The Evolution of the Earth the Formation of This Planet and Its Atmosphere Gave Rise to Life, Which Shaped the Earth’S Subsequent Development

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 past

by 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 under- going meteor bombardment (left ). At this time, it may have been studded with volcanic islands and shrouded by an atmosphere laden with carbon dioxide and heavy with clouds. Three billion years ago its face may have been obscured by an orange haze of methane, the product of early organisms (center). Today clouds, oceans and continents are clearly discernible (right ). This illustration was pre- pared with the help of James F. Kasting of Pennsylvania State University.

66 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. ike the lapis lazuli gem it resembles, standing the origin of life and, perhaps, the blue, cloud-enveloped planet its future. CLAUDE J. ALLéGRE and STEPHEN H. that we recognize immediately Scientists used to believe the rocky SCHNEIDER study various aspects of the L earthÕs geologic history and its climate. from satellite pictures seems remark- planets, including the earth, Mercury, Ve- All•gre is a professor at the University ably stable. Continents and oceans, en- nus and Mars, were created by the rapid of Paris and directs the department of circled by an oxygen-rich atmosphere, gravitational collapse of a dust cloud, a geochemistry at the Institut de Physique support familiar life-forms. Yet this con- deßation giving rise to a dense orb. In du Globe de Paris. He is a foreign mem- stancy is an illusion produced by the the 1960s the Apollo space program ber of the National Academy of Sciences. human experience of time. The earth changed this view. Studies of moon Schneider is a professor in the depart- and its atmosphere are continuously al- craters revealed that these gouges were ment of biological sciences at Stanford tered. Plate tectonics shift the conti- caused by the impact of objects that University and is a senior fellow at the universityÕs Institute of International nents, raise mountains and move the were in great abundance about 4.5 bil- Studies. He is also a senior scientist at ocean ßoor while processes that no one lion years ago. Thereafter, the number the National Center for Atmospheric Re- fully comprehends alter the climate. of impacts appeared to have quickly de- search in Boulder, Colo. Such constant change has character- creased. This observation rejuvenated ized the earth since its beginning some the theory of accretion postulated by 4.5 billion years ago. From the outset, Otto Schmidt. The Russian geophysi- heat and gravity shaped the evolution cist had suggested in 1944 that plan- meant that it took a long time to build of the planet. These forces were gradu- ets grew in size gradually, step by step. up a large planet. A calculation made ally joined by the global eÝects of the According to Schmidt, cosmic dust by George W. Wetherill of the Carnegie emergence of life. Exploring this past lumped together to form particulates, Institution of Washington suggests that oÝers us the only possibility of under- particulates became gravel, gravel be- about 100 million years could pass became small balls, then big balls, then tween the formation of an object mea- tiny planets, or planetesimals, and, Þ- suring 10 kilometers in diameter and nally, dust became the size of the moon. an object the size of the earth. As the planetesimals became larger, The process of accretion had signif- their numbers decreased. Consequent- icant thermal consequences for the ly, the number of collisions between earth, consequences that have forceful- planetesimals, or meteorites, decreased. ly directed its evolution. Large bodies Fewer items available for accretion slamming into the planet produced im-

