The Evolution of the Earth
The formation of this planet and its atmosphere gave
rise to life, which shaped the earth’s subsequent development.
Our future lies in interpreting this geologic pastby Claude J. All•gre and Stephen H. Schneider

EARTH SEEN FROM SPACE has changed dramatically. One hundred million years
after it had formedÑsome 4.35 billion years agoÑthe planet was probably undergoing meteor bombardment (left ). At this time, it may have been studded with volcanic islands and shrouded by an atmosphere laden with carbon dioxide andheavy with clouds. Three billion years ago its face may have been obscured by anorange haze of methane, the product of early organisms (center ). Today clouds,
oceans and continents are clearly discernible (right ). This illustration was prepared with the help of James F. Kasting of Pennsylvania State University.

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Copyright 1994 Scientific American, Inc.

L

ike the lapis lazuli gem it resembles,
the blue, cloud-enveloped planetthat we recognize immediatelyfrom satellite pictures seems remarkably stable. Continents and oceans, encircled by an oxygen-rich atmosphere,
support familiar life-forms. Yet this constancy is an illusion produced by thehuman experience of time. The earth
and its atmosphere are continuously altered. Plate tectonics shift the continents, raise mountains and move theocean ßoor while processes that no one
fully comprehends alter the climate.
Such constant change has characterized the earth since its beginning some
4.5 billion years ago. From the outset,
heat and gravity shaped the evolutionof the planet. These forces were gradually joined by the global eÝects of theemergence of life. Exploring this pastoÝers us the only possibility of under-

Copyright 1992 Scientific American, Inc.

standing the origin of life and, perhaps,
its future.
Scientists used to believe the rocky
planets, including the earth, Mercury, Venus and Mars, were created by the rapid
gravitational collapse of a dust cloud, adeßation giving rise to a dense orb. In
the 1960s the Apollo space programchanged this view. Studies of moon
craters revealed that these gouges werecaused by the impact of objects thatwere in great abundance about 4.5 billion years ago. Thereafter, the numberof impacts appeared to have quickly decreased. This observation rejuvenatedthe theory of accretion postulated by
Otto Schmidt. The Russian geophysicist had suggested in 1944 that planets grew in size gradually, step by step.
According to Schmidt, cosmic dust
lumped together to form particulates,
particulates became gravel, gravel became small balls, then big balls, thentiny planets, or planetesimals, and, Þnally, dust became the size of the moon.
As the planetesimals became larger,
their numbers decreased. Consequently, the number of collisions between
planetesimals, or meteorites, decreased.
Fewer items available for accretion

CLAUDE J. ALLéGRE and STEPHEN H.
SCHNEIDER study various aspects of theearthÕs geologic history and its climate.
All•gre is a professor at the Universityof Paris and directs the department ofgeochemistry at the Institut de Physiquedu Globe de Paris. He is a foreign member of the National Academy of Sciences.
Schneider is a professor in the department of biological sciences at Stanford
University and is a senior fellow at theuniversityÕs Institute of International
Studies. He is also a senior scientist at
the National Center for Atmospheric Research in Boulder, Colo.

meant that it took a long time to build
up a large planet. A calculation madeby George W. Wetherill of the Carnegie
Institution of Washington suggests thatabout 100 million years could pass between the formation of an object measuring 10 kilometers in diameter andan object the size of the earth.
The process of accretion had significant thermal consequences for theearth, consequences that have forcefully directed its evolution. Large bodies
slamming into the planet produced im-mense heat in the interior, melting thecosmic dust found there. The resulting
furnaceÑsituated some 200 to 400 kilometers underground and called a magma oceanÑwas active for millions ofyears, giving rise to volcanic eruptions.
When the earth was young, heat at thesurface caused by volcanism and lavaßows from the interior was supplemented by the constant bombardment ofhuge planetesimals, some of them perhaps the size of the moon or even Mars.
No life was possible during this period.
Beyond clarifying that the earth had
formed through accretion, the Apollo
program compelled scientists to try toreconstruct the subsequent temporaland physical development of the early
earth. This undertaking had been considered impossible by founders of geology, including Charles Lyell, to whom
the following phrase is attributed : No
vestige of a beginning, no prospect foran end. This statement conveys the ideathat the young earth could not be recreated, because its remnants were de-stroyed by its very activity. But the development of isotope geology in the
1960s had rendered this view obsolete.
Their imaginations Þred by Apollo andthe moon Þndings, geochemists began
to apply this technique to understandthe evolution of the earth.

