No Longer Willful, GaiaBecomes Respectable

The Gaia hypothesis, that Earth is a single huge organismintentionally creating an optimum environmentfor itself; has

been made more palatable; interesting science is coming of it

San DiegoG AUA is dead. Even her creator has

rejected this concept of a superor-ganism striving to create and main-

tain conditions perfectly suited to the flour-ishing of life. Her final demise came lastmonth at the first establishment-sponsoredmeeting to focus on her, but, phoenix-like, anew Gaia rose up, stripped of the original'sintention and purpose but still holding swayover Earth.

Eco-extremists had loved the old Gaia,which had been named after the Greekgoddess of Earth, but mainstream scientistsnever would get close to her. The new Gaiais little more appealing to most scientiststhan the old, but the existence of a majorGaia conference sponsored by the AmericanGeophysical Union and attended by numer-ous prominent researchers points up thegrowing interest in the interaction betweenlife and nonlife. Life may not control cli-mate, for example, intentionally or bychance, but it surely participates in the cli-mate system, everyone agrees. Life alsoclearly affects the composition ofthe sea andthe air. Recent discoveries prompted by theGaia hypothesis may even lead to the eluci-dation of new links between life and itssurroundings.

James Lovelock, Gaia's creator, felt com-pelled to hypothesize a complex of control-ling interactions between life and nonlifewhen he considered how remarkably welllife has fared. "The climate and the chemicalproperties ofthe Earth now and throughoutits history," he wrote in his recently reprint-ed 1979 book, "seem always to have beenoptimal for life." This relative stability per-sisted despite the sun brightening 30% sinceEarth formed, volcanoes spewing acid foreons, and green plants polluting the atmo-sphere with toxic oxygen, among other ap-parent abuses. "For this to have happenedby chance is as unlikely as to survive un-scathed a drive blindfold through rush-hourtraffic," Lovelock claimed.

Because the serendipitous interactions ofchemistry and physics would presumably failto ward off the lethal extremes of ice, corro-sion, and poisoning, Lovelock proposed

22 APRIL I988

that, like the human body, life as a wholemaintains optimum conditions for itself, atendency called homeostasis. The sum totalof the living and nonliving components ofthis system Lovelock called Gaia. (The namehas been pronounced guy'a, as in classicalGreek.)The proper study of Earth would thus be

geophysiology, the dissection of the numer-ous circuits through which informationflows in both directions as the living entityof Earth senses changes and directs adjust-ments to counter those changes. As the sunwarmed over the eons, for example, lifewould have intervened. It would have with-drawn from the atmosphere the carbon di-oxide that, through the greenhouse effect,had been warming Earth above the freezingpoint but would threaten to boil the seasunder a warmer sun.

Control of carbon dioxide would be justone ofmany loops through which climate isstill controlled by and for the biosphere,according to the Gaia hypothesis. Therewould be more webs of feedbacks control-ling the composition of seawater, so that itwould not become too salty, and the com-position of the atmosphere, so that its con-tent of oxygen, among a number of gases,would remain within an optimal range.

Gaia had been largely ignored by main-stream scientists for almost 20 years, but itstarted taking some hard knocks soon afterLovelock's engaging introductory talk Mon-day morning. However, by the discussionperiod Monday afternoon Lovelock had al-ready retracted what became known as thestrong version of the Gaia hypothesis.Well into this first discussion, someone

sensed confusion as to what exactly wasbeing debated and asked for a statement ofthe hypothesis. The Gaia hypothesis, Love-lock replied, holds "that the nonliving andliving represent a self-regulating system thatkeeps itself in a constant state," or at leastwithin a limited range of conditions. Butyou said nothing about homeostasis by andfor the biosphere, his questioner noted.Lovelock agreed. "Our thoughts haveevolved over the last 20 years," he said. "Inthe early stages one tended to speak poetical-

S:2

0r

The father of Gala. James Lovelock,PhD. in medicine, chemist, inventor, andindependent scientist, works out of his home'sbarn-turned-laboratory at Coombe Mill, St.Giles on the Heath, Launceston, Cornwall.

