I have now reached an age where my further career is the least of my worries. Yet, I still find it hard sometimes to pronounce that little word–“Gaia.” In the academic world this name is contaminated. Research on Gaia is often discouraged, and it is not good for your curriculum vitae to be associated with this weird idea. The situation is stifling, particularly for young and ambitious scientists. But as concerns about my career evaporated, it was as if a psychic barrier was removed from my brain, and so it finally dawned upon me: of course, the general discomfort is nothing but false shame. Not the idea of Gaia is weird, but the academic world itself. What is the matter?
It all began with the Apollo mission in 1969, when Man set foot on the moon for the first time. I can still remember the excitement of those days. It was the time of the Vietnam War, the Beatles, and the unrest in France. Post-world-war optimism became exhausted, and the frontiers came in sight of our ability to organize the world in an orderly way. But the moon landing seemed to make fun of those doubts. This was the ultimate proof that our technical skills were unlimited. A new era of progress seemed to begin.
And then, all of a sudden, we saw pictures that almost made us forget the moon walks. We looked into a mirror and saw ourselves, for the first time, from space. We knew that the Earth was round, that there were continents and oceans, clouds and ice caps. But this was something quite different. These subtle tinges in light blue, green, and yellow, those white and whirling streaks of clouds; and then that pitch-dark background with fiercely radiating stars… What we saw was our home, our unique native country, rotating in majestic solitude through infinite space. More than four-and-a half billion years ago this heavenly body was born from stellar dust and soon thereafter it had come to life. And we, what else were we than an impertinent breed of the apes? We had barely appeared on the scene, and already we threatened to destroy this very ancient and precious haven.
By now, those pictures of the Earth have become ordinary stereotypes, and it is difficult to remember their original significance. Yet, there can be no doubt that once and for all they have changed the perception of the world for billions of people. In the years following the Apollo voyages we became collectively aware of the vulnerability of the environment and of the risks of unchecked technological and economic growth. This marked the birth of the environmental movement.
It was on the waves of this new élan that the Gaia idea could emerge. The plan of Lovelock, the inventor of Gaia, was intrepid and the terminology was well chosen. Gaia–the power of that name was in the immediate association with the image of our mysterious planet in space. At the same time, Lovelock’s concept contained the elements of a new scientific theory of the Earth. A theory that bridged the traditional gap between the earth and life sciences and offered an opportunity to study our planet as a coherent organized entity. What Lovelock had in mind was a free science that broke with the academic tradition of specialized research and subscribed to latent expectations among the public at large–popularization in optima forma.
Basically, the argument for Gaia was based on two observations (Lovelock 1979, 1988). In the first place, we knew that, owing to biological activity, the Earth’s atmosphere is far removed from thermodynamic equilibrium. The other factor was that life could evolve without interruption during the full 3.7 billion years of its existence. Thus, conditions on Earth were persistently kept within the narrow constraints required for the maintenance of life, despite colossal environmental catastrophes and an increase in solar radiation by some 25%. Lovelock explained these observations by proposing that the Earth be organized. Global regulatory mechanisms, emerging from the multiple interactions between life and its environment, automatically maintain favorable conditions for the long-term survival of the biota. There is no need to invoke forethought or planning. No deus ex machina is required. The system, Gaia, maintains itself like the flame of a candle.
It looked for a while as if Lovelock’s plan to create a new science of the Earth was successful. His books became bestsellers and there was the beginning of a serious scientific discussion. But soon the emerging New Age movement became aware of Gaia. Here was a new goddess to save us from doom. A true Gaia cult was launched, with Gaia meditations, Gaia prayer sessions, and Gaia dance therapies. A brilliant scientific speculation, born out of pure curiosity on the mystery of our unique planet, was transformed into dogma, an article of faith.
One can readily imagine that this interference did not help the scientific discussion on Gaia. But it hardly explains why the idea was so brutally rejected in academic circles. After all, the misuse that the Nazis had made of Darwinism had hardly affected the popularity of that theory. Something else was the matter: the scientific content of Gaia itself was suspect.
