snowball earth scientific american jan 2000

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68 Scientific American January 2000 Snowball Earth

Our human ancestors had it rough. Saber-toothedcats and woolly mammoths may have been day-to-day concerns, but harsh climate was a consum-

ing long-term challenge. During the past million years, theyfaced one ice age after another. At the height of the last icyepisode, 20,000 years ago, glaciers more than two kilometersthick gripped much of North America and Europe. The chill

delivered ice as far south as New York City.Dramatic as it may seem, this extreme climate change pales

in comparison to the catastrophic events that some of our ear-liest microscopic ancestors endured around 600 million yearsago. Just before the appearance of recognizable animal life, ina time period known as the Neoproterozoic, an ice age pre-vailed with such intensity that even the tropics froze over.

Imagine the earth hurtling through space like a cosmic snow-ball for 10 million years or more. Heat escaping from themolten core prevents the oceans from freezing to the bottom,but ice grows a kilometer thick in the –50 degree Celsius cold.All but a tiny fraction of the planet’s primitive organisms die.

Aside from grinding glaciers and groaning sea ice, the only stircomes from a smattering of volcanoes forcing their hot headsabove the frigid surface. Although it seems the planet mightnever wake from its cryogenic slumber, the volcanoes slowlymanufacture an escape from the chill: carbon dioxide.

With the chemical cycles that normally consume carbondioxide halted by the frost, the gas accumulates to record lev-

els. The heat-trapping capacity of carbon dioxide—a green-house gas—warms the planet and begins to melt the ice. Thethaw takes only a few hundred years, but a new problemarises in the meantime: a brutal greenhouse effect. Any crea-tures that survived the icehouse must now endure a hothouse.

As improbable as it may sound, we see clear evidence thatthis striking climate reversal—the most extreme imaginableon this planet—happened as many as four times between 750million and 580 million years ago. Scientists long presumedthat the earth’s climate was never so severe; such intense cli-mate change has been more widely accepted for other planetssuch as Venus [see “Global Climate Change on Venus,” by

Snowball EarthSnowball Earth

Ice entombed our planet hundreds of millionsof years ago, and complex animals evolved inthe greenhouse heat wave that followed

by Paul F. Hoffman and Daniel P. Schrag by Paul F. Hoffman and Daniel P. Schrag

Ice entombed our planet hundreds of millionsof years ago, and complex animals evolved inthe greenhouse heat wave that followed

Copyright 1999 Scientific American, Inc.



Mark A. Bullock and David H. Grinspoon; ScientificAmerican, March 1999]. Hints of a harsh past on the earthbegan cropping up in the early 1960s, but we and our col-leagues have found new evidence in the past eight years thathas helped us weave a more explicit tale that is capturing theattention of geologists, biologists and climatologists alike.

Thick layers of ancient rock hold the only clues to the cli-

mate of the Neoproterozoic. For decades, many of thoseclues appeared rife with contradiction. The first paradox wasthe occurrence of glacial debris near sea level in the tropics.Glaciers near the equator today survive only at 5,000 metersabove sea level or higher, and at the worst of the last ice agethey reached no lower than 4,000 meters. Mixed in with theglacial debris are unusual deposits of iron-rich rock. Thesedeposits should have been able to form only if the Neopro-terozoic oceans and atmosphere contained little or no oxy-gen, but by that time the atmosphere had already evolved tonearly the same mixture of gases as it has today. To confoundmatters, rocks known to form in warm water seem to have

accumulated just after the glaciers receded. If the earth wereever cold enough to ice over completely, how did it warm upagain? In addition, the carbon isotopic signature in the rockshinted at a prolonged drop in biological productivity. Whatcould have caused this dramatic loss of life?

Each of these long-standing enigmas suddenly makes sensewhen we look at them as key plot developments in the tale of a “snowball earth.” The theory has garnered cautious sup-port in the scientific community since we first introduced the

Snowball Earth Scientific American January 2000 69

G L E N A L L I S O N D i g i t a l I m a g e r y © 1 9 9 9 P h o t o D i s c , I n c .