ly. I hope that we are now speaking morescientifically."Without life, would Earth's climate be

wildly out of control?, asked James Kastingof the National Aeronautics and Space Ad-ministration's Ames Research Center. Yes, itwould be, replied Lovelock. Lynn Margulisof Boston University, the staunchest Ameri-can proponent of Gaia, concurred. Are ex-tinction crises, in which 50 or 70% ofspecies disappear, within the limits of Gaia?"Well within," said Lovelock, "the persis-tence of life shows that."Even as the new Gaia hypothesis was

emerging, James Kirchner and John Harteof the University of California at Berkeley,speaking at the Tuesday night session onepistemology, were destroying the old Gaiaand calling into question the whole range ofGaia hypotheses. The traditional, teleologi-cal, strong hypothesis is simply not testable,they said, while the weaker forms, in whichlife merely influences the environment, areso obviously correct that they do not meritstatus as hypotheses.

Lovelock conceded immediately that thestrong Gaia must be abandoned. Kirchner'sanalysis was "a clear-cut demolition ofGaia," he said. "I'm an inventor, one moreintuitive than rational. Being an inventor,it's been very hard to explain what I want to.Most of [Kirchner's] criticisms are of whatwe said a long time ago. We've been at thisfor 20 years."

In an informal session the next day duringa break, Kirchner attacked that defense astrue but inadequate, saying that "you'vechanged, but your old ideas are still outthere. There are people who still thinkyou mean what you said. If you came outin print and said you don't mean that any-

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more, we might get somewhere."It may not be in print yet, but the current

Lovelock-Margulis version of the Gaia hy-pothesis would seem to hold that life hascontrolled its environment within limits nar-row enough that life continued. This is a"homeostatic Gaia," in Kirchner's terms,devoid of purposefulness but still powerful.A vocal contingent of mainstream re-

searchers at the meeting denied that there isany evidence of or even need for such ahomeostatic Gaia; simple, mindless chemis-try and physics can suffice. A central focus ofthis debate was the control of atmosphericcarbon dioxide, a trace gas that plays a majorrole in Earth's greenhouse.Carbon dioxide was probably not always

such a minor component ofthe atmosphere.In Earth's youth the planet was in dire needof a blanket of carbon dioxide, perhaps athousand times heavier than the present one,to hold in the warmth of the feeble sun thatshone then. During the solar system's first 2billion years, Earth would have been frozenover without a strong carbon dioxide green-house; at present, such a greenhouse wouldleave Earth a steamy inferno. That is theparadox of the faint young sun.What Earth needed was not only an early

source ofwarmth but also a way ofgraduallytuning down that warmth in response tothe brightening of the sun. Advocates ofGaia have pointed to the seemingly aggres-sive way the biosphere participates today inthe cycling of carbon dioxide-depositinggreat masses of calcium carbonate shells aslimestone and breaking down the veryrocks. Surely, they argued, such a biospheremust control atmospheric carbon dioxide.No, answered the geophysicists at the

meeting, a mechanism involving material asinanimate as rock and water could make itwork. According to a mechanism suggestedby James Walker and Paul Hays of theUniversity of Michigan and Kasting, a cy-cling of carbon dioxide involving the mo-tion of tectonic plates and the dissolution ofrock by acidified water could have main-tained habitable conditions as the sunbrightened. At one end of the cycle, platetectonics would have kept Earth's share ofcarbon dioxide from being entirely lockedup as calcium carbonate deposits; no controlmechanism could exist if there were nocarbon dioxide gas to work with. But in aworld of drifting and sinking plates likeEarth, a limestone deposit need not be theend of the line. Calcium carbonate depositscan be carried down toward the hot mantlewith the plates as they sink into deep-seatrenches. There some of the carbon dioxideis cooked out of carbonate rocks and re-tumed to the atmosphere through volca-noes.

