Geophysiology: a new look at earth science
1. Planetary medicine
Few of us have avoided the experience of imagining that we were the victims of some fatal but romantic disease. It usually happens after reading a medical textbook and identifying our minor symptoms with those described.
In the affluent parts of the world, society may be undergoing collectively the same experience. The difference is that the apparent hypochondria is about the world itself rather than about their individual selves. The equivalent of the medical textbooks is the ubiquitous doom scenarios. There is no shortage of planetary ailments to identify with, from the psychosociological drama of Orwell's nightmare vision to the dismal prospect of nuclear winters and acid rain.
As in hypochondria, the real problem is not that these global maladies are unreal, but the uncertainty over whether the present symptoms are prodromal of disaster or whether they are no more than the growing pains of the world.
Intelligent hypochondriacs do not consult a biochemist or a molecular biologist about their worries; they go instead to a physician. A good physician knows that hypochondria often masks a real ailment quite different from the one imagined by the patient. Could it be that our very deep concern about the state of the world is a form of global hypochondria? If it is, then we might ask whether it is wise in such an event to seek only the advice of expert scientists like climatologists or biogeochemists. It could be that the real planetary malaise is beyond the understanding of their expertise. It might seem that we have no other options; the practice of planetary medicine does not yet exist.
Let us assume that there is some truth in the foregoing speculation and consider what might be the next step to take. It might involve the establishment of this new profession of planetary medicine.
What would be the qualifications of a planetary physician? If the history of medicine is a guide, they will grow from guesses and empiricism. But early in the history of medicine, physiology, the systems science of individual humans, strongly influenced its further progress. The recognition by Paracelsus that "the poison is the dose" was a physiological enlightenment still to be discovered by those environmentalists who seek the unattainable and pointless goal of zero for pollutants. The discovery, by Harvey, of the circulation of the blood added further to the wisdom of medicine, as did meteorology to our understanding of the earth. The expert sciences of biochemistry and microbiology came much later; it took a long time before their new vision enhanced the practice of medicine.
The purpose of this paper is to introduce geophysiology, a systems approach to the earth sciences. It is the essential theoretical basis for the putative profession of planetary medicine. In no way would geophysiology replace or lessen the importance of the established sciences; it is complementary to them.
The paper will discuss the theoretical basis of geophysiology and conclude by considering how this approach might assist in the design of procedures for the diagnosis and prevention of incipient maladies of our planet. The conclusions will be especially applicable to the humid tropics.
2. Theoretical basis of geophysiology
Notions of the earth as some kind of living system have a long history. In the last century, Dumas and Boussingault described the cycling of the elements like carbon and nitrogen between life and its environment and laid the foundations for the science of biogeochemistry. The first scientific expression of the idea that the sum of the biota might be more than just a catalogue of the species was that of Vernadsky (1945), who used the term 'biosphere' to define the region of the planet where life could be found. The new science of biogeochemistry was extensively developed by Sillen (1966), and by Hutchinson (1954) and, most recently, by Bolin and Cook (1983), McElroy (1983), Garrels (1976), Broecker (1980), and Whitfield (1981).
Geophysiology developed in the late 1960s as an unintended by-product of the space exploration program of NASA. It arose during attempts to design experiments to detect life on other planets, particularly Mars. For the most part, these experiments were geocentric and based on the notion of landing an automated biological or biochemical laboratory on the planet and using it to recognize life by the well-known techniques available to life scientists on earth. Hitchcock and Lovelock (1966) took the opposing view that not only were such experiments likely to fail because of their geocentricity, but also that there was a more certain way of detecting planetary life, whatever its form might be. This alternative approach to life detection came from a systems view of planetary life. In particular, it suggests that if life can be taken to constitute a global entity, its presence would be revealed by a change in the chemical composition of the planet's atmosphere. This change of composition could be compared with that of the abiological steady state of a lifeless planet.