D

ating rocks using so-called radioactive clocks allows geologists to work on old terrains that
do not contain fossils. The hands of aradioactive clock are isotopesÑatomsof the same element that have diÝerent
atomic weightsÑand geologic time ismeasured by the rate of decay of one
isotope into another [see ÒThe Earliest
History of the Earth,Ó by Derek York;SCIENTIFIC AMERICAN, January 1993].
Among the many clocks, those basedon the decay of uranium 238 into lead
206 and of uranium 235 into lead 207
are special. Geochronologists can determine the age of samples by analyzing
only the daughter productÑin this case,leadÑof the radioactive parent, uranium.

How the Earth Got Its CoreT

he differentiation of the planet took place quite quickly after the earth
was formed by the accretion of cosmic dust and meteorites. About 4.4
billion years ago the core—which, with the mantle, drives the geothermal
cycle, including volcanism—appeared; gases emerging from the interior of
the planet also gave rise to a nascent atmosphere. Somewhat later, although
the issue has not been entirely resolved, it seems that continental crust
formed as the various elements segregated into different depths.

UPP

ER MANTLE

LITHOSPHERE
ASTHENOSPHERE
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Isotope geology has permitted geologists to determine that the accretion of
the earth culminated in the diÝerentiation of the planet : the creation of thecoreÑthe source of the earthÕs magnetic ÞeldÑand the beginning of the atmosphere. In 1953 the classic work ofClaire C. Patterson of the California Institute of Technology used the uraniumlead clock to establish an age of 4.55
billion years for the earth and many ofthe meteorites that formed it. Recent
work by one of us (All•gre) on lead isotopes, however, led to a somewhat newinterpretation. As Patterson argued,
some meteorites were indeed formed
about 4.56 billion years ago, and theirdebris constituted the earth. But theearth continued to grow through thebombardment of planetesimals until
some 120 to 150 million years later. At
that timeÑ4.44 to 4.41 billion yearsagoÑthe earth began to retain its atmosphere and create its core. This possibility had already been suggested byBruce R. Doe and Robert E. Zartman of
the U.S. Geological Survey in Denver a
decade ago and is in agreement with
WetherillÕs estimates.
The emergence of the continents
came somewhat later. According to thetheory of plate tectonics, these landmasses are the only part of the earthÕs
crust that is not recycled and, consequently, destroyed during the geothermal cycle driven by the convection in
the mantle. Continents thus provide aform of memory because the record ofearly life can be read in their rocks. The
testimony, however, is not extensive.
Geologic activity, including plate tectonics, erosion and metamorphism, has
destroyed almost all the ancient rocks.
Very few fragments have survived thisgeologic machine.
Nevertheless, in recent years, several
important Þnds have been made, again
using isotope geochemistry. One group,
led by Stephen Moorbath of the University of Oxford, discovered terrain in
West Greenland that is between 3.7 and3.8 billion years old. In addition, Samuel A. Bowring of the Massachusetts Institute of Technology explored a small
area in North AmericaÑthe Acasta
gneissÑthat is 3.96 billion years old.
Ultimately, a quest for the mineralzircon led other researchers to evenmore ancient terrain. Typically foundin continental rocks, zircon is not dissolved during the process of erosion
but is deposited as particles in sediment. A few pieces of zircon can therefore survive for billions of years andcan serve as a witness to the earthÕs
more ancient crust. The search for old
zircons started in Paris with the work ofAnnie Vitrac and Jo‘l R. Lancelot, now

Copyright 1994 Scientific American, Inc.

at the University of Marseilles and the
University of Montpellier, respectively,
as well as with the eÝorts of Moorbathand All•gre. It was a group at the Australian National University in Canberra,
directed by William Compston, that wasÞnally successful. The team discovered
zircons in western Australia that were
between 4.1 and 4.3 billion years old.
Zircons have been crucial not only forunderstanding the age of the continents
but for determining when life Þrst appeared. The earliest fossils of undisputed age were found in Australia andSouth Africa. These relics of blue-green
algae are about 3.5 billion years old.
Manfred Schidlowski of the Max Planck
Institute for Chemistry in Mainz hasstudied the Isua formation in West
Greenland and argues that organic matter existed as many as 3.8 billion years
ago. Because most of the record of earlylife has been destroyed by geologic activity, we cannot say exactly when it ÞrstappearedÑperhaps it arose very quickly, maybe even 4.2 billion years ago.