The most brilliant and entertaining attack on Gaia came from J.W. Kirchner at the 1988 meeting “Scientists on Gaia” in San Diego (Kirchner 1989, 1991). There is not one Gaia, said Kirchner, but there are several. “Influential Gaia” simply implies that life is a significant geological force, whereas in “coevolutionary Gaia” life also adapts to changing conditions by Darwinian processes. “Homeostatic Gaia” goes a step further by stabilizing the global environment with negative feedback loops. “Teleological Gaia” is even more drastic: stability is maintained not only by, but also for the biota. The most extreme form is “optimizing Gaia”, which goes as far as creating optimal conditions for life.
Kirchner then distinguished between “weak Gaia” (including the influential and coevolutionary versions) and “strong Gaia” (the other ones). Weak Gaia has been known for over a hundred years, he argued. Nobody can deny it, it is commonplace and there is no need to give it a new, pretentious name. The strong Gaias, however, are neither good nor false. They are ugly theories, which is a great deal worse. Ugly theories are misleading because they cannot be falsified; they are true whatever the facts. “Optimizing Gaia”, for example, has no meaning unless the optimum is clearly defined. What is optimal for the mixed bag that the biota is: penguins, crocodiles, hot spring bacteria…? Or are we supposed to mean maximum biomass for all the biota together? Then how can we test whether the actual biomass is at its peak?
And on it went. As Kirchner proceeded, the three strong Gaias got uglier and uglier. They melted away until nothing was left. Gone was Lovelock’s dream to create a new science of life and earth and to bring back to life those faded pictures of our unique and lonely planet. All we had was poor weak Gaia. We might just as well remain ordinary geobiologists, biogeologists, or biogeochemists and get on with the job. Or is there a way out? Can we strengthen weak Gaia?
The other day I read “Full House” by Stephen J. Gould (1996). Gould opposes the idea that evolution started with simple organisms and that in the course of geological time ever more complex and “higher” forms of life would have emerged. Of course, we ourselves would then have the honor to represent the pinnacle of evolution. This idea of gradual progress is outdated, says Gould. There is no trend, and there is no progress. All we can see is that life is present on each and every place where there is a niche for it. And there is no sign that this has ever been otherwise. By far the most important component of life is the bacterial world. Bacteria do virtually all the work, while other life forms are of minor importance for the whole. Consequently, evolution is a matter of permutations of a basic theme rather than a development towards an ever-higher level of organization. You may agree with this or not, but you must acknowledge that Gould’s concept has Gaian proportions and, lo and behold, that it is testable.
In
his book, Gould only considers the organisms by themselves, and ignores the
geological context in which they have lived. Even with only weak Gaia available,
we should be able to do better. According to Lovelock, the method of Gaian
science is to follow a top-down systems approach towards life and the Earth.
Figure 1 demonstrates the difference
between the Gouldian and Lovelockian perspectives. Gould’s main concern is
with the biota, while Lovelock concentrates on the “Gaian system”. The
diagram shows the extent of this system on time scales of hundreds of millions
of years. Apart from the biota, the atmosphere, the ocean and the lithosphere
are included. All these entities interact and exchange matter. In addition,
cosmic and endogenic forces influence the system, but the exchange of matter
with these domains is negligibly small on that time scale. To test Gould’s
contention that big organisms don’t matter, we must concentrate on the last
600 Ma, when these organisms were around. I further propose that we focus on the
Urey cycle of calcium silicate-carbonate, because its development over the
Phanerozoic has been studied in considerable depth (e.g.,
Berner 1994) and life
plays an important role in its operation.
The
cycle is shown in
Figure 2. On the right-hand side is the
ocean, and on the left is the continent. The deep earth and the atmosphere are
also included. Three asterisks indicate major processes where life is involved.
Weathering of one mole of calcium silicate on land consumes two moles of
atmospheric CO2 and liberates two moles of bicarbonate, one of
calcium ions and one of silicic acid. Runoff brings the (dissolved) weathering
products into the ocean, and, to keep the balance in order, they leave the
seawater in equivalent amounts. For each mole of calcium silicate weathered on
land, molar amounts of calcium carbonate and silica deposit on the ocean floor
whereas one mole of CO2 escapes into the atmosphere. Subsequently,
endogenic forces, subduction in particular, bring the calcium carbonate and
silica into the deep Earth to reform calcium silicate and CO2. The
calcium silicate moves up to the continental surface so that it is ready for the
next round in the cycle, and volcanoes blow the CO2 into the
atmosphere.