Copyright 1992 Scientific American, Inc. mense heat in the interior, melting the stroyed by its very activity. But the de- Isotope geology has permitted geolo- cosmic dust found there. The resulting velopment of isotope geology in the gists to determine that the accretion of furnaceÑsituated some 200 to 400 kilo- 1960s had rendered this view obsolete. the earth culminated in the diÝerentia- meters underground and called a mag- Their imaginations Þred by Apollo and tion of the planet: the creation of the ma oceanÑwas active for millions of the moon Þndings, geochemists began coreÑthe source of the earthÕs magnet- years, giving rise to volcanic eruptions. to apply this technique to understand ic ÞeldÑand the beginning of the at- When the earth was young, heat at the the evolution of the earth. mosphere. In 1953 the classic work of surface caused by volcanism and lava Claire C. Patterson of the California In- ßows from the interior was supplement- ating rocks using so-called ra- stitute of Technology used the uranium- ed by the constant bombardment of dioactive clocks allows geolo- lead clock to establish an age of 4.55 huge planetesimals, some of them per- D gists to work on old terrains that billion years for the earth and many of haps the size of the moon or even Mars. do not contain fossils. The hands of a the meteorites that formed it. Recent No life was possible during this period. radioactive clock are isotopesÑatoms work by one of us (All•gre) on lead iso- Beyond clarifying that the earth had of the same element that have diÝerent topes, however, led to a somewhat new formed through accretion, the Apollo atomic weightsÑand geologic time is interpretation. As Patterson argued, program compelled scientists to try to measured by the rate of decay of one some meteorites were indeed formed reconstruct the subsequent temporal isotope into another [see ÒThe Earliest about 4.56 billion years ago, and their and physical development of the early History of the Earth,Ó by Derek York; debris constituted the earth. But the earth. This undertaking had been con- SCIENTIFIC AMERICAN, January 1993]. earth continued to grow through the sidered impossible by founders of geol- Among the many clocks, those based bombardment of planetesimals until ogy, including Charles Lyell, to whom on the decay of uranium 238 into lead some 120 to 150 million years later. At the following phrase is attributed: No 206 and of uranium 235 into lead 207 that timeÑ4.44 to 4.41 billion years vestige of a beginning, no prospect for are special. Geochronologists can deter- agoÑthe earth began to retain its at- an end. This statement conveys the idea mine the age of samples by analyzing mosphere and create its core. This pos- that the young earth could not be re- only the daughter productÑin this case, sibility had already been suggested by created, because its remnants were de- leadÑof the radioactive parent, uranium. Bruce R. Doe and Robert E. Zartman of the U.S. Geological Survey in Denver a decade ago and is in agreement with WetherillÕs estimates. How the Earth Got Its Core The emergence of the continents came somewhat later. According to the he differentiation of the planet took place quite quickly after the earth theory of plate tectonics, these land- Twas formed by the accretion of cosmic dust and meteorites. About 4.4 masses are the only part of the earthÕs billion years ago the core—which, with the mantle, drives the geothermal crust that is not recycled and, conse- cycle, including volcanism—appeared; gases emerging from the interior of quently, destroyed during the geother- the planet also gave rise to a nascent atmosphere. Somewhat later, although mal cycle driven by the convection in the issue has not been entirely resolved, it seems that continental crust the mantle. Continents thus provide a formed as the various elements segregated into different depths. form of memory because the record of early life can be read in their rocks. The testimony, however, is not extensive. Geologic activity, including plate tectonics, erosion and metamorphism, has R M A N destroyed almost all the ancient rocks. P P E T L E U Very few fragments have survived this LITHOSPHERE geologic machine. ASTHENOSPHERE Nevertheless, in recent years, several important Þnds have been made, again OLIVINE-RICH W E R M A N T L O L E MATERIAL using isotope geochemistry. One group, led by Stephen Moorbath of the Univer- sity of Oxford, discovered terrain in PEROVSKITE-RICH West Greenland that is between 3.7 and ER MAT IAL 3.8 billion years old. In addition, Sam- uel A. Bowring of the Massachusetts In- stitute of Technology explored a small area in North AmericaÑthe Acasta gneissÑthat is 3.96 billion years old. C O R E Ultimately, a quest for the mineral zircon led other researchers to even more ancient terrain. Typically found in continental rocks, zircon is not dis- 4.4 BILLION solved during the process of erosion YEARS AGO but is deposited as particles in sedi- ment. A few pieces of zircon can therefore survive for billions of years and 4.2 BILLION YEARS AGO can serve as a witness to the earthÕs TO PRESENT more ancient crust. The search for old zircons started in Paris with the work of Annie Vitrac and Jo‘l R. Lancelot, now