O

ne of the most important aspects of the earthÕs evolution
is the formation of the atmosphere, because it is this assemblage ofgases that allowed life to crawl out of
the oceans and to be sustained. Researchers have hypothesized since the1950s that the terrestrial atmospherewas created by gases emerging fromthe interior of the planet. When a volcano spews gases, it is an example of thecontinuous outgassing, as it is called,of the earth. But scientists have questioned whether this process occurredsuddenly about 4.4 billion years ago
when the core diÝerentiated or whether
it took place gradually over time.
To answer this question, All•gre andhis colleagues studied the isotopes ofrare gases. These gasesÑincluding helium, argon and xenonÑhave the peculiarity of being chemically inert, that is,they do not react in nature with otherelements. Two of them are particularly
important for atmospheric studies: argon and xenon. Argon has three isotopes, of which argon 40 is created by
the decay of potassium 40. Xenon has
nine, of which 129 has two diÝerent
origins. Xenon 129 arose as the resultof nucleosynthesis before the earth andsolar system were formed. It was also
created from the decay of radioactive
iodine 129, which does not exist on theearth anymore. This form of iodine waspresent very early on but has diedsince, and xenon 129 has grown at its
expense.
Like most couples, both argon 40 andpotassium 40 and xenon 129 and iodine
129 have stories to tell. They are excel-

Copyright 1994 Scientific American, Inc.

CONTINENTAL SHIFT has altered theface of the planet for nearly a billion
years, as can be seen in the diÝerences
between the positions of the continentsthat we know today and those of 700
million years ago. Pangaea, the superaggregate of early continents, came together about 200 million years ago andthen promptly, in geologic terms, brokeapart. This series was compiled withthe advice of Christopher R. Scotese of
the University of Texas at Arlington.

lent chronometers. Although the atmosphere was formed by the outgassingof the mantle, it does not contain any
potassium 40 or iodine 129. All argon
40 and xenon 129, formed in the earth
and released, are found in the atmosphere today. Xenon was expelled fromthe mantle and retained in the atmosphere; therefore, the atmosphere-mantle ratio of this element allows us toevaluate the age of diÝerentiation. Argon and xenon trapped in the mantle
evolved by the radioactive decay ofpotassium 40 and iodine 129. Thus, if
the total outgassing of the mantle occurred at the beginning of the earthÕs
formation, the atmosphere would notcontain any argon 40 but would contain xenon 129.
The major challenge facing an investigator who wants to measure such ratios of decay is to obtain high concentrations of rare gases in mantle rocks
because they are extremely limited. Fortunately, a natural phenomenon occurs
at mid-oceanic ridges during which volcanic lava transfers some silicates fromthe mantle to the surface. The small
amounts of gases trapped in mantle
minerals rise with the melt to the surface and are concentrated in small vesicles in the outer glassy margin of lava
ßows. This process serves to concentrate the amounts of mantle gases by
a factor of 10 4 or 10 5. Collecting theserocks by dredging the seaßoor andthen crushing them under vacuum in asensitive mass spectrometer allow geochemists to determine the ratios of theisotopes in the mantle. The results arequite surprising. Calculations of the ratios indicate that between 80 and 85
percent of the atmosphere was outgassed in the Þrst one million years; therest was released slowly but constantly
during the next 4.4 billion years.
The composition of this primitive atmosphere was most certainly dominated by carbon dioxide, with nitrogen as
the second most abundant gas. Traceamounts of methane, ammonia, sulfur
dioxide and hydrochloric acid were alsopresent, but there was no oxygen. Except for the presence of abundant water, the atmosphere was similar to that

700 MILLIONYEARS AGOEQUATOR600 MILLIONYEARS AGO

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100 MILLIONYEARS AGOPRESENTSCIENTIFIC AMERICAN October 1994

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4,500 MILLION YEARS AGO

570

505

438

408

360

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PRESENTCOLDERTHAN
PRESENT

PRECAMBRIANORDOVICIANCAMBRIAN

CLIMATE FLUCTUATIONS are apparent over time. Although
the earthÕs early temperature record is quite uncertain, good
estimates can be made starting 400 million years ago, whenof Venus or Mars. The details of theevolution of the original atmosphereare debated, particularly because we do
not know how strong the sun was atthat time. Some facts, however, are notdisputed. It is evident that carbon dioxide played a crucial role. In addition,
many scientists believe the evolving atmosphere contained suÛcient quantities of gases like ammonia and methane to give rise to organic matter.

