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THE GAlA HYPOTHESIS' CAN IT BE TESTED?

James w. KirchnerEnergy and Resources GroupUniversity of California

Berkeley

Abstract. The Gaia hypothesis' entral heme s thatbiological processes homeostatically maintain, on aplanetary scale, geochemical and climatic conditionsfavorable or life. A number of distinct hypotheses avebeen proposed, panning range rom the self-evident othe highly speculative. The self-evident orms of Gaiareiterate he well-documented act hat biological rocessesare critical to biogeochemical cycles, adding thestraightforward though mportan0 oint hat he coupling

between biotic and physical processes should createfeedback oops. The speculative orms of Gaia assert hatbiological processes egulate he physical environment,keeping Earth's climate and surface eochemistry tableand favorable or life. As metaphors, hese versions ofGaia are ntriguing, ntestable, nd, f taken iterally as abasis or research, otentially misleading. As hypotheses,they are ll-defined, unparsimonious, nd unfalsifiable.

INTRODUCTION

"Geology is a particularly alluring field for prematureattempts at the explanation of imperfectly understooddata." (J. D. Dana)

"Geology has to choose etween he rashness f usingimperfect evidence or the sterility of uncorrelated,unexplained acts." (J. W. Gregory)

(quoted y Van Houten 1961, p. 89])

It has been lear or some ime hat biological rocessesare crucial actors n the Earth's surface geochemistry.When Huxley [1877] wrote his textbook n physicalgeography, e could describe he role of green plants nlimiting atmospheric O2 concentrations s a matter ofestablished act. By the time Hutchinson 1954] reviewedthe biogeochemistry f the terrestrial tmosphere, e couldadd CH , N2, NH3,NH4, N20 NO2, H2,8042-, ndformaldehyde o the list of biogenic or biologicallycontrolled tmospheric ubstances. arvey 1957] devotedtwo chapters o biological nfluences n oceanic hemistry.Since he 1950s, fforts on two fronts unraveling ontem-porary biogeochemical ycles nd racing he geophysical,geochemical, nd biological evolution of paleoclimate)have produced n explosion of new insights oo vast toadequately document here; reviews can be found byHolland [1978, 1984], Schopf [1983], and others.Following Berger et al. [1984] we can ist among he most

important nd widely recognized iologically mediatedprocesses hotosynthesis; erobic and anaerobic metabo-lism; denitrification; itrogen ixation; bacterial sulfatereduction nd iron oxidation; mineral dissolution yCO2-enriched roundwater; edimentation f organiccarbon, carbonates, silicates, phosphates, and othermaterials; nd changes n surface lbedo produced ygrowth and decay of vegetation. These processes reateimportant eedback oops linking natural selection nd

evolution within the biota and biologically nducedchanges n the physical environment Cloud, 1968;Schneider ndLonder, 984]. Perhaps ecause he studyof these inkages s inherently nterdisciplinary, hey havenot received the emphasis hat their importance ngeochemical nd geophysical rocesses would seem todemand.

A persistent undercurrent n some related work is theobservation hat the physical environment seems emark-ably well suited o the needs of terrestrial ife [e.g.,Henderson, 913] and that this fact might even reflectorganisms manipulating heir environment o meet theirneeds e.g.,Redfield, 958]. The most ecent, omprehen-sive, and controversial orm of this idea is Lovelock andMargulis' Gaia hypothesis, which states [Lovelock andWatson, 1982, p. 795] that "the climate and the chemicalcomposition f the Earth's surface re kept n homeostasisat an optimum by and for the biosphere" see alsoMargulis and Lovelock 1974], Lovelock nd Margulls[1974a, b] and Lovelock 1979a, 1986b, 1988]). In thisview he biota and he physical nvironment re so ightly

Copyright 989 by the American Geophysical nion

8755-1209/89/89 RG-00256 $5.00

Reviews f Geophysics, 7, 2 / May 1989pages 223-235

Paper number 89RG00256

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coupled hat they can be considered single vast organism[Lovelock, 1986a, b, 1988] called Gaia (after the Greekgoddess f the Earth), and the atmosphere an be consid-ered to be [Lovelock, 1979b, p. 716] "like the fur of a catand the shell of a snail, not living but made by living cellsso as to protect hem against n unfavorable nvironment."

The Gaia hypothesis as eceived a great deal of publicattention. I can only speculate why: it is a hypothesis fgreat generality and vast scope; t has been presented nnontechnical anguage; it addresses matters of bothacademic and practical concern; t suggests colorfulmetaphor hat many find intriguing; and its semantic ndconceptual lasticity llow it to be virtually all things o allpeople. Two groups hat mmediately mbraced Gaia wereenvironmentalists nd, paradoxically, ndustrialists. Theformer argued that harming any part of the planetary"organism" could have far-reaching onsequences, hilethe atter argued hat Gaia's capacity or homeostasis adepollution control unnecessary Schneider, 987].

If verified, the Gaia hypothesis would represent arevolution in biogeochemistry, or it represents fun-damentally different view of the role of life on Earth.Some have spontaneously mbraced t as obvious, andmany have spontaneously ismissed t as bizarre. Others,representing diverse array of specialities, ave wonderedhow the hypothesis could be empirically tested; thisquestion was the focus of the recent AGU ChapmanConference.

My primary purpose here s not to catalogue nd weigh

the evidence or and against he Gaia hypothesis. nstead,I want to address more basic ssue: Is the Gaia hypothe-sis a scientifically testable proposition at all? Thisapproach may disappoint hose who would prefer anintroduction to the relevant biogeochemical andpaleoclimatic heory and data. Nevertheless, he firstquestion o ask of a theory s not whether t is true or false,but what it means and whether it can be tested.

I suspect hat much of the ongoing debate surroundingGaia has arisen because different parties defined thehypothesis n different ways, assumed hat their nterpreta-

tions of the hypothesis ere scientifically meaningful ndempirically estable, nd then proceeded o argue over thedata. My premise s that much of the debate over Galadoes not result from the ambiguity of the geologic ecordor the gaps in our current understanding of globalbiogeochemical ycles, but instead derives rom a lack ofclarity concerning what the Gaia hypothesis means andhow (or whether) t can be tested.

them all into a single coherent statement would be at bestunwise and at worst impossible. Instead, have tried tocompile a taxonomy of the central themes of differentstatements f the Gaia hypothesis n order to state eachprecisely nd o clarify the distinctions etween hem.