TOWERS OF ICE like Argentina’s Moreno Glacier (above)once buried the earth’s continents. Clues about this frozen pasthave surfaced in layers of barren rock such as these hills near thecoast of northwest Namibia (inset ).




idea in the journal Science a year and ahalf ago. If we turn out to be right, the

tale does more than explain the myster-ies of Neoproterozoic climate and chal-lenge long-held assumptions about thelimits of global change. These extremeglaciations occurred just before a rapiddiversification of multicellular life, cul-minating in the so-called Cambrian ex-plosion between 575 and 525 millionyears ago. Ironically, the long periods of isolation and extreme environments ona snowball earth would most likely havespurred on genetic change and couldhelp account for this evolutionary burst.

The search for the surprisingly strong

evidence for these climatic events hastaken us around the world. Althoughwe are now examining Neoproterozoicrocks in Australia, China, the westernU.S. and the Arctic islands of Svalbard,we began our investigations in 1992along the rocky cliffs of Namibia’sSkeleton Coast. In Neoproterozoictimes, this region of southwestern Africawas part of a vast, gently subsidingcontinental shelf located in low south-ern latitudes.

There we see evidence of glaciers inrocks formed from deposits of dirt and

debris left behind when the ice melted.Rocks dominated by calcium- and mag-nesium-carbonate minerals lie justabove the glacial debris and harbor thechemical evidence of the hothouse thatfollowed. After hundreds of millions of years of burial, these now exposedrocks tell the story that scientists firstbegan to piece together 35 years ago.

In 1964 W. Brian Harland of the Uni-versity of Cambridge pointed out thatglacial deposits dot Neoproterozoic rock

outcrops across virtually every continent.By the early 1960s scientists had begun

to accept the idea of plate tectonics,which describes how the planet’s thin,rocky skin is broken into giant piecesthat move atop a churning mass of hotterrock below. Harland suspected that thecontinents had clustered together nearthe equator in the Neoproterozoic, basedon the magnetic orientation of tiny min-eral grains in the glacial rocks. Beforethe rocks hardened, these grains alignedthemselves with the magnetic field anddipped only slightly relative to horizon-tal because of their position near theequator. (If they had formed near the

poles, their magnetic orientation wouldbe nearly vertical.)

Realizing that the glaciers must havecovered the tropics, Harland became thefirst geologist to suggest that the earthhad experienced a great Neoproterozoicice age [see “The Great Infra-CambrianGlaciation,” by W. B. Harland andM.J.S. Rudwick; Scientific American,August 1964]. Although some of Har-land’s contemporaries were skeptical

about the reliability of the magneticdata, other scientists have since shownthat Harland’s hunch was correct. Butno one was able to find an explanationfor how glaciers could have survived thetropical heat.

At the time Harland was announcinghis ideas about Neoproterozoic glaciers,physicists were developing the firstmathematical models of the earth’s cli-mate. Mikhail Budyko of the LeningradGeophysical Observatory found a wayto explain tropical glaciers using equa-tions that describe the way solar radia-

tion interacts with the earth’s surfaceand atmosphere to control climate.Some geographic surfaces reflect moreof the sun’s incoming energy than oth-ers, a quantifiable characteristic knownas albedo. White snow reflects the mostsolar energy and has a high albedo,darker-colored seawater has a low albe-do, and land surfaces have intermediatevalues that depend on the types and dis-tribution of vegetation.

The more radiation the planet reflects,the cooler the temperature. With theirhigh albedo, snow and ice cool the at-

mosphere and thus stabilize their ownexistence. Budyko knew that this phe-


EARTH’S LANDMASSES were most likely clustered near the equator during the globalglaciations that took place around 600 million years ago. Although the continents havesince shifted position, relics of the debris left behind when the ice melted are exposed atdozens of points on the present land surface, including what is now Namibia (red dot ).

H E I D I N O L A N D

AUSTRALIASOUTH CHINA

ANTARCTICA

AFRICA

INDIA

KAZAKHSTANNORTH AMERICA

NORTHERNEUROPE

SIBERIA

SOUTH AMERICAEASTERN

SOUTH AMERICA

C O U R T E S Y O F D A N I E L P .