Note that all the compounds are recycled and nothing is lost–the system has the potential to be in a steady state. For example, while weathering removes 2 moles of CO2 from the atmosphere, calcification returns one of them and volcanic outgassing the other. Of course, the real system is likely to deviate from the steady state over time. For one thing, the scheme consists of two compound processes, which operate on entirely different time scales. Whereas weathering and sedimentation equilibrate within a few thousand years, the processes in the deep earth work on time scales of millions of years. Hence, these two processes are uncoupled, so that the quantity of CO2 released by the volcanoes is independent of what happens on the surface of the continents and in the oceans.
Now we approach the crux of the argument. Suppose that the Earth is cool, and that the volcanoes blow a large quantity of CO2 into the atmosphere. Climate will warm up as a result. At higher temperatures all chemical reactions speed up. This general rule will also apply to weathering. Hence, this process will readily consume the extra CO2. We have here a regulatory mechanism of the global climate, a thermostat for the entire earth. Deviations of the CO2-content in the atmosphere are automatically neutralized.
The thermostat in the Urey cycle has often been invoked as an example of homeostatic Gaia. Yet, it would also work if we were to remove all life from the Earth while leaving the rest intact. (In fact, it has been suggested that the strength of the thermostat on a dead Earth would have been sufficient to keep the temperatures within the constraints required for life. Another blow for Gaia!). All life can do is to modulate the basic scheme; it does not drive the system or alter its basic structure. Let’s see how life modulates weathering.
As soon as rocks appear on the continental surface, they are immediately overgrown by microorganisms, lichens and vascular plants. These organisms concur in extracting the nutrients on which they depend from the minerals. They help fragmenting the rocks and suck the precious nutrients out of them until a soil is left over. As the retrieved nutrients are passed on from one organism to the next one, they distribute themselves over the entire continent and finally fertilize the ocean. Weathering is the mining for life, and we all join in the eating. As to calcium silicate weathering, the organisms suck the required CO2 out of the air and pump it into the ground. The asterisk in Figure 2 means that the life speeds up the degradation of calcium silicate many times.
Now, suppose again that the earth is cold. The weatherers are in poor shape. But here comes the sudden outpouring of CO2 from the volcanoes. The temperature goes up and an immediate outburst of biotic activity follows, so that the weathering rate jumps up. The thermostat is far more efficient than without life. Here we have Gaia!
Paleontologist Steve Gould thinks that there is no trend in evolution. Yet, how limited is his perspective. He only considers the organisms per se and ignores our top-down systems approach. Let us now look at the evolution of the global thermostat during the Phanerozoic.
Robert Berner (1994) modeled the cycle to make a rough estimate of the concentration of atmospheric CO2 during the past 600 million years. His conclusion is staggering. In the period from 600 to 450 million years ago the concentration of atmospheric CO2 was 15–20 times as high as today. Then, between 450 and 300 million years ago, there is a sudden decrease until the present level is reached. At this stage we are in the Permo-Carboniferous ice age. Subsequently, CO2 goes up a bit, but then it comes down again until the Pleistocene.
What might have caused the spectacular decrease of CO2 between 450 and 300 million years? There are various factors in play, but Berner shows that the origin and evolution of the vascular plants must have been crucial. It is exactly in this period that extensive woods begin to cover the continents. The plants with their deep roots enormously enhance the weathering rate of calcium silicate. Berner shows that this enhancement of weathering rates by the vascular plants has been the main factor in bringing down CO2 levels.
Is there a trend or not?
The same applies to the other two asterisks in Figure 2. Consider the deposition of silica on the ocean floor. Some 600 million years ago silica simply precipitated from the seawater. We still find thick crusts of it in the Proterozoic rocks. It is easy to imagine that those spontaneous incrustations were most uncomfortable for creatures that lived in the ocean. They threatened to destroy all the delicate tissues that were exposed to the water. But what do we see? In the course of geological time, ever more organisms appear in the sea, which are capable to suck the dissolved silica from the water and use it in the production of their finely sculptured skeletons. I think of the sponges, the radiolarians and, more recently, the diatoms, with their gigantic blooms in the Recent ocean. These delicate structures do not precipitate spontaneously from the seawater, but are assembled with utmost precision by the living cell. In the Recent ocean all silica is sucked out of the water and removed by the silicifying organisms. The hazard is overcome. Spontaneous precipitation no longer occurs. The seawater itself is strongly undersaturated–it is almost a silica vacuum.