Coevolutionary aiaThe biota influences its abiotic environment, and theenvironment in turn influences the evolution of the biota

by Darwinian processes. Watson and Lovelock 1983, p.284] state,

The biota have effected profound changes on theenvironment of the surface of the earth. At the sametime, that environment has imposed constraints n thebiota, so that life and the environment may be consid-ered as two parts of a coupled system... perturbationsof one will affect the other and this may in turn feedback on the original change. The feedback may tend

either o enhance r to diminish he initial perturbation,depending n whether ts sign s positive or negative.

Homeostatic Gaia

The biota nfluences he abiotic world n a way that sstabilizing. The dominant inkages between he biota andthe physical world are negative eedback oops.

Lovelock nd Margulis [1974a, p. 93]

From the fossil record it can be deduced that stableoptimal conditions or the biosphere ave prevailed or

thousands f millions of years. We believe that theseproperties of the terrestrial atmosphere are bestinterpreted s evidence of homeostasis n a planetaryscale maintained y life on the surface.

Lovelock nd Margulis [1974b, p. 3]

The notion of the biosphere s an active adaptivecontrol system ble to maintain he earth n homeostasiswe are calling he "Gaia" Hypothesis.

Lovelock 1988, p. 13]

Through Gaia theory, see he Earth and he ife it bearsas a system, system hat has the capacity o regulatethe temperature and the composition of the Earth'ssurface nd o keep t comfortable or living organisms.

Geophysiological aiaThe biosphere an be compared with a single mmense

organism which, like other organisms, may exhibit bothhomeostatic and unstable behavior.

A TAXONOMY OF GAlA HYPOTHESESLovelock 1986b, pp. 12 and 19]

Defining the Gaia hypothesis s difficult. So manylogically distinct theories have been put forth under thesingle banner of "the Gaia hypothesis" hat compressing

Gala theory suggests hat we inhabit and are part of aquasi-living entity that has the capacity for globalhomeostasis. his s the basis or geophysiology... a


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systems pproach o Earth science. It is the essentialtheoretical asis or the putative profession f planetarymedicine.


Gaia theory predicts that the climate and chemical

composition f the Earth are kept in homeostasis orlong periods until some internal contradiction orexternal orce causes jump to a new stable state.

Optimizing GaiaThe biota manipulates ts physical environment n ways

that create biologically avorable onditions.


We argue hat it is unlikely that chance lone accountsfor the fact that temperature, H and the presence fcompounds of nutrient elements have been, forimmense eriods of time, ust those optimal or surfacelife. Rather we present he "Gaia hypothesis" he deathat energy is expended by the biota to activelymaintain hese optima.

Lovelock nd Watson 1982, p. 795]

The Gaia hypothesis... postulates hat the climate andchemical omposition f the Earth's surface re kept inhomeostasis t an optimum by and or the biosphere.


Planetary ife must be able to regulate ts climate andchemical state... the greater part of our own environ-ment on earth s always perfect and comfortable or life.The energy of sunlight s so well shared hat regulationis, effectively, ree of charge.

Lovelock 1979a, p. 127]

The most mportant roperty f Gaia s the tendency ooptimize conditions or all terrestrial ife.

Some may take issue with this taxonomy. This is not

the only way to classify he Gaia hypotheses, r perhapseven he best. My point s simply hat t needs o be done,because he single erm "Gaia" has been used o refer bothto observations hat many find self-evident nd to proposi-tions hat many ind extremely peculative.

This wide range of possibilities, long with the engagingplasticity of the terminology, means hat individuals anmake of Gaia whatever hey wish. The unsympathetic anridicule he notion of global optimal control. The sympa-thetic can point out that the biota and the abiotic environ-ment are obviously nterrelated n any number of ways.The uncritical particularly among he public and he press)can take data detailing these interrelationships ndmisinterpret hem as scientific validation of their fancifulextrapolations f Gaia. And in scientific discourse great

Kirchner: THE GAlA HYPOTHESIS'225

deal of unnecessary rgument may result from a simplemisunderstanding f which hypothesis s on the table atany given time.

The wide variety of Gaia hypotheses reates a risk ofmisinterpreting vidence or one version as somehow lsoproving another orm of Gaia as well. For example, t has

long been known that the atmosphere s in greater chemicaldisequilibrium han would be produced y extraterrestrialradiation lone [e.g., Lewis and Randall, 1923]. This is, asHutchinson [1954], Sillen [1966], and others have noted,evidence that biological processes are important deter-minants of Earth's atmospheric chemistry. Lovelock[1965] and Hitchcock nd Lovelock 1967] pointed out thatspectroscopic ata should detect chemical disequilibriumin the atmospheres f other planets if life is present.Lovelock's 1975] prediction hat life would not be foundon Mars was subsequently upported by data from theViking program. Atmospheric hemical disequilibrium snot, however, evidence that the biota manipulates heatmosphere n any particular way or for any particularreason.

The many Gaia hypotheses make it easy to create alsedichotomies. Rejecting one null hypothesis e.g., that thebiota has no effect on its physical environment) does notprove an alternative hypothesis e.g., that the biota and thephysical environment orm a goal-seeking, homeostaticcybernetic ontrol system) unless he two hypotheses remutually exclusive and ointly exhaustive which, in thisexample, hey are not). Nonetheless, ovelock and Watson

[1982, p. 799] (see also Lovelock 1979a], Lovelock andMargulis [1974a, b], and Margulis and Lovelock 1974])propose hat "a test for Gaia is to consider what wouldhappen f life were now deleted rom the Earth." This is,of course, a test for life, not a test for Gaia. A test for Gaiashould look for evidence that a lifeless Earth would be

different n the way that Gaia says a lifeless Earth shouldbe different (that it should be less homeostatic, orexample), not just different somehow. Many argumentsfor Gaia are based on rejecting he hypothesis hat bioticprocesses re completely irrelevant to the physical

environment; his hypothesis as already been ejected ymany nvestigators or quite some ime.