S C H R A G

WESTAFRICA




nomenon, called the ice-albedo feed-back, helps modern polar ice sheets togrow. But his climate simulations alsorevealed that this feedback can run outof control. When ice formed at latitudeslower than around 30 degrees north orsouth of the equator, the planet’s albedobegan to rise at a faster rate because di-rect sunlight was striking a larger surface

area of ice per degree of latitude. Thefeedback became so strong in his simula-tion that surface temperatures plummet-ed and the entire planet froze over.

Frozen and Fried

Budyko’s simulation ignited interestin the fledgling science of climate

modeling, but even he did not believethe earth could have actually experi-enced a runaway freeze. Almost every-one assumed that such a catastrophewould have extinguished all life, and

yet signs of microscopic algae in rocksup to one billion years old closely re-semble modern forms and imply a con-

tinuity of life. Also, once the earth hadentered a deep freeze, the high albedoof its icy veneer would have driven sur-face temperatures so low that it seemedthere would have been no means of es-cape. Had such a glaciation occurred,Budyko and others reasoned, it wouldhave been permanent.

The first of these objections began to

fade in the late 1970s with the discoveryof remarkable communities of organ-isms living in places once thought tooharsh to harbor life. Seafloor hot springssupport microbes that thrive on chemi-cals rather than sunlight. The kind of volcanic activity that feeds the hotsprings would have continued unabatedin a snowball earth. Survival prospectsseem even rosier for psychrophilic, orcold-loving, organisms of the kind livingtoday in the intensely cold and drymountain valleys of East Antarctica.Cyanobacteria and certain kinds of algae

occupy habitats such as snow, porousrock and the surfaces of dust particles en-cased in floating ice.

The key to the second problem—re-versing the runaway freeze—is carbondioxide. In a span as short as a humanlifetime, the amount of carbon dioxidein the atmosphere can change as plantsconsume the gas for photosynthesis andas animals breathe it out during respi-ration. Moreover, human activities suchas burning fossil fuels have rapidly

loaded the air with carbon dioxidesince the beginning of the IndustrialRevolution in the late 1700s. In theearth’s lifetime, however, these carbonsources and sinks become irrelevantcompared with geologic processes.

Carbon dioxide is one of several gas-es emitted from volcanoes. Normallythis endless supply of carbon is offsetby the erosion of silicate rocks: Thechemical breakdown of the rocks con-verts carbon dioxide to bicarbonate,which is washed to the oceans. Therebicarbonate combines with calcium

and magnesium ions to produce car-bonate sediments, which store a greatdeal of carbon [see “Modeling the Geo-


ROCKY CLIFFS along Namibia’s Skele-ton Coast (left ) have provided some of the best evidence for the snowball earthhypothesis. Authors Schrag (far left ) and

Hoffman point to a rock layer that repre-sents the abrupt end of a 700-million-year-old snowball event. The light-colored boulder in the rock between them proba- bly once traveled within an iceberg andfell to the muddy seafloor when the icemelted. Pure carbonate layers stackedabove the glacial deposits precipitated inthe warm, shallow seas of the hothouseaftermath. These “cap” carbonates are theonly Neoproterozoic rocks that exhibitlarge crystal fans, which accompany rapidcarbonate accumulation (above).

C A P C A R B O N A T E S

G L A C I A L D E P O S I T S

C O U R T E S Y O F D A N I E L P .

S C H R A G

C O U R T E S Y O F G A L E N P I P P A H A L V E R S O N

C R Y S T A L F A N S




chemical Carbon Cycle,” by R. A. Bern-er and A. C. Lasaga; Scientific Amer-ican, March 1989].

In 1992 Joseph L. Kirschvink, a geo-biologist at the California Institute of Technology, pointed out that during aglobal glaciation, an event he termed asnowball earth, shifting tectonic plateswould continue to build volcanoes and

to supply the atmosphere with carbondioxide. At the same time, the liquidwater needed to erode rocks and burythe carbon would be trapped in ice.With nowhere to go, carbon dioxidewould collect to incredibly high levels—

high enough, Kirschvink proposed, toheat the planet and end the global freeze.