The reasoning behind this idea was that the planetary biota would be obliged to use any mobile medium available to them as a source of essential nutrients and as a sink for the disposal of the products of their metabolism. Such activity would render a planet with life as recognizably different from a lifeless one. At that time there was a fairly detailed compositional analysis by infrared astronomy of the Mars and Venus atmospheres, and it revealed both planets to have atmospheres not far from chemical equilibrium. Therefore, they were probably lifeless. By contrast, earth's atmosphere, viewed in this way, was seen to be far from equilibrium, with oxidizing and reducing gases coexisting in what was clearly an unstable state and that, nevertheless, was maintained steady by life. In the infrared, earth radiates its signature of life so clearly as to be detectable from well outside the solar system. The success of this approach to life detection forced our attention back to the earth and to the nature of the system that could hold so unstable an atmosphere in a steady state that was even more remarkably just right for life.
In the early 1970s, Lynn Margulis and I introduced the Gaia hypothesis. It postulated the earth to be a self-regulating system comprising the biota and their environment, with the capacity to maintain the climate and the chemical composition at a steady state favorable for life.
Most earth scientists today would accept that the atmosphere is a biological product, and this is a tribute to the success of biogeochemistry. But most would disagree that the biota in any way control atmospheric composition or any of the important variables (such as global temperature and surface redox potential) which depend on the atmosphere. The principal objection to Gaia or the geophysiological approach is that it is teleological. That is, the regulation of the climate or chemical composition on a planetary scale would require some kind of forecasting or clairvoyance by the biota. I will now try to show that this objection is wrong and that geophysiological regulation requires neither foresight nor planning. It is in fact a simple consequence of Darwinian natural selection. The evolution of the species is not independent of the evolution of their environment. The two evolutionary processes are in fact tightly coupled. Life and its environment evolve together as a single system so that not only does the species that leaves the most progeny tend to inherit the environment, but also the environment that favors the most progeny is itself sustained. What then is the mechanism of this geophysiological regulation?
Figure 1. Top - Geophysiological Model. A diagram drawn from control theory to illustrate as a single system the active feedback between the biota and their environment. In the diagram, the biota are represented as an amplifier connected to a sensor that recognizes any departure from the operating point of the system. Physical or chemical variables, such as temperature or oxygen concentration, coming from the environment or from external inputs, such as the sun or pollution, are summed and compared with the operating point of the system. If there is a difference, biota respond by active feedback so as to oppose it and to keep the system in homeostasis. The system also has the capacity to evolve, thus moving the operating point to a new steady value. This form of systems evolution is called homeorhesis.
Bottom - Biogeochemical Model. A diagram taken from biogeochemistry to illustrate the mass and energy transfer between the biota and their environment. In biogeochemistry the system is treated as a set of linked but separate parts. A change in one part, say the atmosphere, can alter the conditions of another part, such as the biota or the oceans, but the feedback between them, whether negative or positive, is taken to be passive rather than active and responsive. In biogeochemical systems, the evolution of the biota and the evolution of the environment are usually considered as separate and uncoupled processes.
Let us accept for a moment that the biota can profoundly influence their environment. The converse is also self-evidently true; that is, organisms are affected by the environment. To take atmospheric composition as an example, plants and animals are obviously dependent upon the oxygen, carbon dioxide, and nitrogen of the air, but they also produce all three of these gases. In other words, life and its environment are two parts of a closely coupled system where these two components are arranged in a feedback loop (Fig. 1, top). Perturbations of one will affect the other, and this in turn will feed back on the original change. The feedback may be negative and oppose the change, or positive, and enhance it, but it will not in general be nonexistent. Geophysiological models are distinguished from the conventional models of biogeochemistry (Fig. 1, bottom) by their amplified active feedback. As a consequence, they are more powerful and can adjust their operating points as the system evolves, a process termed 'homeorhesis' by Waddington (1976). Biogeochemical models are puny by comparison, and the operating point is commonly fixed by the chemical and physical constraints of the system.
What properties does this close coupling of life and its environment confer on the whole system? Does it explain the homeostasis and homeorhesis that is observed? The difficulty is that the diagram (Fig. 1, top) is much too simple; in reality, the biota and the environment are vastly complex entitles, interconnected in multiple ways, and there is hardly a single aspect of their interaction that can confidently be described by a mathematical equation. It occurred to me that a drastic simplification was needed, namely, reduction of the environment to a single variable, temperature, and of the biota to a single species, daisies. I first described Daisyworld in 1982, and I am indebted to my colleague Andrew Watson for the clear graphic way of expressing it illustrated in Fig. 2.
