earths atmosphere. Researchers have speculatedthat a GRB caused the Ordovician mass extinction 450million years ago, because the pattern of species extinc-tion is consistent with what one might expect from aGRB. But there is no direct evidence that a GRB (or asupernova) has ever affected life on Earth.

A Hubble Space Telescope survey of GRB host galaxiesindicates that bursts are exceedingly rare in mature spiralgalaxies such as the Milky Way. The chances of our planetbeing in a nearby GRBs line of fire are much lower thanthe odds that Ill win the decathlon gold medal at the 2012Summer Olympics. And there are no supernova progeni-tors close enough to Earth to substantially harm our

biosphere should any of these stars explode in the nextseveral centuries (S&T: March 2007, page 26).

The threat posed by wandering stars, pulsars, blackholes, and other interstellar exotica is even more minus-cule, given their low density in space. But extraterrestrialobjects are not the only causes for concern. Geologistshave discovered several ticking time bombs lurking withinour planet.

INCOMING!Asteroid

fragments

heading toward

Earth could

wreak regional

havoc.

As dramatized reasonably accurately in the TV movie

Supervolcano, Yellowstone National Park contains avolcanic caldera roughly 45 miles across, and lurkingbeneath it is a giant, seething mass of molten rock. TheYellowstone supervolcano last went off 640,000 years ago,pumping 1,000 cubic kilometers of volcanic ash and gasesinto the atmosphere, covering much of North America.

I

STOCKPHOTO.COM

/DUUUNA

SOURCE: ISTOCKPHOTO.COM / PJARD (STATUE OF LIBERTY) & MIKEUK (OCEAN)

S&T Photo Illustration Casey Reed



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This material blotted out the Sun, cooling the planetleast several years. If Yellowstone blows its stack nexat least half of the U.S. will be buried under volcanicand the vast amounts of atmospheric material will snout agriculture around much of the world, perhaps cing the human population by tens of percent.

The last three Yellowstone eruptions were spacedroughly 700,000 years apart, which sounds ominousFortunately, there is no large-scale ground deformatigreatly increased seismic or geyser activity that signaimpending supereruption. Given the rarity of these clysms, were probably safe for tens of thousands of y

And thats a good thing. About 74,000 years ago, LToba, a 20-by-60-mile-wide supervolcano on the Indosian island of Sumatra, exploded and perhaps gave Hsapiens its closest shave with extinction. Genetic stud

Majr ahfa frmYwn ruin

2.1 miin ar ag

Majrahfa frm

MunS. Hn

1980

Major ashfallfrom

Yellowstoneeruption

640,000years ago

Major ashfall fromYellowstone eruption

2.1 million years ago

Majorashfall from

MountSt. Helens

1980

MunS. Hn

Yellowstone

Majr ahfa frmYwn ruin

2.1 miin ar ag

Majrahfa frm

MunS. Hn

1980

Majr ahfafrm

Ywnruin

640,000ar ag

500m

iles

1,000

miles

ASHFALL COMPARED The 1980 Mount St. Helens eruption devastated 600

square kilometers of forest, but its volume of ejected material was puny com-

pared to a Yellowstone supereruption.

YellowstoneNational Park

Caldera rim

Od Faihfu

30miles

2 7 0 m i lCann

10miles

IDAHO

MONTANA

WYOMING

Magmachamber

Plume

YELLOWSTONE TIME BOMB The Yellowstone caldera is about 45 miles across.

A magma chamber containing hundreds of cubic miles of molten rock underlies the

National Park. If the supervolcano vents this material into the atmosphere, it will precipitate a global ecological crisis that will radically alter the course of civilization.

Mount St. Helens, / . cubic mile

Mount Pinatubo, / . cubic miles

Krakatoa, / . cubic miles

Mount Mazama (Crater Lake, Oregon)~, years ago / cubic miles

Tambora cubic miles

Yellowstone caldera. million years ago cubic miles

Yellowstone caldera, years ago cubic miles

Yellowstone caldera. million years ago cubic miles

Volcanic Eruption Volumes

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indicate that the human population passed through abottleneck sometime between 70,000 and 80,000 yearsago, when there were perhaps only a few thousand livingindividuals. Although there is no proven link betweenthe Lake Toba supereruption and the genetic bottleneck,

the close association in time may not be coincidental. Arecent computer simulation indicates that the eruptionproduced a decade-long volcanic winter that severelyaffected global ecosystems, posing a formidable challengeto Paleolithic human societies.