The other end of the cycle provides thefeedback mechanism, the thermostat in thiscase. Atmospheric carbon dioxide dissolvesin rainwater, forms carbonic acid, and dis-solves calcium-containing silicate rock. Thefreed calcium and neutralized acid is lost tothe sea. This is chemical weathering. If thetemperature rises for any reason, chemicalweathering accelerates, but ocean water alsoevaporates faster, and the rain falls faster.The more rain, the more carbon dioxide isremoved to be lost through even fasterweathering. With carbon dioxide decreas-ing, the temperature rise is countered. Atemperature drop has the opposite effect,slowing the removal of carbon dioxide,which allows volcanoes to increase atmo-spheric carbon dioxide and thus tempera-ture.

'...there is nocompelling reason tosuppose that thebisphere controls thewhole system."This negative feedback between tempera-

ture and atmospheric carbon dioxide-whether temperature goes up or down car-bon dioxide counters the change-can oper-ate in the absence of life. Those inclinedtoward Gaian mechanisms pointed out thatplants greatly accelerate the rate ofweather-ing and thus the removal of carbon dioxide.Plants do this by increasing soil concentra-tions of carbon dioxide up to 40 times overthat now in the atmosphere. Perhaps thatcould provide a useful feedback, it wasargued.

Tyler Volk of New York University con-structed a global model with such a feedbackin it. It includes plants in its chemical weath-ering because they not only lead to carbondioxide in the soil, but they also respond tochanges in atmospheric carbon dioxide,which makes for a feedback loop. Volkfound that a doubling of plant productivityunder increased carbon dioxide could leadto a cooling of 1C. There being practicallimits on plant productivity, he noted, thisfeedback could be significant but not domi-nant.That is the way much of the week went-

the "Earth is alive" camp, the principalsbeing Lovelock, Margulis, and AndrewWatson of the Marine Biological Associa-tion's laboratory at Plymouth, England,suggesting that life's obvious influence onits surroundings must extend to control aswell, while the geochemists insisted that

they have found no need to invoke life as adominant force. Even in the case of oxygen,which is solely a product of photosynthesis,control resides in a balance of inorganicprocesses, according to this tinking.

Kasting and Heinrich Holland of Har-vard University summed up the anti-Gaiasentiment in a joint statement of their own:'The biosphere is one of several factors thataffect the composition of the ocean andatmosphere. Its effects are more importantin some cases than others, but there is nocompelling reason to suppose that the bio-sphere controls the whole system." By thisthinking, life has done so well against somany obstacles because it readily adapts toconditions as it finds them, not because itexerts control over them.

Gaia, even in its more modest guise, mayhave failed to entice the geochemists, but anumber of researchers expressed some em-pathy for the concept, especially those deal-ing with the trace gases of the atmosphere.In the trace gas discussion period, RalphCicerone, a prominent researcher from theNational Center for Atmospheric Research(NCAR) in Boulder, commented that "I'vebeen stimulated so much by Lovelock's andMargulis's work the past 10 years."One of Cicerone's interests has been

methane. Seventy percent of the methaneproduced comes from microorganisms. It isa greenhouse gas, and through the web ofreactions linking atmospheric trace chemi-cals it influences a variety of processes, frompollutant removal and stratospheric ozonedestruction to the fate ofoxygen. The possi-bilities for life-nonlife interactions are obvi-ously numerous.Another area where "the Gaian view raises

some interesting questions," Cicerone said,is atmospheric nitrogen. If chemistry had itsway, that gas would be transformed tonitrate dissolved in the ocean, its most stablestate, leaving Earth with precious little at-mospheric pressure. Denitifying bacteriareturn nitrate-nitrogen to its gaseous state,saving the day in the Gaian view, but Cice-rone is as yet unsure whether that is thewhole story.Another trace gas chemist admitting to

some Gaian tinking was Peter Liss of theUniversity of East Anglia. Lovelock "is thestimulus of many of the measurements thatmy group has made," he noted. Liss's cur-rent interest is dimethylsulfide or DMS.The recognition ofDMS as the geochemi-

cal link in a possible feedback loop betweenlife in the sea and climate is probably themost respectable and most promising prod-uct of the Gaian approach to understandingEarth. The story of DMS illustrates whatGaia, even as an overstatement, has donebest-stimulate productive offbeat thinking

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and cross-discipline contacts.Lovelock started things offwhen he took

his latest invention to sea in 1971 in searchofDMS. His electron-capture detector, nowin wide use in everything from. pesticide-analyzing gas chromatographs to explosives-sniffing bomb detectors, is an exquisitelysensitive tool for studying DMS. Lovelockand his colleagues suspected DMS producedby phytoplankton was the missing marinesource of sulfur for the land. He found theDMS in seawater, which was later found tobe ubiquitous by Meinrat Andreae of theMax Planck Institute in Mainz. But whatwas it doing in the middle of the ocean? ByGaian thinking it presumably has a largerpurpose.