Kirschvink had originally promotedthe idea of a Neoproterozoic deep freezein part because of mysterious iron de-posits found mixed with the glacial de-bris. These rare deposits are found

much earlier in earth history when theoceans (and atmosphere) containedvery little oxygen and iron could read-ily dissolve. (Iron is virtually insolublein the presence of oxygen.) Kirschvinkreasoned that millions of years of icecover would deprive the oceans of oxygen, so that dissolved iron expelledfrom seafloor hot springs could accu-

mulate in the water. Once a carbondioxide–induced greenhouse effect be-gan melting the ice, oxygen would againmix with the seawater and force theiron to precipitate out with the debrisonce carried by the sea ice and glaciers.

With this greenhouse scenario in mind,climate modelers Kenneth Caldeira of Lawrence Livermore National Labora-tory and James F. Kasting of Pennsylva-nia State University estimated in 1992that overcoming the runaway freezewould require roughly 350 times the

present-day concentration of carbondioxide. Assuming volcanoes of theNeoproterozoic belched out gases at thesame rate as they do today, the planetwould have remained locked in ice forup to tens of millions of years beforeenough carbon dioxide could accumu-late to begin melting the sea ice. Asnowball earth would be not only the

most severe conceivable ice age, it wouldbe the most prolonged.

Carbonate Clues

Kirschvink was unaware of twoemerging lines of evidence that

would strongly support his snowballearth hypothesis. The first is that theNeoproterozoic glacial deposits are al-most everywhere blanketed by carbon-ate rocks. Such rocks typically form inwarm, shallow seas, such as the Ba-


Breakup of a single landmass 770 million years ago leavessmall continents scattered near the equator. Formerly land-locked areas are now closer to oceanic sources of moisture.Increased rainfall scrubs more heat-trapping carbon dioxideout of the air and erodes continental rocks more quickly.Consequently, global temperatures fall, and large ice packsform in the polar oceans.The white ice reflects more solar en-ergy than does darker seawater, driving temperatures evenlower. This feedback cycle triggers an unstoppable coolingeffect that will engulf the planet in ice within a millennium.

Average global temperatures plummet to –50 degrees Cel-sius shortly after the runaway freeze begins.The oceans iceover to an average depth of more than a kilometer, limitedonly by heat emanating slowly from the earth’s interior.Mostmicroscopic marine organisms die, but a few cling to lifearound volcanic hot springs. The cold, dry air arrests thegrowth of land glaciers,creating vast deserts of windblownsand.With no rainfall,carbon dioxide emitted from volcanoesis not removed from the atmosphere.As carbon dioxide ac-cumulates,the planet warms and sea ice slowly thins.

Stage 1

Snowball Earth PrologueStage 2

Snowball Earthat Its Coldest

VOLCANO

CARBON DIOXIDE

HOTSPRING

SEA ICE

SANDDUNES

EVOLUTION OF A SNOWBALL EARTH EVENT . . .




hama Banks in what is now the AtlanticOcean. If the ice and warm water hadoccurred millions of years apart, no onewould have been surprised. But thetransition from glacial deposits to these“cap” carbonates is abrupt and lacksevidence that significant time passed be-tween when the glaciers dropped theirlast loads and when the carbonates

formed. Geologists were stumped to ex-plain so sudden a change from glacial totropical climates.

Pondering our field observations fromNamibia, we realized that this change isno paradox. Thick sequences of carbon-ate rocks are the expected consequenceof the extreme greenhouse conditionsunique to the transient aftermath of asnowball earth. If the earth froze over,an ultrahigh carbon dioxide atmospherewould be needed to raise temperaturesto the melting point at the equator. Once

melting begins, low-albedo seawater re-places high-albedo ice and the runawayfreeze is reversed [see illustration below].The greenhouse atmosphere helps todrive surface temperatures upward to al-most 50 degrees C, according to calcula-tions made last summer by climate mod-eler Raymond T. Pierrehumbert of theUniversity of Chicago.

Resumed evaporation also helps towarm the atmosphere because watervapor is a powerful greenhouse gas,and a swollen reservoir of moisture inthe atmosphere would drive an en-hanced water cycle. Torrential rainwould scrub some of the carbon diox-ide out of the air in the form of carbon-ic acid, which would rapidly erode therock debris left bare as the glaciers sub-sided. Chemical erosion productswouldquickly build up in the ocean water,leading to the precipitation of carbon-

ate sediment that would rapidly accu-mulate on the seafloor and later be-come rock. Structures preserved in theNamibian cap carbonates indicate thatthey accumulated extremely rapidly,perhaps in only a few thousand years.For example, crystals of the mineralaragonite, clusters of which are as tallas a person, could precipitate only from

seawater highly saturated in calciumcarbonate.