Is there a trend or not?
Like silica, vast amounts of carbonate mineral precipitated spontaneously out of the sea 600 million years ago, without any strong intervention of organisms ( Grotzinger and Knoll 1999). Here too, the spontaneous incrustations formed a hazard for life in the ocean. But the biotic response was slightly different than in the case of silica. In the course of evolution a curious, almost internally contradictory development emerged. Today, the ocean water is replete with potent inhibitors of the spontaneous precipitation of carbonates. Among these “crystal poisons” are muci, and these substances are excreted by innumerable marine organisms (Marin et al. 1996). Thus, the precursors of limestone are not sucked away, and the sea is not a vacuum of calcium and bicarbonate, as was the case for silica. The open ocean has become “anticalcifying” – it cannot get rid of its limestone spontaneously because the formation of it is arrested. Inside their cells and tissues, however, some of the marine organisms make microscopic vesicles where the limestone is allowed to precipitate. Hence, all limestone production in the open sea is biologically regulated at present. Marin et al. (1996) observed that the same anticalcifying mucus that protects the soft tissues from incrustation helps organizing skeleton formation by keeping crystallization in check. Thus, in the course of evolution the production of limestone was brought under a strict biological regime. Major events marking the transition from spontaneous to controlled precipitation were the Cambrian event and the emergence of pelagic calcification in the Mesozoic.
Is there a trend or not?
The list of trends can easily be extended with, for instance, the growing involvement of life on cycling of water and rocks, or the unfolding of biogeochemical pathways. In all these cases we see the same general pattern emerging: the grip of the biota on the rest of the system becomes larger and larger as time progresses, despite catastrophes and extinction events. In terms of Figure 1, this implies that the arrows to and from the biota become progressively thicker. One may speculate that the development of human society is well in agreement with this general picture. The trends in the Urey cycle are particularly revealing, because they result from the evolution of eukaryotic organisms, including plants and animals. Remember that Gould specifically denied the existence of trends in the evolution of these organisms. Thus, from a Gaia point of view Gould’s position seems to be untenable. In a recent paper on directionality in the history of life, Knoll and Bambach (in press) reach a similar conclusion.
Where does this bring us in terms of Kirchner’s five Gaias? I feel that our observation does not fit any of his categories and propose to add a sixth goddess to the list: “Strengthening Gaia”. This hypothesis would imply that, on the multimillion years scale, the interactions between the biota and its geological environment become stronger with time. The question then is where to put it in the ascending spectrum of strength and ugliness. Clearly, Strengthening Gaia is compatible with the two weak Gaias, Influential and Coevolutionary. The biota influences its environment and adapts to changing conditions by Darwinian evolution. But there is more to Strengthening Gaia. In Figure 1, Influential and Coevolutionary Gaia simply mean that the arrows to and from the biota exist, not that they become progressively thicker. Weak Gaia is not strong enough to explain the observations.
Although further critical reflection is required, Strengthening Gaia appears to be consistent with Darwinian theory. We would have to assume that, as the biota influences its environment, it adds new niches to the existing repertoire and then adapts to them. This brings about a subsequent round of environmental changes and adaptations, and so on. Thus, although Strengthening Gaia is a great deal stronger than the weak Influential and Coevolutionary Gaias, it is not an ugly hypothesis, but one that can be tested.
Can we move up any further along the scale into strong Gaia without risking Kirschner’s epistemological anathema? After all, the concept of Strengthening Gaia is only descriptive, as it merely implies an upward trend in biological involvement over time. By itself, Strengthening Gaia has no implications for the nature of this involvement.