In order to proceed must briefly summarize somerelevant epistemology nd scientific methodology. Thesepoints are familiar to many researchers, ut it will aid thepresent iscussion f they are stated xplicitly.

HYPOTHESES: TRUE, FALSE, AND UNTEsTABLE

The day-to-day business f science consists f testinghypotheses, ut some hypotheses annot be tested. Somehypotheses re untestable n practice, hough he practicalimpossibility of performing an exact test is not crucial fsurrogate ests an be devised. For example, ne may hope


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that a full-scale test of the nuclear winter hypothesis willnever be carried out, but some of its basic mechanisms canbe observed t other scales, such as in large forest fires.Other hypotheses for example, he "hypothesis" hat in4000 B.C. a devious god assembled he geological ecordin a way that gives the impression hat the Earth is mucholder) are unrestable n principle; hese can obstruct heprogress f science. n this paper, unless therwise oted,the terms "testable" and "unrestable" will refer to

testability n principle.The minimal criteria of testability can be stated

concisely. In order to be testable a hypothesis must beclear, and its terms must be unambiguous. It must beintelligible n terms of observable henomena. And mostimportantly, t must generate predictions f two kinds:confirmatory predictions (phenomena hat should beobserved f the hypothesis s true and that would not bepredicted by the existing body of accepted heory) andfalsifying predictions phenomena hat should be observedif the hypothesis s false).

A hypothesis hat is unrestable s much ess useful hanone that is merely false. A false hypothesis, nce knownto be false, at least helps to restrict he sphere of pos-sibilities. Unrestable theories, on the other hand, are atbest empty and at worst misleading, nd in the minds ofthe unwary hey can be entrancing. A few examples willsuffice o make his point.

1. Ill-defined hypotheses re unrestable ecause heycan be endlessly einterpreted o fit almost any data, but for

the same reason hey cannot contain specific empiricalinformation; hey exclude no possibilities.

2. Tautological ypotheses re unrestable ecause heyare true by definition, but as such, hey contain no newinformation; their conclusions re entirely contained ntheir premises.

3. Unfalsifiable hypotheses re unrestable ecause heymake no falsifying predictions, ut as such, hey have noempirical content; onfirmation f the hypothesis oes notrestrict the sphere of possibilities because he set ofexcluded data data hat would have been nconsistent ith

the hypothesis) s empty.Note that hypotheses n each of these lasses which arenot ogically distinct, ut are stated his way n the nterestsof simplicity) will usually show good agreement withexperimental ata. That is why they are misleading. Onebelieves hey fit the data so well because hey capture heessence f the problem, when n fact they fit because heyare independent f the empirical facts. For the samereason hey are entrancing; ne thinks t remarkable hatthe predictions re always confirmed. Attempts o testsuch theories are ultimately futile, not because hey aredoomed to failure, but because they are doomed toinevitable nd nconsequential uccess.


Some criteria or judging the usefulness f hypothesescan also be stated. Some hypotheses, while testable andperhaps even correct, are simply not very useful. Themore specific he confirmatory predictions erived rom ahypothesis, nd he more general he falsifying predictions,the more ogical content he hypothesis ontains Popper,1959, p. 113]. Hypotheses re useful to the degree hatthey are ogically distinct rom other heories; f a hypothe-sis simply restates widely accepted principles, or can bedirectly derived rom them, a search or confirmatory atawould be an inefficient use of scarce resources.

Hypotheses re also useful n proportion o the phenomenathat they alone can predict or explain and, perhaps moreimportantly, n inverse proportion to what they mustassume. This s the well-known Law of Parsimony: f twohypotheses xplain he same body of data equally well,prefer the theory that imposes he lightest burden ofassumptions Poincare, 1905, pp. 146 and 151]. Whilethis principle cannot be precisely codified (and in aborderline ase may reveal the choice among competingtheories o be a matter of aesthetic aste), t remains a basictenet of the scientific enterprise.

GAlA AS METAPHOR

Some have suggested hat Gaia is simply a metaphorrather than a hypothesis. Metaphors are unrestable, utthey can nevertheless e scientifically useful; they can

suggest new lines of analysis, such as the search forbiogenic ources f cloud condensation uclei [Charlson tal., 1987]. Indeed, much of science, elying as it does onreasoning by analogy, is deeply metaphorical. Asscientific metaphors o, Gaia is unusually colorful, rich,and evocative, nd hope hat t will spur many nterestingand fruitful speculations. But metaphors nd hypothesesare two different hings, and it is important not to confusethem.

Why is it misleading o treat a metaphor s an empiri-cally testable roposition? As hypotheses, etaphors re

ill-defined because hey can be reinterpreted o explainalmost any observed ehavior; hey fail to specify n whatsense the metaphor is true. One can agree withShakespeare hat all the world is a stage, n some sense(e.g., its inhabitants an be viewed as playing out theirroles), but t is not a stage n all senses e.g., t is not madeof flooring and does not have a row of footlights at itsedge). There is, doubtless, some sense n which thebiosphere an be considered s an organism Lovelock,1986a, b, 1988], but this analogy s not scientificallyverifiable without some stipulation f how it does or doesnot apply. Proposals of specific ways in which thebiosphere ight act like a global organism e.g., t might


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exhibit homeostasis) re, of course, hypotheses nd may betestable. The metaphor tself, however, s untestable. thas no empirical content beyond he data that suggest hespecific ways n which t does or does not apply.

Boston 1989] has noted hat Gaia can be interpreted sillustrating he principle of emergent properties, which

holds that a complex system can have properties hatwould not be predicted from studies of its isolatedcomponents. As a cautionary note to those who viewsystems implistically, his principle s well taken. As ahypothesis er se, however, t is ill-defined. Even simpleabiotic systems can exhibit emergent properties; t isprecisely which properties are predicted and whichmechanisms reate hem hat constitutes hypothesis.