Cap carbonates harbor a second lineof evidence that supports Kirschvink’ssnowball escape scenario. They containan unusual pattern in the ratio of twoisotopes of carbon: common carbon 12and rare carbon 13, which has an extraneutron in its nucleus. The same pat-terns are observed in cap carbonatesworldwide, but no one thought to in-terpret them in terms of a snowballearth. Along with Alan Jay Kaufman,

Scientific American January 2000 73

D A V I D F I E R S T E I N

Concentrations of carbon dioxide in the atmosphere increase1,000-fold as a result of some 10 million years of normal vol-canic activity. The ongoing greenhouse warming effectpushes temperatures to the melting point at the equator. Asthe planet heats up,moisture from sea ice sublimating nearthe equator refreezes at higher elevations and feeds thegrowth of land glaciers. The open water that eventuallyforms in the tropics absorbs more solar energy and initiatesa faster rise in global temperatures.In a matter of centuries,abrutally hot,wet world will supplant the deep freeze.

As tropical oceans thaw, seawater evaporates and worksalong with carbon dioxide to produce even more intensegreenhouse conditions. Surface temperatures soar to morethan 50 degrees Celsius,driving an intense cycle of evapora-tion and rainfall.Torrents of carbonic acid rain erode the rock debris left in the wake of the retreating glaciers. Swollenrivers wash bicarbonate and other ions into the oceans,where they form carbonate sediment. New life-forms—en-gendered by prolonged genetic isolation and selective pres-sure—populate the world as global climate returns to normal.

Stage 3

Snowball Earthas It Thaws

Stage 4

Hothouse Aftermath

GLACIERS CARBONATESEDIMENT

Snowball Earth

. . . AND ITS HOTHOUSE AFTERMATH




an isotope geochemist now at the Uni-versity of Maryland, and Harvard Uni-versity graduate student Galen PippaHalverson, we have discovered that theisotopic variation is consistent overmany hundreds of kilometers of ex-posed rock in northern Namibia.

Carbon dioxide moving into theoceans from volcanoes is about 1 per-cent carbon 13; the rest is carbon 12. If the formation of carbonate rocks werethe only process removing carbon fromthe oceans, then the rock would have the

same fraction of carbon 13 as thatwhich comes out of volcanoes. But thesoft tissues of algae and bacteria growingin seawater also use carbon from the wa-ter around them, and their photosynthet-ic machinery prefers carbon 12 to carbon13. Consequently, the carbon that is leftto build carbonate rocks in a life-filledocean such as we have today has a high-er ratio of carbon 13 to carbon 12 thandoes the carbon fresh out of a volcano.

The carbon isotopes in the Neopro-terozoic rocks of Namibia record a dif-ferent situation. Just before the glacial

deposits, the amount of carbon 13plummets to levels equivalent to the vol-canic source, a drop we think recordsdecreasing biological productivity as iceencrusted the oceans at high latitudesand the earth teetered on the edge of arunaway freeze. Once the oceans icedover completely, productivity wouldhave essentially ceased, but no carbonrecord of this time interval exists be-cause calcium carbonate could not haveformed in an ice-covered ocean. Thisdrop in carbon 13 persists through thecap carbonates atop the glacial deposits

and then gradually rebounds to higherlevels of carbon 13 several hundred me-ters above, presumably recording therecovery of life at the end of the hot-house period.

Abrupt variation in this carbon iso-tope record shows up in carbonaterocks that represent other times of massextinction, but none are as large or aslong-lived. Even the meteorite impactthat killed off the dinosaurs 65 millionyears ago did not bring about such a

prolonged collapse in biological activity.Overall, the snowball earth hypothe-

sis explains many extraordinary obser-vations in the geologic record of theNeoproterozoic world: the carbon iso-topic variations associated with theglacial deposits, the paradox of cap car-bonates, the evidence for long-livedglaciers at sea level in the tropics, and theassociated iron deposits. The strength of

the hypothesis is that it simultaneouslyexplains all these salient features, noneof which had satisfactory independentexplanations. What is more, we believethis hypothesis sheds light on the earlyevolution of animal life.