Figure 2. The effect of daisy cover on the mean temperature of Daisyworld, curve (A) and the effect of temperature on the growth of daisies, curve (B). In this example, the daisies are taken to be lighter in color than the bare planetary surface so that increasing daisy cover reflects more sunlight and so lowers the mean temperature. The left hand intersection of curves A and B is a stable equilibrium between daisies and temperature. The dashed curve (Al) is drawn for a lower solar luminosity than with curve (A). If the daisy cover did not respond to temperature, the difference in planetary temperature for the two solar luminosities would be (dT), about 16°C. If, as is more normal, the daisies responded to the cooler sun by covering less of the planet, the temperature difference would be (dTl), about 2°C. This simple responsive coupling between life and its environment is the basis of geophysiological regulation.
3. Daisyworld model
Daisyworld is a cloudless planet with a negligible atmospheric greenhouse that bears life only in the form of daisies. To start with, let us assume that the daisies are white. Because they are lighter than the ground in which they grow, they tend to increase the albedo of their locality, and, as a consequence, are cooler than a comparable area of bare ground. Where the daisies cover a substantial proportion of the planetary surface they will influence the mean surface temperature of the planet. The variation will be as illustrated by the curve (A) in Fig. 2. The parallel dotted line shows how the curve (A) might shift if there were a change in some external variable that influenced the planetary temperature. An example of such a variable is the output of radiant energy from Daisyworld's sun.
Like most plant life, the daisies grow best over a restricted range of temperatures. The growth rate peaks near 20°C and falls to zero below 5°C and above 40°C. As a function of temperature, the steady-state population of daisies will be as in the curve (B) of Fig. 2. Curves (A) and (B) relate the temperature to the Daisyworld population at steady state, and the steady state of the whole system must be specified by the point of intersection of these two curves. In the example, it can be seen that there are two possible steady-state solutions. It turns out that the solution where the derivatives of the two curves have opposite signs is unconditionally stable, whereas the other solution is unstable. If the system is initialized at some arbitrary point, it will normally settle down at the stable solution.
What happens to this stable solution when some change of the external environment alters the planetary temperature? Suppose, for example, that the sun warms up as our sun is said to be doing. If the daisy population is artificially held constant, the planetary temperature will simply follow the change of heat output of the sun; there will be a much larger temperature change than if we allow the daisy population to grow to its new natural steady state. In this new steady state, the daisy population has changed so as to oppose the effect of a change in solar output.
Very few assumptions are made in this model. It is not necessary to invoke foresight or purpose on the part of the daisies. It is merely assumed that the growth of daisies can affect the mean planetary temperature and vice versa. Note that the mechanism works equally well whatever direction the effect is. Black daisies would have done as well; as long as the Daisyworld albedo is different from that of the bare ground, some thermostasis will result. The assumption that growth is restricted to a narrow range of temperatures is crucial to the working of the mechanism, but all main-stream life is observed to be limited within this same narrow range; indeed, the peaked growth curve (B) is common to other variables besides temperature, for example, pH and the abundance of nutrients.
In a recent paper, Watson & Lovelock (1983), this model is discussed in depth, and we emphasize there that the exercise was conducted not because we thought that daisies or any other colored plants regulate the earth's temperature by this mechanism, but because it is easily understood as a model of the close coupling between the biota and the environment. The daisy populations were modeled by differential equations borrowed directly from theoretical ecology, Carter and Prince (1981).
Daisyworld models have a novel and wholly unexpected property. Their mathematical solution is not limited, as are similar simple ecological models, to two species only. Indeed, the number of species that can be accommodated appears to be limited only by the speed and size of the computer used and by the patience of the user. The inclusion of feedback from the environment appears to stabilize the system of differential equations used to model the growth and competition of the species. Theoretical ecology models have nearly always ignored the environment of their imaginary species, just as geophysical and geochemical models have tended to ignore the biota. Daisyworld models are admittedly primitive and, as yet, limited to a few species and a single environmental variable. But they are models of an active system where the biota and the environment are closely coupled, and they do share with the real world the same strong tendency to homeostasis and stability.