If Gaia is meant o be simply a metaphor r a restate-ment of the principle of emergent roperties, t is puzzlingthat its proponents make so many references o the Gaia

hypothesis, ests for Gaia, and proofs of Gaia [e.g.,Lovelock, 1983, 1988; Lovelock and Watson, 1982;Lovelock and Margulis, 1974b; Margulis and Lovelock,1974]. If Gaia is not ntended s a scientific proposition, tis misleading o refer o it as f it were one.

COEVOLUTIONARY GAlA

The idea that the biota and the physical environmentcoevolve is not original or unique to Gaia. To myknowledge, t was first stated by Spencer 1844, p. 93],who held that the biogenic ncrease n atmospheric xygenconcentration made possible a "more perfect mental andbodily development" n the biota. While Spencer'saccount s simultaneously rander and cruder than thecontemporary iew, few would dispute ts general remise;biotic processes hape he physical environment, hich nturn guides biotic evolution. ndeed, he fact that he bioticand physical worlds are interrelated s by now so welldocumented [Hutchinson, 1954; Berkner and Marshall,1964; Cloud, 1968; Holland, 1964, 1978, 1984; Schneiderand Londer, 1984] that it would seem odd to call it a

hypothesis at all. An observation hat is so widelyrecognized lacks the tentative character of a truehypothesis.

Coevolutionary aia asserts ot ust that the biota andthe physical nvironment re interrelated ut also hat thisrelationship s characterized by negative and positivefeedback [Lovelock and Watson, 1982; Watson andLovelock, 1983]. While the observation that interrelatedsystem components xhibit feedback s useful for thoseunfamiliar with feedback processes, t also followsanalytically rom elementary heorems f systems nalysis

and has no specific empirical content. An empirical est stherefore nnecessary. f the biota and the physical world

are nterrelated that s, each has effects on the other), thereare no other possibilities xcept hat feedback, positive ornegative, haracterizes his relationship.

HOMEOSTATIC GAlA

Gaia could be taken simply to mean that some of theinteractions etween he biota and the physical environ-ment are stabilizing (i.e., characterized by negativefeedback). If this is so, it is not surprising. Given thatthese eedback oops are numerous nd that each must beeither stabilizing or destabilizing, t is highly likely thatsome are stabilizing. Homeostatic Gaia can, however, bestated in two nontrivial versions: a weak form (thedominant interactions between the biotic and the abiotic

worlds are stabilizing) and a somewhat stronger claim

(these interactions make Earth's physical environmentsignificantly more stable han it would have been withoutlife).

Note that the simple act of climatic homeostasis to theextent that climatic and paleoclimatic data do or do notindicate table conditions, topic well beyond his paper'sscope) s not at issue. Gaia concerns mechanisms hroughwhich homeostasis s thought o be maintained, ot ust thefact that it exists. Evidence of homeostasis is therefore not

compelling vidence or Gaia; how can we tell whether heclimate has been stable because of biotic processes r inspite of them?

The point has been made repeatedly for example, byLovelock and Watson [1982], Lovelock [1979a], andMargulis and Lovelock 1974]) that crude physical modelsof climate cannot xplain why the Earth was warm enoughto support ife during the Precambrian, when the Sun isthought o have been approximately 5% dimmer han t isnow [Sagan and Mullen, 1972; Newman and Rood, 1977].This in itself is not a test of Gaia. The limitations of

simple models (which assume that the Precambrianatmosphere had the same composition as at present,despite lear evidence or higher partial pressures f one or

more greenhouse ases) do not necessarily mply thatbiological control mechanisms [e.g., Lovelock andWhifi'eld, 982] are needed o resolve he "faint youngSun" paradox. The simplest olution o the paradox maybe refined geophysical limate models, such as those ofBerner et al. [1983] and Walker et al. [1981]. It is easy ospeculate about hypothetical biological mechanisms hatresolve the faint young Sun paradox and other incon-gruities n the climatic history of the Earth. The hard worklies n specifying plausible, arsimonious, uantitativelyrealistic mechanism ufficient o dispose of the paradox

and proposing nd completing test of that theory. Oneattempt to include quantitatively realistic biological


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feedbacks n models of global temperature egulation[Volk, 1987] concluded hat their effect would be small. Awide variety of hypothetical ontrol mechanisms ould bespeculatively nvoked to explain any given discrepancybetween existing theory and data, but they should betestable gainst more han ust the gaps hey were selectedto fill.

.Lovelock nd Margulis 1974a, p. 100-101] andMarguli and Lovelock 1974, pp. 479-486] proposedwide range of biological eedback mechanisms hat mightcontrol he climate. Conspicuously bsent rom the bulk ofthe ensuing esearch, owever, s any mention of processesthat might destabilize he climate. For example, n areview of the role of oceanic phytoplankton n producingcloud condensation uclei and altering he global radiationbudget (the Gaian mechanism f climate regulation hathas eceived he greatest ttention o date), Charlson t al.[1987] note i n passing hat it is unclear whether theresulting eedback loop is stabilizing or destabilizing.They then discuss t length how stabilizing eedback mightarise and how it might be instrumental n regulatingclimate , they do not address he possible origins andconsequenc.sof destabilizing eedback. ithout nowingwhat biological mechanisms ay undermine omeostasis,it is difficult to make a balanced assessment of the role of

the biota.

Recently, ice core data have shown that variations nbiogenic cloud condensation uclei and carbon dioxideserve to amplify, not damp, the temperature xcursionsduring glacial periods. Levels of nonmarine sulfate (aproxy for biogenic dimethyl sulfide, a precursor f cloudcondensation uclei [Charlson et al., 1987]) are muchhigher during glacial periods than during interglacials[Legrand et al., 1988]. Similarly, carbon dioxide evelsare significantly higher during interglacials han glacials[Barnola et al., 1987; Jouzel et al., 1987]. These are thetwo most climatically nfluential biogenic compounds orwhich long-term records are available. Those recordsindicate hat both of these biologically mediated eedbackrelationships are either (1) ineffective in offsetting

nonbiological positive feedback processes controllingthese compounds Schwartz, 1988] or (2) actively des-tabilizing n their own right. Neither of these possibilitiessupports he notion of homeostasis hrough biologicalcontrol of climate.