Still, the problem of the sun remains unresolved. One hypothesis holds that during the Archeanera, which lasted from about 4.5 to 2.5
billion years ago, the sunÕs power was
only 75 percent of what it is today. Thispossibility raises a dilemma: How could
life have survived in the relatively cold
climate that should accompany a weaker sun? A solution to the faint early sun
paradox, as it is called, was oÝered byCarl Sagan and George Mullen of Cornell University in 1970. The two scientists suggested that methane and ammonia, which are very eÝective at trapping infrared radiation, were quite abundant. These gases could have created asuper-greenhouse eÝect. The idea wascriticized on the basis that such gases
were highly reactive and have short
lifetimes in the atmosphere.
In the late 1970s Veerabhadran Ra70

CARBONIFEROUS

fossils were more abundantly preserved. As climate shifted,
so did lifeÑsuggesting feedback between the two. The datesof these evolutions remain unclear as well, but their order ismanathan, now at the Scripps Institution of Oceanography, and Robert D.
Cess and Tobias Owen of the State University of New York at Stony Brook proposed another solution. They postulated that there was no need for methane
in the early atmosphere because carbon
dioxide was abundant enough to bringabout the super-greenhouse eÝect.
Again this argument raised a diÝerent
question: How much carbon dioxide
was there in the early atmosphere? Terrestrial carbon dioxide is now buried incarbonate rocks, such as limestone, although it is not clear when it becametrapped there. Today calcium carbonate
is created primarily during biologicalactivity; in the Archean period, carbon
may have been primarily removed during inorganic reactions.
The rapid outgassing of the planet
liberated voluminous quantities of water from the mantle, creating the oceans
and the hydrologic cycle. The acids thatwere probably present in the atmosphere eroded rocks, forming carbonate-rich rocks. The relative importanceof such a mechanism is, however, debated. Heinrich D. Holland of Harvard University believes the amount of carbon
dioxide in the atmosphere rapidly decreased during the Archean and stayed
at a low level.
Understanding the carbon dioxide

SCIENTIFIC AMERICAN October 1994

DEVONIAN
SILURIAN

content of the early atmosphere is pivotal to understanding the mechanismsof climatic control. Two sometimes conßicting camps have put forth ideas on
how this process works. The Þrst groupholds that global temperatures and carbon dioxide were controlled by inorganic geochemical feedbacks; the second
asserts that they were controlled by biological removal.
James C. G. Walker, James F. Kasting
and Paul B. Hays, then at the Universityof Michigan, proposed the inorganic
model in 1981. They postulated thatlevels of the gas were high at the outsetof the Archean and did not fall precipitously. The trio suggested that as the climate warmed, more water evaporated,
and the hydrologic cycle became morevigorous, increasing precipitation andrunoÝ. The carbon dioxide in the atmosphere mixed with rainwater to createcarbonic acid runoÝ, exposing minerals at the surface to weathering. Silicate
minerals combined with carbon that
had been in the atmosphere, sequestering it in sedimentary rocks. Less carbon dioxide in the atmosphere meant,in turn, less of a greenhouse eÝect. The
inorganic negative feedback processoÝset the increase in solar energy.
This solution contrasts with a second
paradigm: biological removal. One theory advanced by James E. Lovelock, an

Copyright 1994 Scientific American, Inc.