68 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. at the University of Marseilles and the CONTINENTAL SHIFT has altered the 700 MILLION University of Montpellier, respectively, face of the planet for nearly a billion YEARS AGO as well as with the eÝorts of Moorbath years, as can be seen in the diÝerences and All•gre. It was a group at the Aus- between the positions of the continents EQUATOR tralian National University in Canberra, that we know today and those of 700 million years ago. Pangaea, the superag- directed by William Compston, that was gregate of early continents, came to- Þnally successful. The team discovered gether about 200 million years ago and zircons in western Australia that were then promptly, in geologic terms, broke between 4.1 and 4.3 billion years old. apart. This series was compiled with 600 MILLION Zircons have been crucial not only for the advice of Christopher R. Scotese of YEARS AGO understanding the age of the continents the University of Texas at Arlington. but for determining when life Þrst appeared. The earliest fossils of undis- puted age were found in Australia and lent chronometers. Although the atmo- South Africa. These relics of blue-green sphere was formed by the outgassing algae are about 3.5 billion years old. of the mantle, it does not contain any Manfred Schidlowski of the Max Planck potassium 40 or iodine 129. All argon Institute for Chemistry in Mainz has 40 and xenon 129, formed in the earth 500 MILLION studied the Isua formation in West and released, are found in the atmo- YEARS AGO Greenland and argues that organic mat- sphere today. Xenon was expelled from ter existed as many as 3.8 billion years the mantle and retained in the atmo- ago. Because most of the record of early sphere; therefore, the atmosphere-man- life has been destroyed by geologic ac- tle ratio of this element allows us to tivity, we cannot say exactly when it Þrst evaluate the age of diÝerentiation. Ar- appearedÑperhaps it arose very quick- gon and xenon trapped in the mantle ly, maybe even 4.2 billion years ago. evolved by the radioactive decay of 400 MILLION potassium 40 and iodine 129. Thus, if YEARS AGO ne of the most important as- the total outgassing of the mantle oc- pects of the earthÕs evolution curred at the beginning of the earthÕs O is the formation of the atmo- formation, the atmosphere would not sphere, because it is this assemblage of contain any argon 40 but would con- gases that allowed life to crawl out of tain xenon 129. the oceans and to be sustained. Re- The major challenge facing an inves- searchers have hypothesized since the tigator who wants to measure such ra- 1950s that the terrestrial atmosphere tios of decay is to obtain high concen- 300 MILLION was created by gases emerging from trations of rare gases in mantle rocks YEARS AGO the interior of the planet. When a volca- because they are extremely limited. For- no spews gases, it is an example of the tunately, a natural phenomenon occurs continuous outgassing, as it is called, at mid-oceanic ridges during which vol- of the earth. But scientists have ques- canic lava transfers some silicates from tioned whether this process occurred the mantle to the surface. The small suddenly about 4.4 billion years ago amounts of gases trapped in mantle when the core diÝerentiated or whether minerals rise with the melt to the sur- 200 MILLION it took place gradually over time. face and are concentrated in small vesi- YEARS AGO To answer this question, All•gre and cles in the outer glassy margin of lava his colleagues studied the isotopes of ßows. This process serves to concen- rare gases. These gasesÑincluding heli- trate the amounts of mantle gases by um, argon and xenonÑhave the pecu- a factor of 104 or 105. Collecting these liarity of being chemically inert, that is, rocks by dredging the seaßoor and they do not react in nature with other then crushing them under vacuum in a elements. Two of them are particularly sensitive mass spectrometer allow geo- important for atmospheric studies: ar- chemists to determine the ratios of the 100 MILLION gon and xenon. Argon has three iso- isotopes in the mantle. The results are YEARS AGO topes, of which argon 40 is created by quite surprising. Calculations of the ra- the decay of potassium 40. Xenon has tios indicate that between 80 and 85 nine, of which 129 has two diÝerent percent of the atmosphere was out- origins. Xenon 129 arose as the result gassed in the Þrst one million years; the of nucleosynthesis before the earth and rest was released slowly but constantly solar system were formed. It was also during the next 4.4 billion years. created from the decay of radioactive The composition of this primitive at- iodine 129, which does not exist on the mosphere was most certainly dominat- PRESENT earth anymore. This form of iodine was ed by carbon dioxide, with nitrogen as present very early on but has died the second most abundant gas. Trace since, and xenon 129 has grown at its amounts of methane, ammonia, sulfur expense. dioxide and hydrochloric acid were also Like most couples, both argon 40 and present, but there was no oxygen. Ex- potassium 40 and xenon 129 and iodine cept for the presence of abundant wa- 129 have stories to tell. They are excel- ter, the atmosphere was similar to that