The media has recently focused a great deal of attentionon a more typical volcano, Cumbre Vieja, on La Palma inthe Canary Islands. The 6,400-foot-high mountain appearsto be unstable, leading to fears of a massive landslide thatcould trigger a mega-tsunami that would cross the Atlanticin 6 to 7 hours. Large cities such as New York could not beevacuated quickly enough, so giant waves sweeping overAtlantic coastlines would drown millions of people and

leave behind incalculable economic destruction. But recentwork by geologists at the Delft University of Technology inThe Netherlands strongly suggests that Cumbre Vieja willremain stable for the next 10,000 years.

Another inevitable geological threat comes from a mag-netic-field reversal. Earths north and south poles occasion-ally flip polarity, but without any regular periodicity. Themost recent reversal occurred 780,000 years ago, which islonger than the normal timescale between events. Are weoverdue? Earths magnetic-field strength has weakenedby about a third in the past 2,000 years, raising legitimateconcerns that we might be heading toward a big switcherooin the next 1,000 to 2,000 years.

Although we dont really know what happens duringthese reversals, most geologists think the global dipolefield shuts down temporarily, and then it turns back onwith opposite polarity. During the time when the field isshut down, Earth would lose its magnetic shield. But wedstill have our atmosphere to absorb the blows from thesolar wind and cosmic radiation.

Theres no evidence that a magnetic reversal has ever

Jaara

Equar

SUMBAWA

Krakatoa

LakeToba

TamboraJAVA

INDIAN

OCEAN

SOUTH CHINA

SE A

JAVA SEA

M A L A Y S A

I N D O E I A

SUMATR

A

BORN

EO

6hou

rs

7

hours

3hou

rs

30min

ANARY

LA DS

Landslide

MOROC

CO

MAURI

AN

A

W

T

N

AHARA

LAKE TOBA The most

recent supervolcanic erup-

tion occurred in Lake Toba

(far left), Sumatra, about

74,000 years ago. The cata-

strophic 1-gigaton explosion

(3,000 times the energy of

the 1980 Mount St. Helens

eruption) might have cooledglobal temperatures 3 to 5C

( to F) for several years,

a volcanic winter that may

have brought early modern

humans to the brink of

extinction.

TSUNAMI IN

WAITING?

The satellite image

on the left shows

La Palma, in the

Canary Islands. At

the southern end

lies the volcano

Cumbre Vieja,

whose west-

ern flanks may

someday collapseinto the Atlantic,

triggering a mega-

tsunami (above)

that will inundate

coastlines in

northern Africa,

Europe, and the

Americas.

S&T:CASEYREED(3)

NASA/LANDSAT

NASA/GSFC/MITI/ERSDAC/JAR

US/JAPANASTERSCIENCETEAM



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caused a large-scale extinction, and our ancestors, Homoerectus, breezed through several reversals without anyevidence of a population decline. A reversal wont threatenhumanitys existence, but it would disrupt civilization.Without a magnetic shield, powerful solar storms willknock out satellites and power grids. Airliners will beforced to fly at lower altitudes to protect passengers fromspace radiation. Aurorae will occur all over Earth.

A more near-term possible disaster is a global pan-demic, such as the 191820 influenza outbreak that killedtens of millions of people (more than from World War I).Many dangerous strains of bacteria are evolving resis-tance to antibiotics, leading to concerns that a superbug,carried swiftly around the world by airline passengers,could kill millions of people. China, for example, recentlyquarantined travelers to prevent the spread of swine flu.

These large natural disasters are so rare that I donlose sleep. What worries me most are man-made podisasters, such as environmental degradation, climachange, bioterrorism, and nuclear war, especially as nations acquire these dreadful weapons. I concur wiBritish astrophysicist Martin Rees, who writes in hisOur Final Hour, Through malign intent, or throughadventure, 21st-century technology could jeopardise potential, foreclosing its human or posthuman futur

Anyone who has metS&T Editor in ChiefRobert Naeye

knows he has no hope of winning a decathlon gold meda

Millions of years ago

= North up = South up

Timeline of Earths Magnetic Polarity

5 4 3 2 1

MAGENTIC REVERSALS Earths magnetic field has flipp

polarity about two dozen times over the past 5 million years

There is no obvious pattern, but we seem to be overdue.

SOURCE: USGS / E. EDWARD MANKINEN / G. BRENT


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