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PALEOCENECRETACEOUS

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5

OLIGOCENE
EOCENE

more apparent. First a primordial soup formed, then primitive organisms, such as algae, stromatolites and jellyÞsh,arose; spiny Þsh were followed by the ichthyostega, perhaps

originator of the Gaia hypothesis, assumed that photosynthesizing microorganisms, such as phytoplankton,
would be very productive in a highÐcarbon dioxide environment. These creatures slowly removed carbon dioxidefrom the air and oceans, converting it
into calcium carbonate sediments. Critics retorted that phytoplankton had
not even evolved for most of the timethat the earth has had life. ( The Gaia
hypothesis holds that life on the earth
has the capacity to regulate temperature and the composition of the earthÕs
surface and to keep it comfortable forliving organisms.)
More recently, Tyler Volk of New York
University and David W. Schwartzmanof Howard University proposed another Gaian solution. They noted that bacteria increase carbon dioxide content
in soils by breaking down organic matter and by generating humic acids. Bothactivities accelerate weathering, removing carbon dioxide from the atmosphere.
On this point, however, the controversy
becomes acute. Some geochemists, including Kasting, now at Pennsylvania
State University, and Holland, postulatethat while life may account for somecarbon dioxide removal after the Archean, inorganic geochemical processes
can explain most of the sequestering.
These researchers view life as a rather

24

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MIOCENE

0.01

0.0

HOLOCENE
PLEISTOCENE

the Þrst creature to crawl from ocean onto land. The rest of
the story is well known: dinosaurs appeared and died out,
their place taken by mammals.

weak climatic stabilizing mechanism
for the bulk of geologic time.
The issue of carbon remains criticalto the story of how life inßuenced theatmosphere. Carbon burial is a key tothe vital process of building up atmospheric oxygen concentrationsÑa prerequisite for the development of certain life-forms. In addition, global warming may be taking place now as a resultof humans releasing this carbon. For
one or two billion years, algae in theoceans produced oxygen. But because
this gas is highly reactive and because
there were many reduced minerals in
the ancient oceansÑiron, for example,
is easily oxidizedÑ much of the oxygen
produced by living creatures simply got
used up before it could reach the atmosphere, where it would have encountered gases that would react with it.
Even if evolutionary processes had given rise to more complicated life-forms
during this anaerobic era, they would
have had no oxygen. Furthermore, unÞltered ultraviolet sunlight would havelikely killed them if they left the ocean.
Researchers such as Walker and Preston
Cloud, then at the University of California at Santa Barbara, have suggested
that only about two billion years ago,
after most of the reduced minerals inthe sea were oxidized, did atmospheric
oxygen accumulate. Between one andtwo billion years ago oxygen reachedcurrent levels, creating a niche for
evolving life.
By examining the stability of certain
minerals, such as iron oxide or uranium oxide, Holland has shown that theoxygen content of the Archean atmosphere was low, before two billion years
ago. It is largely agreed that the presentday oxygen content of 20 percent is theresult of photosynthetic activity. Still,the question is whether the oxygen content in the atmosphere increased gradually over time or suddenly. Recent studies indicate that the increase of oxygen
started abruptly between 2.1 and 2.03
billion years ago, and the present situation was reached 1.5 billion years ago.
The presence of oxygen in the atmosphere had another major beneÞt for
an organism trying to live at or above
the surface: it Þltered ultraviolet radiation. Ultraviolet radiation breaks down
many moleculesÑfrom DNA and oxygen to the chloroßuorocarbons that are
implicated in stratospheric ozone depletion. Such energy splits oxygen into
the highly unstable atomic form O,
which can combine back into O2 andinto the very special molecule O3, orozone. Ozone, in turn, absorbs ultraviolet radiation. It was not until oxygen wasabundant enough in the atmosphere toallow the formation of ozone that life

SCIENTIFIC AMERICAN October 1994

71

even had a chance to get a root-hold ora foothold on land. It is not a coincidence that the rapid evolution of life
from prokaryotes (single-celled organisms with no nucleus) to eukaryotes(single-celled organisms with a nucleus) to metazoa (multicelled organisms)
took place in the billion-year-long eraof oxygen and ozone.
Although the atmosphere was reaching a fairly stable level of oxygen during
this period, the climate was hardly uniform. There were long stages of relative
warmth or coolness during the transition to modern geologic time. The composition of fossil plankton shells that
lived near the ocean ßoor provides ameasure of bottom water temperatures.
The record suggests that over the past
100 million years bottom waters cooledby nearly 15 degrees Celsius. Sea levels
dropped by hundreds of meters, andcontinents drifted apart. Inland seas
mostly disappeared, and the climate
cooled an average of 10 to 15 degrees
C. Roughly 20 million years ago permanent ice appears to have built up on
Antarctica.
About two to three million years ago
the paleoclimatic record starts to show
signiÞcant expansions and contractionsof warm and cold periods on 40,000year or so cycles. This periodicity is interesting because it corresponds to thetime it takes the earth to complete an
oscillation of the tilt of its axis of rotation. It has long been speculated, andrecently calculated, that known chang-