WARMER THAN PRESENT

COLDER THAN PRESENT

PRECAMBRIAN ORDOVICIAN DEVONIAN CAMBRIAN SILURIAN CARBONIFEROUS

CLIMATE FLUCTUATIONS are apparent over time. Although fossils were more abundantly preserved. As climate shifted, the earthÕs early temperature record is quite uncertain, good so did lifeÑsuggesting feedback between the two. The dates estimates can be made starting 400 million years ago, when of these evolutions remain unclear as well, but their order is

of Venus or Mars. The details of the manathan, now at the Scripps Institu- content of the early atmosphere is piv- evolution of the original atmosphere tion of Oceanography, and Robert D. otal to understanding the mechanisms are debated, particularly because we do Cess and Tobias Owen of the State Uni- of climatic control. Two sometimes con- not know how strong the sun was at versity of New York at Stony Brook pro- ßicting camps have put forth ideas on that time. Some facts, however, are not posed another solution. They postulat- how this process works. The Þrst group disputed. It is evident that carbon diox- ed that there was no need for methane holds that global temperatures and car- ide played a crucial role. In addition, in the early atmosphere because carbon bon dioxide were controlled by inorgan- many scientists believe the evolving at- dioxide was abundant enough to bring ic geochemical feedbacks; the second mosphere contained suÛcient quanti- about the super-greenhouse eÝect. asserts that they were controlled by bi- ties of gases like ammonia and meth- Again this argument raised a diÝerent ological removal. ane to give rise to organic matter. question: How much carbon dioxide James C. G. Walker, James F. Kasting was there in the early atmosphere? Ter- and Paul B. Hays, then at the University till, the problem of the sun re- restrial carbon dioxide is now buried in of Michigan, proposed the inorganic mains unresolved. One hypothe- carbonate rocks, such as limestone, al- model in 1981. They postulated that Ssis holds that during the Archean though it is not clear when it became levels of the gas were high at the outset era, which lasted from about 4.5 to 2.5 trapped there. Today calcium carbonate of the Archean and did not fall precipi- billion years ago, the sunÕs power was is created primarily during biological tously. The trio suggested that as the cli- only 75 percent of what it is today. This activity; in the Archean period, carbon mate warmed, more water evaporated, possibility raises a dilemma: How could may have been primarily removed dur- and the hydrologic cycle became more life have survived in the relatively cold ing inorganic reactions. vigorous, increasing precipitation and climate that should accompany a weak- The rapid outgassing of the planet runoÝ. The carbon dioxide in the atmo- er sun? A solution to the faint early sun liberated voluminous quantities of wa- sphere mixed with rainwater to create paradox, as it is called, was oÝered by ter from the mantle, creating the oceans carbonic acid runoÝ, exposing miner- Carl Sagan and George Mullen of Cor- and the hydrologic cycle. The acids that als at the surface to weathering. Silicate nell University in 1970. The two sci- were probably present in the atmo- minerals combined with carbon that entists suggested that methane and am- sphere eroded rocks, forming carbon- had been in the atmosphere, sequester- monia, which are very eÝective at trap- ate-rich rocks. The relative importance ing it in sedimentary rocks. Less car- ping infrared radiation, were quite abun- of such a mechanism is, however, debat- bon dioxide in the atmosphere meant, dant. These gases could have created a ed. Heinrich D. Holland of Harvard Uni- in turn, less of a greenhouse eÝect. The super-greenhouse eÝect. The idea was versity believes the amount of carbon inorganic negative feedback process criticized on the basis that such gases dioxide in the atmosphere rapidly de- oÝset the increase in solar energy. were highly reactive and have short creased during the Archean and stayed This solution contrasts with a second lifetimes in the atmosphere. at a low level. paradigm: biological removal. One the- In the late 1970s Veerabhadran Ra- Understanding the carbon dioxide ory advanced by James E. Lovelock, an

PERMIAN JURASSIC PALEOCENE OLIGOCENE PLIOCENE HOLOCENE TRIASSIC CRETACEOUS EOCENE MIOCENE PLEISTOCENE

more apparent. First a primordial soup formed, then primi- the Þrst creature to crawl from ocean onto land. The rest of tive organisms, such as algae, stromatolites and jellyÞsh, the story is well known: dinosaurs appeared and died out, arose; spiny Þsh were followed by the ichthyostega, perhaps their place taken by mammals.