Lashof 1989] has estimated limatic and biogeochemi-cal feedback esponses o an anthropogenic reenhousewarming. Of the five biologically mediated eedbacks hathe examines, only one (an increase n photosynthesisresulting rom direct CO2 fertilization) ffsets he effectsof an anthropogenic oubling f atmospheric O2. The

other four biologically mediated processes a decrease nvegetation lbedo as the tree ine moves north, an increasein plant respiration and metabolism of soil carbon, anincrease n methanogenesis, nd shifts in ocean biota


resulting n increased O 2 release) would amplify hetemperature ise expected rom a greenhouse arming.

It is generally agreed that oxygen production byblue-green "algae" (recently reclassified as bacteria)changed he redox balance f the Precambrian tmospherefrom reducing o oxidizing, n a demonstration f biologi-

cal control hat was clearly not homeostatic and whichvirtually exterminated he microbes esponsible or it).Lovelock 1979a, p. 31] and Lovelock and Margulis[1974b, p. 8] cite the fact that terrestrial ife survived hisevent as evidence or Gaia's esilient daptation o change.If the most destabilizing iotic event n Earth's history anbe construed s evidence or Gaia, and he elative stabilitysince then can also be cited as evidence for Gaia, onewonders what conceivable vents ould not be interpretedas supporting he Gaia hypothesis. f there are none, Gaiacannot be tested gainst he geologic ecord.

If Gaia stabilizes nd Gaia alestabilizes since Gaia is afeedback ystem, hose are the only two possibilities), sthere ny possible ehavior hat s not Gaian? Some mightobject that the biota's destabilizing effect in thePrecambrian ndicates hat Gaia had not fully matured, utsuch n objection s openly autological; t defines Gaia nterms of homeostasis and then asserts that Gaia ishomeostatic.

In the absence of clear evidence that biotic mechanisms

do in fact stabilize he global environment, t seems wise orefrain rom assuming hat they ought o do so. It is wiserstill, I think, to avoid the assumption hat biotic mecha-nisms should be generally ither stabilizing or destabiliz-ing, or that he stability properties f these mechanisms retheir most nteresting r important eatures.

Before addressing he hypotheses hat I have termedgeophysiological aia and optimizing Gaia, would ike topause here to comment on mathematical models of Gaiaand whether hey demonstrate iological homeostasis fthe environment.

MODELS OF GAlA

An interesting utgrowth of the Gaia hypothesis asbeen a mathematical odel demonstrating hat homeostaticbehavior an be achieved n a biotic system without anyform of conscious control. The Daisyworld model[Watson nd Lovelock, 983; Lovelock, 988], designed obe heuristic ather than realistic, describes planet onwhich temperature s controlled y the albedo, which isdetermined y the color of the daisies growing on thesurface. This model's purpose s not to describe ow the

Earth's emperature ight actually have been egulated nthe ace of changing olar uminosity; nstead, aisyworldis intended o explore he consequences f strong ouplingbetween he biota and he physical nvironment.


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The theory behind Daisyworld can be outlined asfollows. Biotic growth curves as a function f environ-mental variables uch s temperature) re characteristicallypeaked. f the variable f interest s a function f bioticpopulations, here s some ange of changes n externalconditions for which the effect of the induced shift in

biotic populations ffsets he mposed hange Figure 1).

8O

60

>,, 40

e 20.

T1

T2

0 10 20 30 40 50

Mean surface temperature (C)

Figure 1. The effect of temperature n he population f daisies(B) and he effect of daisy cover on the mean emperature n theDaisyworld model see ext) or two values f solar uminosity(A and A1). This example ssumes hat only white daisies represent; ncreasing aisy cover aises lbedo nd owers meantemperature. f solar uminosity ncreases rom A to A1, theresulting emperature ise increases aisy cover and raisesalbedo, imiting he emperature ise rom T to T1. If daisy overdid not respond o temperature, he shift n luminosity wouldresult n a temperature ncrease o T2. Parameter alues sed rethose of Watson and Lovelock 1983]. These curves differ fromthose of Lovelock 1986b, p. 15] because he latter are mis-labeled. What Lovelock refers to as "daisy population" sactually , one actor n the growth ate equation Watson ndLovelock, 1983, equation 1)]. What Lovelock refers to as"mean emperature" s actually he temperature f the daisies.Mean temperature s a function of the daisy temperature, hetemperature f bare ground, nd he ractional aisy over. Themodel s very stable ecause he selected arameters ake daisycover a nearly vertical unction f temperature t the operatingpoint T. Daisy cover shifts rom zero o 45% in response o atemperature hange f only IC.

This esult s general nd holds whether he daisies armor cool the surface. Figure 2 shows he behavior f themodel when both ight and dark daisies re assumed o bepresent; he surface emperature s held nearly constantover wide excursions n solar uminosity.

It is easy o see how the result n Figure 2 is obtained.Black and white daisies are assumed o have exactly thesame growth esponse o temperature, ut black daisies reassumed o be 10C warmer han white daisies n equiva-lent ambient conditions. Consequently, lack daisiesthrive in cooler ambient conditions, ower the albedo, andwarm the surface, while the opposite s true of white

daisies.Watson and Lovelock's conclusion hat "regardless of

the directions of the eedbacks, he model always showsgreater stability with daisies han it does without hem"

[Watson and Lovelock, 1983, p. 288] (italics in theoriginal) ndicates selectivity of emphasis. There is arange of luminosities or which he temperature/luminositycurve is flatter than it would have been without daisies, butthere are also ranges where it is much steeper as simplecontinuity between the endpoints requires). If the

operating oint s near either end of the stable ange, smallshifts in luminosity can have drastic consequences.Perturbations ould also cause the system to suddenlyjump from one path of the hysteresis oops o another. Thesense n which such behavior ndicates greater stability"is unclear.