Survival and Redemption of Life

In the 1960s Martin J. S. Rudwick,working with Brian Harland, pro-

posed that the climate recovery follow-ing a huge Neoproterozoic glaciationpaved the way for the explosive radia-

tion of multicellular animal life soonthereafter. Eukaryotes—cells that havea membrane-bound nucleus and fromwhich all plants and animals descend-ed—had emerged more than one billionyears earlier, but the most complex or-ganisms that had evolved when the firstNeoproterozoic glaciation hit were fila-mentous algae and unicellular proto-zoa. It has always been a mystery whyit took so long for these primitive or-ganisms to diversify into the 11 animal

body plans that show up suddenly inthe fossil record during the Cambrianexplosion [see illustration on this page].

A series of global freeze-fry eventswould have imposed an environmentalfilter on the evolution of life. All extanteukaryotes would thus stem from thesurvivors of the Neoproterozoic calam-ity. Some measure of the extent of eu-karyotic extinctions may be evident in

universal “trees of life.” Phylogenetictrees indicate how various groups of or-ganisms evolved from one another,based on their degrees of similarity.These days biologists commonly drawthese trees by looking at the sequencesof nucleic acids in living organisms.

Most such trees depict the eukaryotes’phylogeny as a delayed radiation crown-ing a long, unbranched stem. The lack of early branching could mean that mosteukaryotic lineages were “pruned” dur-ing the snowball earth episodes. Thecreatures that survived the glacial epi-

sodes may have taken refuge at hotsprings both on the seafloor and near thesurface of the ice where photosynthesiscould be maintained.

The steep and variable temperatureand chemical gradients endemic to eph-emeral hot springs would preselect forsurvival in the hellish aftermath tocome. In the face of varying environ-mental stress, many organisms respondwith wholesale genetic alterations. Se-vere stress encourages a great degree of


ALL ANIMALS descended from the first eukaryotes, cells with a membrane- bound nucleus, which appeared about two billion years ago. By the time of the first snowball earth episode more than one billion years later, eukary-otes had not developed beyond unicellular protozoa and filamentous algae.But despite the extreme climate, which may have “pruned” the eukaryotetree (dashed lines), all 11 animal phyla ever to inhabit the earth emergedwithin a narrow window of time in the aftermath of the last snowball event.The prolonged genetic isolation and selective pressure intrinsic to a snowball earth could be responsible for this explosion of new life-forms.

B A C T E R

I A

ARCHAEA

SNOWBALL

EARTH

EVENTS

3 , 5 0

0

2 , 5 0

0 7 0

0

Time (millions of years ago)

6 0 0

5 0 0

8 0 0

9 0 0

3 , 0 0

0

2 , 0 0

0

1 , 5 0

0 0

Mollusks

Platyhelminths

Priapulids

Nematodes

Arthropods

Brachiopods

Chordates

Echinoderms

Cnidarians

Poriferans

Annelids

E U K A

R Y O T E S

EMERGENCE

OF ANIMALS

x

x

H E I D I N O L A N D




genetic change in a short time, be-cause organisms that can mostquickly alter their genes will havethe most opportunities to acquiretraits that will help them adaptand proliferate.

Hot-spring communities widelyseparated geographically on the icysurface of the globe would accumu-

late genetic diversity over millionsof years. When two groups thatstart off the same are isolated fromeach other long enough under dif-ferent conditions, chances are thatat some point the extent of geneticmutation will produce a new spe-cies. Repopulations occurring aftereach glaciation would come aboutunder unusual and rapidly chang-ing selective pressures quite differ-ent from those preceding the gla-ciation; such conditions would alsofavor the emergence of new life-

forms.Martin Rudwick may not have

gone far enough with his inferencethat climatic amelioration follow-ing the great Neoproterozoic iceage paved the way for early animal evo-lution. The extreme climatic events them-selves may have played an active role inspawning multicellular animal life.