ATMOSPHERIC COMPOSITION, shown by the relative concentration of various
gases, has been greatly inßuenced by life on the earth. The early atmosphere had
fairly high concentrations of water and carbon dioxide and, some experts believe,
methane, ammonia and nitrogen. After the emergence of living organisms, the oxygen that is so vital to our survival became more plentiful. Today carbon dioxide,
methane and water exist only in trace amounts in the atmosphere.

ro; and ÒWhat Caused the Mass Extinction? A Volcanic Eruption,Ó by Vincent
E. Courtillot; SCIENTIFIC AMERICAN, October 1990]. It is remarkable that despite violent, episodic perturbations, theclimate has been buÝered enough tosustain life for 3.5 billion years.
One of the most pivotal climatic discoveries of the past 20 years has comefrom ice cores in Greenland and Antarctica. When snow falls on these frozen continents, the air between thesnow grains is trapped as bubbles. The
snow is gradually compressed into ice,along with its captured gases. Some ofthese records can go back as far as
200,000 years; scientists can analyze
the chemical content of ice and bubbles
from sections of ice that lie as deep as
2,000 meters below the surface.
The ice-core borers have determinedthat the air breathed by ancient Egyptians and Anasazi Indians was very
similar to that which we inhale todayÑ
except for a host of air pollutants introduced over the past 100 or 200 years.
Principal among these added gases, or
pollutants, are extra carbon dioxideand methane. The former has increased
25 percent as a result of industrialization and deforestation; the latter has
doubled because of agriculture, land
use and energy production. The concern that increased amounts of thesegases might trap enough heat to cause
global warming is at the heart of theclimate debate [see ÒThe Changing Climate,Ó by Stephen H. Schneider; SCIENTIFIC AMERICAN, September 1989].
The ice cores have shown that sustained natural rates of worldwide temperature change are typically about one
degree C per millennium. These shifts
are still signiÞcant enough to have radically altered where species live and tohave potentially contributed to the extinction of such charismatic megafauna
as mammoths and saber-toothed tigers.
But a most extraordinary story fromthe ice cores is not the relative stabilityof the climate during the past 10,000
years. It appears that during the heightof the last ice age 20,000 years ago
there was between 30 and 40 percent
less carbon dioxide and 50 percent lessmethane in the air than there has been
during our period, the Holocene. ThisÞnding suggests a positive feedback
between carbon dioxide, methane andclimatic change.
The reasoning that supports the ideaof this destabilizing feedback system
goes as follows. When the world wascolder, there was less concentration ofgreenhouse gases, and so less heat wastrapped. As the earth warmed up, carbon dioxide and methane levels increased, accelerating the warming. If

74

Copyright 1994 Scientific American, Inc.

es in orbital geometry could alter theamount of sunlight coming in between
winter and summer by about 10 percent or so and could be responsible for
initiating or ending ice ages.

M

ost interesting and perplexingis the discovery that between
600,000 and 800,000 years
ago the dominant cycle switched from40,000-year periods to 100,000-year intervals with very large ßuctuations. The
last major phase of glaciation ended
about 10,000 years ago. At its height
20,000 years ago, ice sheets a mile thickcovered much of northern Europe andNorth America. Glaciers expanded inhigh plateaus and mountains throughout the world. Enough ice was locked up
on land to cause sea levels to drop morethan 100 meters below where they aretoday. Massive ice sheets scoured theland and revamped the ecological faceof the earth, which was Þve degrees C
cooler on average than it is currently.
The precise causes of these changes
are not yet sorted out. Volcanic eruptions may have played a signiÞcant roleas shown by the eÝect of El Chich—n in
Mexico and Mount Pinatubo in the Philippines. Tectonic events, such as the development of the Himalayas, may inßuence world climate. Even the impact
of comets can inßuence short-term climatic trends with catastrophic consequences for life [see ÒWhat Caused theMass Extinction? An Extraterrestrial Impact,Ó by Walter Alvarez and Frank Asa-

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TIME (BILLIONS OF YEARS AGO)SCIENTIFIC AMERICAN October 1994