originator of the Gaia hypothesis, as- weak climatic stabilizing mechanism two billion years ago oxygen reached sumed that photosynthesizing micro- for the bulk of geologic time. current levels, creating a niche for organisms, such as phytoplankton, The issue of carbon remains critical evolving life. would be very productive in a highÐcar- to the story of how life inßuenced the By examining the stability of certain bon dioxide environment. These crea- atmosphere. Carbon burial is a key to minerals, such as iron oxide or urani- tures slowly removed carbon dioxide the vital process of building up atmo- um oxide, Holland has shown that the from the air and oceans, converting it spheric oxygen concentrationsÑa pre- oxygen content of the Archean atmo- into calcium carbonate sediments. Crit- requisite for the development of cer- sphere was low, before two billion years ics retorted that phytoplankton had tain life-forms. In addition, global warm- ago. It is largely agreed that the present- not even evolved for most of the time ing may be taking place now as a result day oxygen content of 20 percent is the that the earth has had life. (The Gaia of humans releasing this carbon. For result of photosynthetic activity. Still, hypothesis holds that life on the earth one or two billion years, algae in the the question is whether the oxygen con- has the capacity to regulate tempera- oceans produced oxygen. But because tent in the atmosphere increased grad- ture and the composition of the earthÕs this gas is highly reactive and because ually over time or suddenly. Recent stud- surface and to keep it comfortable for there were many reduced minerals in ies indicate that the increase of oxygen living organisms.) the ancient oceansÑiron, for example, started abruptly between 2.1 and 2.03 More recently, Tyler Volk of New York is easily oxidizedÑ much of the oxygen billion years ago, and the present situa- University and David W. Schwartzman produced by living creatures simply got tion was reached 1.5 billion years ago. of Howard University proposed anoth- used up before it could reach the atmo- The presence of oxygen in the atmo- er Gaian solution. They noted that bac- sphere, where it would have encoun- sphere had another major beneÞt for teria increase carbon dioxide content tered gases that would react with it. an organism trying to live at or above in soils by breaking down organic mat- Even if evolutionary processes had giv- the surface: it Þltered ultraviolet radia- ter and by generating humic acids. Both en rise to more complicated life-forms tion. Ultraviolet radiation breaks down activities accelerate weathering, remov- during this anaerobic era, they would many moleculesÑfrom DNA and oxy- ing carbon dioxide from the atmosphere. have had no oxygen. Furthermore, un- gen to the chloroßuorocarbons that are On this point, however, the controversy Þltered ultraviolet sunlight would have implicated in stratospheric ozone de- becomes acute. Some geochemists, in- likely killed them if they left the ocean. pletion. Such energy splits oxygen into cluding Kasting, now at Pennsylvania Researchers such as Walker and Preston the highly unstable atomic form O, State University, and Holland, postulate Cloud, then at the University of Califor- which can combine back into O2 and that while life may account for some nia at Santa Barbara, have suggested into the very special molecule O3, or carbon dioxide removal after the Ar- that only about two billion years ago, ozone. Ozone, in turn, absorbs ultravio- chean, inorganic geochemical processes after most of the reduced minerals in let radiation. It was not until oxygen was can explain most of the sequestering. the sea were oxidized, did atmospheric abundant enough in the atmosphere to These researchers view life as a rather oxygen accumulate. Between one and allow the formation of ozone that life