We have shown how the worldwideglacial deposits and carbonate rocks inthe Neoproterozoic record point to anextraordinary type of climatic event, asnowball earth followed by a briefer

but equally noxious greenhouse world.But what caused these calamities in thefirst place, and why has the world beenspared such events in more recent histo-ry? The first possibility to consider isthat the Neoproterozoic sun was weak-er by approximately 6 percent, makingthe earth more susceptible to a globalfreeze. The slow warming of our sun asit ages might explain why no snowball

event has occurred since that time. Butconvincing geologic evidence suggeststhat no such glaciations occurred in thebillion or so years before the Neopro-terozoic, when the sun was even cooler.

The unusual configuration of conti-nents near the equator during Neopro-terozoic times may better explain howsnowball events get rolling [see illustra-

tion on page 70]. When the continentsare nearer the poles, as they are today,carbon dioxide in the atmosphere re-mains in high enough concentrations tokeep the planet warm. When global tem-peratures drop enough that glaciers coverthe high-latitude continents, as they do inAntarctica and Greenland, the ice sheetsprevent chemical erosion of the rocks be-neath the ice. With the carbon burial pro-

cess stifled, the carbon dioxide inthe atmosphere stabilizes at a levelhigh enough to fend off the ad-vancing ice sheets. If all the conti-nents cluster in the tropics, on theother hand, they would remainice-free even as the earth grewcolder and approached the criti-cal threshold for a runaway freeze.

The carbon dioxide “safetyswitch” would fail because car-bon burial continues unchecked.

We may never know the truetrigger for a snowball earth, aswe have but simple theories forthe ultimate forcing of climatechange, even in recent times. Butwe should be wary of the planet’scapacity for extreme change. Forthe past million years, the earthhas been in its coldest state sinceanimals first appeared, but eventhe greatest advance of glaciers

20,000 years ago was far from thecritical threshold needed to plungethe earth into a snowball state.Certainly during the next severalhundred years, we will be more

concerned with humanity’s effects on cli-mate as the earth heats up in response tocarbon dioxide emissions [see “The Hu-man Impact on Climate Change,” byThomas R. Karl and Kevin E. Trenberth;Scientific American, December 1999].But could a frozen world be in our moredistant future?

We are still some 80,000 years from

the peak of the next ice age, so our firstchance for an answer is far in the fu-ture. It is difficult to say where theearth’s climate will drift over millions of years. If the trend of the past millionyears continues and if the polar conti-nental safety switch were to fail, wemay once again experience a global icecatastrophe that would inevitably joltlife in some new direction.


The Authors

PAUL F. HOFFMAN and DANIEL P. SCHRAG, both at HarvardUniversity, bring complementary expertise to bear on the snowballearth hypothesis. Hoffman is a field geologist who has long studied an-cient rocks to unravel the earth’s early history. He led the series of ex-peditions to northwestern Namibia that turned up evidence for Neo-proterozoic snowball earth events. Schrag is a geochemical oceanogra-pher who uses the chemical and isotopic variations of coral reefs,deep-sea sediments and carbonate rocks to study climate on timescalesranging from months to millions of years. Together they were able tointerpret the geologic and geochemical evidence from Namibia and toexplore the implications of a snowball earth and its aftermath.

Further Information

Origin and Early Evolution of the Metazoa. Edited by J. H. Lipps and P. W. Signor. Plenum Publishing, 1992.The Origin of Animal Body Plans. D. Erwin, J. Valentineand D. Jablonski in American Scientist, Vol. 85, No. 2, pages126–137; March–April 1997.

A Neoproterozoic Snowball Earth. P. F. Hoffman, A. J.Kaufman, G. P. Halverson and D. P. Schrag in Science, Vol.281, pages 1342–1346; August 28, 1998.

The First Ice Age. Kristin Leutwyler. Available only at www.sciam.com/2000/0100issue/0100hoffman.html on the ScientificAmerican Web site.

Some say the world will end in fire,Some say in ice.

From what I’ve tasted of desireI hold with those who favor fire.

But if it had to perish twice,I think I know enough of hateTo say that for destruction ice

Is also great And would suffice.

—Robert Frost,

Fire and Ice (1923)

SA


snowball earth scientific american jan 2000

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