Likewise, the claim [Lovelock, 1988, p. 46] that "in noway is the stability of Daisyworld dependent on anidiosyncratic choice of initial values, or rate constants"needs o be qualified. Daisyworld exhibits stability nearthe optimal temperature or daisies because the black

daisies, which warm the surface, hrive n cooler empera-tures than the white daisies, which cool it. But what if,instead, he optimal temperature or black daisies were15C igher han hat or whitedaisies? A higher ptimaltemperature might explain why the black daisies evolvedto be 10C "warmer" n color while sharing he sameenvironment as white daisies.) If this were true, blackdaisies would thrive in warmer temperatures nd wouldtend to keep the surface warm (and vice versa for whitedaisies). The result s a tug-of-war between he two typesof daisies. As Figure 3 illustrates, Daisyworld egulatedin this way is a hysteresis oop, where each value of solarluminosity gives two equilibrium temperatures eachrepresenting omplete xtinction f one or the other ype ofdaisies) hat lie well outside he optimal temperatures oreither species. Climatic perturbations ould ikely resultin unstable ransitions etween he upper equilibrium, nwhich a too hot Daisyworld ries o keep tself too hot, andthe ower equilibrium, n which a too cold Daisyworld riesto keep tself too cold.

What is most interesting about this "pathological"Daisyworld hat have described s that the available datasuggest that the biotic feedbacks regulating climate

actually work this way. As pointed out above, ice coreanalyses ndicate that during glacial periods, biologicalprocesses roduce less carbon dioxide and more cloudcondensation uclei. In other words, biological processesappear to make the planet colder when it is cold andwarmer when it is warm. Dickinson and Hanson [1984]and Hansen et al. [1984] have estimated how Earth's

vegetation patterns would shift in response o globaltemperature hanges; hey found that planetary vegetationalbedo would shift in ways that exaggerate, ather thanoffset, the imposed emperature hange. Of course, he

Daisyworld model s heuristic nd s not meant o implythat the Earth's temperature s actually regulated bychanges n vegetation albedo. Nevertheless, o the extentthat vegetation lbedo actually does nfluence climate, he


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230' Kirchner: THE GAIA HYPOTHESIS 27,2/REVIEWSOFGEOPHYSICS

80

60

, 40

0.5 o .

.

0.5 1.0 1.5

Solar luminosity (arbitrary units)

Figure 2. Mean surface emperature nd daisy populations or

the Daisyworld model ncorporating oth black and white daisiesover a range of solar luminosity adapted rom Watson andLovelock [1983]). Watson and Lovelock do not show thehysteresis oops hat arise when solar uminosity ecreases; heyare shown here to illustrate he existence f multiple equilibria nsome ranges of luminosity. Black daisies will be warmer than

the ambient emperature, nd white daisies ooler, owing o their

coloration; lack daisies hrive in ambient emperatures round15 to 25C and warm the surface, while white daisies thrive inambient emperatures round 20 to 30C and cool the surface.As a result, over a wide range of solar uminosity he surface smaintained t temperatures avorable or one or both daisies.

available data indicate hat it does so in the way that thepathological Daisyworld (instead of Lovelock andWatson's Daisyworld) ays t should.

Do models such as Daisyworld constitute Lovelock,1983, p. 66] "a cybemetic roof of the Gaia hypothesis"?Of course, clever modelers can make a model do almostwhatever hey want, but the point will try to make here sslightly more subtle. Models are often used o derive heconsequences f hypotheses. When the model's compo-nents can be independently erified or have been derivedfrom well-grounded mechanistic heory (as in climatemodeling), the results can be particularly useful incomparing heory and observations. owever, he resultsof heuristic models ike Daisyworld cannot be directlytested gainst eal world dam. Consequently, he theory stested, not by comparing model and real world behavior,but by comparing model results o the behavior predicted

by the Gaia hypothesis. That kind of test will inevitablysucceed barring ogical or mathematical rrors) because tis not comparing a theory with data, but comparing a

theory in words) with itself (in mathematical erms). Suchmodels an be used o derive and illuminate he implica-tions of the Gaia hypothesis, but not to establish tsvalidity.

GEOPHYSIOLOGICAL GAlA

Lovelock [1986b, 1988] has recently proposed"geophysiology," he representation f the biosphere s asingle organism, as a systems pproach o Earth science.In this view the biosphere, ike an organism, ill generallymaintain nternal homeostasis n the face of changingexternal orces. Just as physiology ddresses he function-ing of whole organisms, geophysiology s intended tounify the constituent parts of the biosphere and thedisciplines hat address hem. According to Lovelock

[1988, preface], "specialties, like biogeochemistry,theoretical cology, and evolutionary iology, all exist, butthey have no more to offer the concerned nvironmental


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27,2/REVIEWSOFGEOPHYSICS Kirchner: THE GAIA HYPOTHESIS o 231

80

60o

, 4o-

20

o0.5

80

1.0 1.5

0 ' , .... -- 0.5 1.0 1.5

, 40

0.5 1.0 1.5

Solar luminosity (arbitrary units)

Figure 3. Daisyworld xhibiting pathological" ehavior s aresult of fixing he optimal emperature f black daisies t 15Cabove hat of white daisies. Doing so reverses he bias mplicit nWatson and Lovelock's 1983] assumption hat black daisies are10C "blacker" than white daisies. Note the existence of two

equilibria, both outside he ranges of ambient emperatures hat

are favorable or either daisy color (20-30C for white, and30-40C for black). At either equilibrium, one of the daisyspecies s driven o extinction.

physician r the patient han could he analogous cienceof biochemistry or microbiology in the nineteenthcentury."

Some systems evel approach o Earth science sdesperately eeded, ut should his be its basis? t may bemisleading o use the same language we apply to or-ganisms when discussing he biosphere s a planetaryorganism. To borrow a point rom evolutionary iologistE. Mayr, there may be no such hing as a healthy herd ofdeer, but only a herd of healthy deer. Now, a herd (or aplanet) may be said o be healthy, but not n the same sensethat one of its members s healthy.