Copyright 1994 Scientific American, Inc. SCIENTIFIC AMERICAN October 1994 71 even had a chance to get a root-hold or es in orbital geometry could alter the ro; and ÒWhat Caused the Mass Extinc- a foothold on land. It is not a coinci- amount of sunlight coming in between tion? A Volcanic Eruption,Ó by Vincent dence that the rapid evolution of life winter and summer by about 10 per- E. Courtillot; SCIENTIFIC AMERICAN, Oc- from prokaryotes (single-celled organ- cent or so and could be responsible for tober 1990]. It is remarkable that de- isms with no nucleus) to eukaryotes initiating or ending ice ages. spite violent, episodic perturbations, the (single-celled organisms with a nucle- climate has been buÝered enough to us) to metazoa (multicelled organisms) ost interesting and perplexing sustain life for 3.5 billion years. took place in the billion-year-long era is the discovery that between One of the most pivotal climatic dis- of oxygen and ozone. M 600,000 and 800,000 years coveries of the past 20 years has come Although the atmosphere was reach- ago the dominant cycle switched from from ice cores in Greenland and Ant- ing a fairly stable level of oxygen during 40,000-year periods to 100,000-year in- arctica. When snow falls on these fro- this period, the climate was hardly uni- tervals with very large ßuctuations. The zen continents, the air between the form. There were long stages of relative last major phase of glaciation ended snow grains is trapped as bubbles. The warmth or coolness during the transi- about 10,000 years ago. At its height snow is gradually compressed into ice, tion to modern geologic time. The com- 20,000 years ago, ice sheets a mile thick along with its captured gases. Some of position of fossil plankton shells that covered much of northern Europe and these records can go back as far as lived near the ocean ßoor provides a North America. Glaciers expanded in 200,000 years; scientists can analyze measure of bottom water temperatures. high plateaus and mountains through- the chemical content of ice and bubbles The record suggests that over the past out the world. Enough ice was locked up from sections of ice that lie as deep as 100 million years bottom waters cooled on land to cause sea levels to drop more 2,000 meters below the surface. by nearly 15 degrees Celsius. Sea levels than 100 meters below where they are The ice-core borers have determined dropped by hundreds of meters, and today. Massive ice sheets scoured the that the air breathed by ancient Egyp- continents drifted apart. Inland seas land and revamped the ecological face tians and Anasazi Indians was very mostly disappeared, and the climate of the earth, which was Þve degrees C similar to that which we inhale todayÑ cooled an average of 10 to 15 degrees cooler on average than it is currently. except for a host of air pollutants intro- C. Roughly 20 million years ago perma- The precise causes of these changes duced over the past 100 or 200 years. nent ice appears to have built up on are not yet sorted out. Volcanic erup- Principal among these added gases, or Antarctica. tions may have played a signiÞcant role pollutants, are extra carbon dioxide About two to three million years ago as shown by the eÝect of El Chich—n in and methane. The former has increased the paleoclimatic record starts to show Mexico and Mount Pinatubo in the Phil- 25 percent as a result of industrializa- signiÞcant expansions and contractions ippines. Tectonic events, such as the de- tion and deforestation; the latter has of warm and cold periods on 40,000- velopment of the Himalayas, may in- doubled because of agriculture, land year or so cycles. This periodicity is in- ßuence world climate. Even the impact use and energy production. The con- teresting because it corresponds to the of comets can inßuence short-term cli- cern that increased amounts of these time it takes the earth to complete an matic trends with catastrophic conse- gases might trap enough heat to cause oscillation of the tilt of its axis of rota- quences for life [see ÒWhat Caused the global warming is at the heart of the tion. It has long been speculated, and Mass Extinction? An Extraterrestrial Im- climate debate [see ÒThe Changing Cli- recently calculated, that known chang- pact,Ó by Walter Alvarez and Frank Asa- mate,Ó by Stephen H. Schneider; SCIEN- TIFIC AMERICAN, September 1989]. The ice cores have shown that sus- 100 tained natural rates of worldwide temperature change are typically about one METHANE, AMMONIA degree C per millennium. These shifts

) are still signiÞcant enough to have rad- T S

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C have potentially contributed to the ex- I R R

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P ATMOSPHERE V (

as mammoths and saber-toothed tigers.

F UNKNOWN S O E But a most extraordinary story from

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A the ice cores is not the relative stability O 50 NITROGEN I G

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O 25 less carbon dioxide and 50 percent less M C T methane in the air than there has been A CARBON DIOXIDE OXYGEN during our period, the Holocene. This Þnding suggests a positive feedback between carbon dioxide, methane and 0 climatic change. 4.5 4 3 2 1 The reasoning that supports the idea TIME (BILLIONS OF YEARS AGO) of this destabilizing feedback system goes as follows. When the world was ATMOSPHERIC COMPOSITION, shown by the relative concentration of various gases, has been greatly inßuenced by life on the earth. The early atmosphere had colder, there was less concentration of fairly high concentrations of water and carbon dioxide and, some experts believe, greenhouse gases, and so less heat was methane, ammonia and nitrogen. After the emergence of living organisms, the oxy- trapped. As the earth warmed up, car- gen that is so vital to our survival became more plentiful. Today carbon dioxide, bon dioxide and methane levels in- methane and water exist only in trace amounts in the atmosphere. creased, accelerating the warming. If