Lovelock was stymied until 1984 when hevisited the University ofWashington to givea taLk. He remarked to Robert Charlson, aspecialist in atmospheric particles and cloudphysics, that 'your colleagues are giving mea hard time because I have no mechanism"for linking DMS and the sulfate aerosol thatit forms in the atmosphere to climate.

Charlson confirmed that the aerosol alonewould not suffice, but he came up with anamnplifier that would make it work. Thesubmicrometer aerosol could form douddroplets that are 100 times larger than theoriginal aerosol and thus influence the re-flectivity of marine douds. The more reflec-tive the clouds, the cooler the surface, whichcould dose the loop by affecting the produc-tion ofDMS (see box).The four principals now involved-Charl-

son, Lovelock, Andreae, and Stephen War-ren, also at the University ofWashington-then published a paper proposing that theDMS-climate link has tended to cool Earth,perhaps against the brightening sun or in-creasing greenhouse. If DMS productionwere to cease, Earth might warm by severaldegrees, they said. But among their assump-tions was one that the reflectivity of marinedouds is sensitive enough to changes in thenumber of fine aerosol partides that canform doud droplets. Calculations indictedthat it was, but Charlson was suggesting inhis talks that it should be demonstratedexperimentally by adding DMS or aerosolpartides to marine air and looking forchanges in cloud reflectivity.At one talk, James Coakley of NCAR

stood to say that the experiment had alreadybeen done, with positive results. Satelliteimages had revealed long plumes of doudintensified by the addition of smoke parti-des from ship stacks. Marine douds canbecome brighter ifthey have more partides.The ripples from Lovelock's 1971 detec-

tion of DMS are still spreading. Severalmeeting attendees made intentionally vaguemention of a forthcoming paper by Michel

22 APRIL I988

Legrand and Robert Delmas of the Labora-tory of Glaciology and Geophysics of theEnvironment in Grenoble. They reportedlyhave found a record of a DMS proxy, meth-ylsulfonic acid, in an Antarctic ice core thatindicates above-average production duringthe most recent ice age, at least near Antarc-tica. If true, this feedback loop would seemto enhance ice ages. Lovelock suggestedwith a smile that Gaia might thus prefer acooler climate than the present one.The DMS-dlimate link has some hurdles

to surmount. Most important will be sortingout the nature of the so-called "Gaianswitch," if any, connecting changes in solarradiation reaching the surface and changesin the production of DMS. The direction,much less the magnitude, of the change is

totally unknown. But the great attraction ofthis proposed feedback is its testability.Identifying testable Gaian hypotheses,which have been in painfully short supply,was the primary purpose of the meeting.Now that Gaia has been cloaked in morefashionable garb, there should be more test-able links between the living and the nonliv-ing worlds. * RICHARD A KERRR

ADDITIONAL READING

R. Charlson, J. Lovelock, M. Andreae, S. Warren,"Oceanic phytoplanion, atmospheric sulfur, cloud albe-do and climate," Nature 326, 655 (1987).

J. Kasting, 0. Toon, J. Pollack, "How climate evolvedon the terrestrial planets," Sci. Am. 258, 90 (Feb. 1988).

L. Lovelock, Gaia: A New Look at Lipf on Earth(Oxford Univ. Press, Oxford, 1979; reprinted with anew preface, 1987).

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interesting science is coming of itpalatable;organism intentionally creating an optimum environment for itself; has been made more

No Longer Willful, Gaia Becomes Respectable: The Gaia hypothesis, that Earth is a single huge

RICHARD A. KERR

DOI: 10.1126/science.240.4851.393 (4851), 393-395.240Science 

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