It is important to distinguish geophysiological aiafrom an Earth-as-organism metaphor. Consider, forcomparison, the Earth-as-mechanism metaphor thatunderlies ontemporary atural science. The success fthis mechanistic framework has demonstrated the effec-

tiveness f thinking about he natural world as if it were a

machine whether or not it actually is, in any sense, amechanism). would agree with the proponents f Gaiathat it may be useful to attempt o speculate bout the

natural world as if it were an organism. But the questionof whether he Earth actually is an organism s neitherscientifically meaningful or scientifically nswerable.

As a hypothesis, he biosphere/organism etaphor s,like all metaphors, ll-defined. In which senses s the Earthlike an organism, nd how can we tell how far to take theanalogy? The vast diversity of living organisms, rom mento microbes and plants to pachyderms, means hat anyconceivable phenomenon ould be compared o somebehavior f some organism. For example, eophysiologyexplains he sudden edox shift at the end of the Arcbean,dramatic xample of biological destabilization, n terms ofpuberty Lovelock, 988, p. 99]. Similarly, geophysiologyreinterprets he ice core data (which suggest, s describedabove, hat biological eedback einforces he temperatureshifts between glacial ages and interglacials, ather thanhomeostatically pposing hem) in the following way[Lovelock, 988, pp. 136 and 150]: "the glacial cool s the

preferred state of Gaia, [and] the interglacials ike thepresent ne represent ome emporary ailure of regulation,a fevered state of the planet... [during glacials] cloud


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232 Kirchner: THE GAIA HYPOTHESIS 27,2/REVIEWSOFGEOPHYSICS

cover and low carbon dioxide operated n synchrony spart of a geophysiological rocess o keep he Earth cool."

Geophysiology an be endlessly einterpreted, nd forthat reason t is unfalsifiable. Any conceivable ehaviorcould be taken to be physiological, articularly f onedefines he physiologically preferred" state t will and f

one considers hysiological reakdown e.g., "feverish"interglacials) o be evidence or physiological ehavior.Geophysiology redicts Lovelock, 988, p. 13] "homeo-stasis or long periods ntil some nternal ontradiction rexternal orce causes jump to a new stable state." Butexternal forces and internal contradictions are alwaysprevalent; ny nstability ould perhaps rroneously, incea balance of "internal contradictions" an be stabilizing)be attributed o any number of such causes. Moreover,given hat stasis s defined as the absence f jumps, andjumps are defined s the absence f stasis, eophysiology

seems o predict simply hat the biosphere ill exhibithomeostasis xcept when t does not. It is futile to try tofalsify such a hypothesis, r to try to derive meaningfulpredictions rom t.

To some, an endlessly nterpretable heory s appealingbecause t can be used o explain virtually anything. Butfor the same eason t predicts othing, nd ts explanationsare empty. If any conceivable henomena an be ex-plained y geophysiology, hen geophysiology ays implythat "anything s possible." The value of a scientifictheory, by contrast, ies precisely n limiting he sphere fthe possible, hereby llowing ne o separate he plausiblefrom the mplausible.

If the purpose of geophysiology ere simply to spurinteresting peculations bout he climatic history of theEarth, one might not object. But Lovelock 1986b, p. 12](see also Lovelock [1988, pp. 152-182]) expresslyadvocates eophysiology s "the essential heoretical asisfor the putative rofession f planetary medicine" o cureEarth's environmental lls. With no means of testing heunderlying heory, how can one tell whether he cures trecommends re he planetary quivalent f penicillin r ofbloodletting? The risk of creating atrogenic global

maladies hrough reatments hat have no testable basisshould not be underestimated. It is hard to see how

geophysiology ould help us to distinguish etween"planetary medicine" nd planetary alpractice.

there is some "objective function" that those conditionsoptimize. Every conceivable environment can be"favorable," if one has complete reedom o specify whatit is favorable for.

What could possibly be optimal for the whole bio-sphere? One can define an optimal environment or a

given organism n a number f ways. But what conditionsare optimal, or even simply favorable, for a diversecollection f organisms hat have conflicting equirements?One response s that current conditions re optimal, butthis creates n obvious autology y defining optimality nterms of prevalent conditions and then asserting hatprevalent conditions (assumed to be maintained bybiological ontrol) re, n fact, optimal.

Stability and optimality for the agent supplying hehomeostasis) re mutually exclusive. The population f anorganism hat makes ts environment more suitable oritself will grow, whereupon t dominates more of theenvironment and grows still further; this behavior isdestabilizing. Stability s achieved y organisms foulingtheir own nests" and thereby limiting their populationgrowth.

Consider he Daisyworld model in a world with onlywhite daisies. At a stable point he daisies upply homeos-tasis because f the temperature ises, more daisies growand the albedo increases. This implies that a highertemperature ould be more suitable or white daisies, sothe temperature t the stable point is cooler han optimal,and the daisies' growth response revents a warming thatwould be favorable or them. At the peak of the daisygrowth curve, here s no homeostasis; f the temperatureincreases, aisies die, and the temperature ncreases tillfurther, and more daisies die. The optimum is reachedonly n an unstable ransition etween stable suboptimumand total extinction.

A common objection o Gaian optimization s that it isteleological; t would require some sort of foresight ndplanning Doolittle, 1981]. It is important o note hat onecan form a mechanistic scenario for the evolution of

favorable conditions without any "engineering," ust as

ants and termites have evolved strategies or regulating hetemperature f their nests without being old how to do it:


OPTIMIZING GAlA

The most speculative ersion of the Gaia hypothesisholds that biotic control of the physical environmentcreates conditions hat are favorable, or even optimal, or

the biosphere. This hypothesis s ill-defined unless onecan specify what constitutes avorable or optimal condi-tions. It can be demonstrated hat for any set of conditions,

Although the environmental control mechanisms relikely to be subtle and complex, we believe theirevolution can be comprehended roadly in terms ofNeodarwinian hought ... Analogous with theevolution of local environmental or internal control, inthe evolution of atmospheric omeostasis hose speciesof organisms hat retain or alter conditions ptimizingtheir timess (i.e., proportion of offspring eft to thesubsequent eneration) eave more of the same. In thisway conditions re retained or altered o their benefit.