74 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. life had a hand in this story, it would have been to drive, rather than to op- pose, 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 greenhouse gases. One hypothesis suggests that increased nutrient runoÝ from the continental shelves that were exposed as sea levels fell fertilized phytoplankton. This nutrient input could have created a larger biomass of such marine species. Because the calcium carbonate shell makes up most of the mass of many phytoplankton species, increased pro- ductivity would remove carbon dioxide from 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 biological pump may have oÝset the negative feedback caused by the destruction of the forests. Great amounts of carbon can be stored in soils, however, so the demise of forests may have led to the sequestering of carbon in the ground. ICE CORES from Greenland or Antarctica have provided scientists with a swatch What is signiÞcant is the idea that the cut from the earthÕs atmospheric history. As snow is compressed into ice, air bub- feedback was positive. By studying the bles trapped between the ßakes are preserved. By analyzing the gases in these tiny transition from the highÐcarbon diox- chambers, researchers can determine the composition of the atmosphere up to al- ide, low-oxygen atmosphere of the Ar- most 200,000 years ago. chean to the era of great evolutionary progress about a half a billion years ago, it becomes clear that life may have The theory of heat trappingÑcodiÞed warm the earth by one degree or Þve. been a factor in the stabilization of cli- in mathematical models of the climateÑ To make the critical projections of fu- mate. In another exampleÑduring the suggests that if carbon dioxide levels ture climatic change needed to under- ice ages and interglacial cyclesÑlife double sometime in the middle of the stand the fate of ecosystems on this seems to have the opposite function: next century, the world will warm be- earth, we must dig through land, sea accelerating the change rather than di- tween one and Þve degrees C. The mild and ice to uncover as much of the geo- minishing it. This observation has led end of that range entails warming at logic, paleoclimatic and paleoecological one of us (Schneider) to contend that the rate of one degree per 100 yearsÑa records as we can. These records pro- climate and life coevolved rather than factor of 10 faster than the one degree vide the backdrop against which to cal- life serving solely as a negative feed- per 1,000 years that has historically ibrate the crude instruments we must back on climate. been the average rate of natural change use to peer into a shadowy environ- on a global scale. Should the higher end mental future, a future increasingly in- f we humans consider ourselves of the range occur, then we could see ßuenced by us. part of lifeÑthat is, part of the nat- rates of climatic change 50 times faster I ural systemÑthen it could be ar- than natural average conditions. Change gued that our collective impact on the at this rate would almost certainly force FURTHER READING earth means we may have a signiÞcant many species to attempt to move their HOW TO BUILD A HABITABLE PLANET. coevolutionary role in the future of the ranges, just as they did from the ice Wallace Broecker. Lamont-Doherty Geo- planet. The current trends of popula- ageÐinterglacial transition between logical Observatory Press, 1990. tion growth, the demands for increased 10,000 and 15,000 years ago. Not only SCIENTISTS ON GAIA. Edited by Stephen standards of living and the use of tech- would species have to respond to cli- H. Schneider and Penelope J. Boston. nology and organizations to attain these matic change at rates 10 to 50 times MIT Press, 1991. growth-oriented goals all contribute to faster, but few would have undisturbed, FROM STONE TO STAR: A VIEW OF MOD- pollution. When the price of polluting open migration routes as they did at ERN GEOLOGY. Claude J. All•gre. Har- is low and the atmosphere is used as a the end of the ice age and the onset of vard University Press, 1992. EARTHÕS EARLY ATMOSPHERE. James F. free sewer, carbon dioxide, methane, the interglacial era. It is for these rea- Kasting in Science, Vol. 259, pages 920Ð chloroßuorocarbons, nitrous oxides, sul- sons that it is essential to understand 926; February 12, 1993. fur oxides and other toxics can build up. whether doubling carbon dioxide will