We have seen that the role of life in the Urey cycle has regulatory overtones. Silicic acid entering the ocean prompts blooms of silicifyers. They scavenge this nutrient and remove it from the ocean before it can produce harmful crusts on delicate tissues. As more silicic acid enters the ocean, more is taken out by the scavengers (unless some other nutrient, such as iron, becomes limiting). Similarly, anticalcification and subsequent calcification seem to make the ocean a more hospitable place than it was in the Precambrian. Do we have to invoke teleological Gaia to explain the emergence of these cleansing faculties? I don’t think so. They began as local adaptations, giving selective advantage to ancestral species. Their present oceanwide regulatory power emerged as a corollary to the success of the offspring. As to the vascular plants, one may envisage that their success was not primarily caused by their capability to lower atmospheric CO2. It rather was enhanced retrieval of energy and nutrients that locally gave them selective advantage. Whereas originally the honing of the global thermostat was insignificant, it became incorporated in life’s regulatory repertoire as vegetation spread over the continents.
It is time to propose Regulating Gaia, the seventh on the list. This goddess is even stronger than Strengthening Gaia, but here, too, we remain on safe Darwinian ground. Strengthening and Regulating Gaia can both be tested and falsified. In fact, they prompt us to give special attention to instances of weakening and deregulation. Examples are the demise of anaerobic metabolism in response to the oxygenation of the atmosphere and the snowball Earth event (Hoffman and Schrag 2000). Lovelock and Kump (1994) propose another case in point, related to the Urey cycle. They argue that, if the burning of fossil fuel were continued long enough, we might arrive at a runaway greenhouse world where the vegetation cover decreases with increasing global temperatures. Are such instances exceptions to the rule, mere hiccups on an ascending trend towards increasing global regulation? It is on our answer to this question that the fate of Strengthening and Regulating Gaia depends.
What about the classical strong Gaias–Homeostatic, Teleological and Optimizing? I agree with Kirchner that these concepts are confusing, as they imply a predetermined purpose in nature. It is useful to realize however that they refer to a higher level of organization than the individual regulatory mechanisms discussed so far: they allude to the emergent behavior of all regulatory mechanisms taken together.
My feeling is that there is something extraordinarily important to learn here, but that we are not yet in a position to bring it properly under words. We are now approaching the heart of Gaia, the great intuition of Lovelock. The ecologist William Hamilton (cited by Morton 1999) compared Lovelock with Copernicus, who discovered that the Earth turns around the sun, but could not formulate a mechanism to clarify his observation. We had to wait for Newton to hear the explanation. Who will be the Newton for Gaia?
Gaia gives us the choice between two attitudes. We can follow Kirchner and throw the goddess into the dustbin. Or we take a more adventurous stand and regard Gaia as a signpost, a guiding principle for future discoveries. I opt for the latter choice. We must gratefully acknowledge Kirchner for keeping our feet on the ground. But to me Lovelock remains the visionary who leads us ahead. So, let us reclaim Gaia from overcritical epistomologists.
Again I evoke the shock of recognition sensed by millions of people when for the first time they saw the Earth from space. I also remind the new science of Gaia, by which Lovelock tried to keep that unique experience alive.
At this point it is good to remember the original purpose of Academia–to give substance to the fundamental values underlying society and lead the public debate on matters that are of concern to us all. Particularly in these times of rapidly changing value systems there is an urgent need for the critical voice of science.
I mention three points where science, and in particular the study of Gaia, might further such a debate. In the first place there are the widespread evils of fundamentalism and dogmatism. Science brings the notion that absolute truth does not exist and that modesty in our dealings with the world around us is more in place. Gaia is not true, it only is a useful concept, a guiding principle if we wish to become better acquainted with the planet that we inhabit.
In the second place, I point to the scourges of racism and nationalism that re-emerge time and again and caused untold damage and suffering. In a recent publication on the future of education, the French philosopher Edgar Morin (1999) wrote that we must stop considering Serbia, France, England or America as God’s own country, and that only the Earth itself deserves this name. I think that the debate on Gaia can only reinforce this consciousness. How compelling is the idea that Gaia was not implanted ready for use on our planet, but that it grew and evolved, in labor and pain, over billions of years. The world on which we are allowed to live is a haven, entirely unique in the universe.
Finally, Gaia sharpens our view on the vulnerability of life’s environment and of the way in which life counters serious disturbances. A Dutch poet wrote: “The weak forces win in the end.” So, let us learn to respect both the persistence and the weaknesses of Gaia.
In this editorial I argue that Gaia has been part of science right from the original formulation of the idea. If New Age has claimed it for its own purposes, all we have to do is to claim it back. We should not tolerate outside interference with a healthy scientific concept.