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27,2/REVIEWSOFGEOPHYSICS Kirchner: THEG^I^HYPOTHESlS o233

Margulis and Lovelock 1974, p. 486]

It is a recognized act of biology that environments reregulated on a much more local scale... We merelysuggest extrapolation of these ideas to the entireatmosphere-biosphere ystem.

Lovelock 1986b, p. 13]

Life and the environment volve ogether s a singlesystem o that not only does he species hat eaves hemost progeny end to inherit the environment ut alsothe environment hat favors he most progeny s itselfsustained.

The problem with these rguments s not that they aredemonstrably rong, but that they are radically unpar-simonious nd, given ordinary Darwinian emoval of theunfit, hey are unfalsifiable. or global ontrol o arise byevolution, an organism's effects on "the entireatmosphere-biosphere ystem" would need o be signifi-cant enough o affect ts own reproductive uccess. his ishighly mplausible, s Doolittle [1981] has pointed out:"It is difficult to accept hat behaviors whose effects...will not be felt for thousands f generations an be selectedfor, especially when the first beneficiaries f those effectsmay be organisms which are not themselves esponsiblefor them." Recently, Lovelock 1988, pp. 126-151] hasemphasized aian mechanisms rising hrough ehaviorsthat benefit organisms irectly, n other han Gaian ways.But if the connection etween ndividual eproductiveadvantage nd Gaian altruism s only coincidental, wewould expect Gaian and non-Gaian behavior o arise withequal probability. Darwin said it all much morestraightforwardly: he environment nd the biota are wellmatched ecause rganisms hat are poorly dapteA ill beunderrepresented n the next generation's ene pool.

Given that the unfit are selected gainst, laims thatGaia creates a favorable environment are unfalsifiable.

Whether r not Gaia optimizes, we would still observe hatthe Earth's organisms match its environment, ecauseorganisms hat do not suit the environment do not thrive

and will not be noticed. Holland 1984, p. 539] put t quitesimply:

The geologic ecord seems much more in accord withthe view that the organisms hat are better able tocompete have come to dominate, and that the Earth'snear-surface nvironments nd processes ave accom-modated hemselves o the changes wrought bybiological evolution... We live on an Earth that s thebest of all possible worlds only for those who are welladapted o its current state.

When, at the recent AGU Chapman onference n theGaia hypothesis, raised n objection o Gaia, arguing hatthe Earth's physical environment as obviously not

favorable or penguins, was met with the response hatArctic animals are eccentric, xotic, ringe species. That,of course, s exactly the point; on a much colder Earth,penguins and latter-day mastodons, erhaps) would bemore prominent, nd scientists ould peer out from heirfur coats o observe hat the environment eemed deallysuited to their needs.

CONCLUSIONS

The proponents f the Gaia hypothesis ave made hreevaluable ontributions. irst, hey have eiterated he pointoriginally made by those investigating iogeochemicalcycles, namely hat biological agents play a vital role increating he Earth's physical and chemical environmentand that those nterested n the surface chemistry ndphysics f the Earth cannot rbitrarily gnore biologicalprocesses. econd, hey have suggested ome mechanismsby which the biota may be particularly mportant ndetermining he Earth's climate. Third, they have createdan engaging nd colorful metaphor hat some have used asa unifying heme and a motivator or research. n myview, however, this metaphor if taken literally) isultimately misleading and will needlessly color one'simpressions f how the biota and he physical nvironmentinteract. Attempts o test this metaphor s a scientificproposition ill be, in my opinion, ultimately utile.

Some have viewed Gaia as a dating but viablehypothesis. My view is that each of the different Gaiahypotheses s either daring or viable, but not both.Coevolutionary aia s testable ut not daring; t simplyrecasts, n different anguage, he ong-standing ndwidelyaccepted observation hat biological processes nd thephysical environment nfluence one another. The claimthat hese nterrelationships re generally, r even univer-sally, stabilizing homeostatic aia) may be testable ndmay well spur some nteresting esearch nto the feedbackmechanisms ontrolling limate. The scanty ata available

to date, however, end to refute the hypothesis tself.Geophysiological aia, by contrast, annot e refuted ydata, not because t is inescapably orrect, ut because t isill-defined nd can be reinterpreted o explain lmost nyimaginable henomena. Optimizing Gaia is similarlyill-defined, nd given simple Darwinian elimination f theunfit, t is both unparsimonious nd unfalsifiable.

Gaia s crippled y its great generality; t searches or asimple capsule escription f the role of life on Earth. Thekey fact, that the biota and the physical worlds areinterconnected, as realized ong ago. The day-to-daytask of figuring out "how the world works" consists fdocumenting hese interconnections nd their conse-quences. here s a lot of difficult, mportant cience o be


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234 Kirchner' THE GAIA HYPOTHESIS 27,2/REVIEWS OF GEOPHYSICS

done here, and we should get on with it. An untestablebelief that these elationships ught o have some generalform or function may only blind us to what they actuallyare and actually do. What is "out there," what can beobserved, measured, ypothesized, erified, and falsified,is biogeochemistry, atural selection, evolution, and so

forth. Gaia may be a grand vision, but it is not the kind ofvision hat can be scientifically alidated.

Some may object that to abandon he empirical searchfor Gaia is to forgo the chance of discovering grand,ultimate truth. With Poincare [1905, p. 163] I wouldrespond,

To those who feel that we are going too far in ourlimitations of the domain accessible to the scientist, Ireply: These questions which we forbid you toinvestigate, and which you so regret, are not onlyinsoluble, hey are llusory and devoid of meaning.

ACKNOWLEDGMENTS. J. Harte first suggested hat ataxonomy of the Gaia hypotheses would be useful and mademany vital comments. am grateful o J. E. Lovelock, H. D.Holland, S. H. Schneider, M. E. Power, and B. A. Roy for theirsuggestions, o the anonymous eviewers or their comments, ndto the William and Flora Hewlett Foundation for financial

support.Ann Henderson-Sellers as the editor responsible or this

paper. She hanks . G. Cogley and P. Boston or their assistancein evaluating his paper.

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James W. Kirchner, Energy and Resources Group, Uni-versity of California, Berkeley, CA 94720.

can it be tested

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