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1

life had a hand in this story, it would
have been to drive, rather than to oppose, climatic change. Once again,
though, this picture is incomplete.
Nevertheless, most scientists would
agree that life could well be the principal factor in the positive feedback between climatic change and greenhousegases. One hypothesis suggests that increased nutrient runoÝ from the continental shelves that were exposed assea levels fell fertilized phytoplankton.
This nutrient input could have createda larger biomass of such marine species.
Because the calcium carbonate shell
makes up most of the mass of manyphytoplankton species, increased productivity would remove carbon dioxidefrom the oceans and eventually the atmosphere. At the same time, boreal
forests that account for about 10 to 20
percent of the carbon in the atmosphere
were decimated during the ice ages.
Carbon from these high-latitude forests
could have been released to the atmosphere, yet the atmosphere had less of
this gas then. Thus, the positive feedback system of the oceanÕs biologicalpump may have oÝset the negative
feedback caused by the destruction of
the forests. Great amounts of carbon
can be stored in soils, however, so thedemise of forests may have led to thesequestering of carbon in the ground.
What is signiÞcant is the idea that the
feedback was positive. By studying thetransition from the highÐcarbon dioxide, low-oxygen atmosphere of the Archean to the era of great evolutionary
progress about a half a billion years
ago, it becomes clear that life may havebeen a factor in the stabilization of climate. In another exampleÑduring theice ages and interglacial cyclesÑlife
seems to have the opposite function:
accelerating the change rather than diminishing it. This observation has led
one of us ( Schneider ) to contend thatclimate and life coevolved rather thanlife serving solely as a negative feedback on climate.

If we humans consider ourselves
part of lifeÑthat is, part of the natural systemÑthen it could be argued that our collective impact on theearth means we may have a signiÞcant
coevolutionary role in the future of theplanet. The current trends of population growth, the demands for increased
standards of living and the use of technology and organizations to attain thesegrowth-oriented goals all contribute topollution. When the price of polluting
is low and the atmosphere is used as afree sewer, carbon dioxide, methane,chloroßuorocarbons, nitrous oxides, sulfur oxides and other toxics can build up.

Copyright 1994 Scientific American, Inc.

ICE CORES from Greenland or Antarctica have provided scientists with a swatch
cut from the earthÕs atmospheric history. As snow is compressed into ice, air bubbles trapped between the ßakes are preserved. By analyzing the gases in these tiny
chambers, researchers can determine the composition of the atmosphere up to almost 200,000 years ago.

The theory of heat trappingÑcodiÞed
in mathematical models of the climateÑ
suggests that if carbon dioxide levels
double sometime in the middle of thenext century, the world will warm between one and Þve degrees C. The mildend of that range entails warming at
the rate of one degree per 100 yearsÑa
factor of 10 faster than the one degree
per 1,000 years that has historically
been the average rate of natural change
on a global scale. Should the higher endof the range occur, then we could see
rates of climatic change 50 times fasterthan natural average conditions. Change
at this rate would almost certainly force
many species to attempt to move theirranges, just as they did from the iceageÐinterglacial transition between
10,000 and 15,000 years ago. Not only
would species have to respond to climatic change at rates 10 to 50 times
faster, but few would have undisturbed,
open migration routes as they did atthe end of the ice age and the onset of
the interglacial era. It is for these reasons that it is essential to understandwhether doubling carbon dioxide will

warm the earth by one degree or Þve.
To make the critical projections of future climatic change needed to understand the fate of ecosystems on thisearth, we must dig through land, sea
and ice to uncover as much of the geologic, paleoclimatic and paleoecologicalrecords as we can. These records provide the backdrop against which to calibrate the crude instruments we must
use to peer into a shadowy environmental future, a future increasingly inßuenced by us.

FURTHER READINGHOW TO BUILD A HABITABLE PLANET.
Wallace Broecker. Lamont-Doherty Geological Observatory Press, 1990.
SCIENTISTS ON GAIA. Edited by Stephen
H. Schneider and Penelope J. Boston.
MIT Press, 1991.
FROM STONE TO STAR: A VIEW OF MODERN GEOLOGY. Claude J. All•gre. Harvard University Press, 1992.
EARTHÕS EARLY ATMOSPHERE. James F.
Kasting in Science, Vol. 259, pages 920Ð
926; February 12, 1993.

SCIENTIFIC AMERICAN October 199475