The power of the Daisyworld models is perhaps best illustrated by the imaginary world depicted in Fig. 3. It illustrates the time history of a planet where thermostasis is maintained during the progressive increase of luminosity of its sun and in spite of repeated disasters that destroy a substantial proportion of the daisies. In addition, the world is populated also by rabbits that eat the daisies and by foxes that cull the rabbits. The health of a self-regulating system is measured by its capacity to resist perturbation and by the rapidity and smoothness of the return to normal. The system illustrated in Fig. 3 (top) was perturbed four times during the course of its evolution by the abrupt but temporary deletion of 40 percent of the plant population.
These four perturbations were effectively resisted and the system rapidly recovered its former stable state. Figure 3 (bottom) illustrates the variation of the population of the species of this imaginary world. Between the perturbations, both the environment and the populations are seen to be stable. At the disturbances, the changes in the population and in the temperature take place in synchrony. The model is not concerned with the cause of the perturbations, and these could have been either internally or externally generated. This response resembles that described by S. J. Gould in his hypothesis of the punctuated evolution of the species. Gaia models are limited neither to daisies nor to the regulation of temperature by albedo change. Other environmental variables, such as the pH of the soil or sea or the abundance of oxygen and other elements, can plausibly be shown to keep within a narrow range by the same homeostatic processes that were illustrated in Fig. 2.
The regulation of the climate as a consequence of an evolutionary feedback system, involving atmospheric CO2 and the weathering of the rocks by the biota, has already been described by Lovelock and Whitfield (1982) and Lovelock and Watson (1982). The model is based on that of Walker et al. (1981), who assumed that when life started the climate was warm enough, in spite of a cooler sun, on account of a much higher concentration of CO2 in the air. It was thought to make up between 10 and 30 percent of the atmosphere. As the sun evolves and increases its flux of radiation, the temperature is kept constant by a progressive decrease of CO2. The process of CO2 removal is the weathering of calcium silicate rocks. The Gaian variant of Walker's model assumes that the biota are actively engaged in the process of weathering and that the rate of this process is directly related to the biomass at any time. If conditions are too cold, the rate of weathering declines, and as a consequence of the constant input of CO2 by degassing from the earth's interior, the CO2 partial pressure rises.
Models of this kind about CO2 and climate could add to the current interest in this most important environmental concern. They are based on an active feedback-control system, and with such systems, it is possible to predict instability and oscillation enhanced by positive feedback; these instabilities are most probable when the system nears the limits of its capacity to regulate. It is interesting to compare this prediction with the observations of the correlation between CO2 and climate that characterized the last glaciation, particularly the sudden and apparently synchronous rise of both CO2 and temperature at the termination of the glaciation. The exact sequence of these events is still uncertain but few doubt that the end was sudden and that both CO2 and temperature rose substantially on a global scale. These rapid changes some 12,000 years ago cannot be explained by geochemical or geophysical theory alone. They suggest a sudden change of biomass; most probably the sudden death of a proportion of the marine phytoplankton, an event that would reduce the rate of pumping of CO2 from the air. The geophysiological prediction of oscillatory instability near the limits of regulation fits with these observations. It is well known that glaciations are in synchrony with variations of solar illumination consequent upon the earth's orbital position and inclination, the Milankovich effect. This alone cannot account for the rapid reversal of the glaciation. But the Milankovich effect could be the trigger that synchronizes an otherwise free-running geophysiological oscillator.
Figure 3. (top) The evolution of the temperature on an imaginary Daisyworld populated by daisies. The color of Daisyworld changes in response to temperature over the range 5-40°C. It is also populated by rabbits that graze upon the daisies and by foxes that hunt the rabbits. In addition, at intervals Daisyworld is perturbed by catastrophes that cause the death of 40 percent of the daisy population. It is assumed that Daisyworld is warmed by a star that increases its luminosity linearly with time. When the planetary temperature exceeds 5°C, the daisies grow rapidly and are dark colored in response to the initial low temperature. The mean temperature rises by positive feedback until the operating temperature for homeostasis is reached. Homeostasis is maintained and the four perturbations resisted. The capacity of the system to restore homeostasis after a disturbance is seen to decline as the increasing solar output carries the system nearer to its limit. The dashed line illustrates the planetary temperature expected in the absence of life. (bottom) The populations of daisies (A), rabbits (R) and foxes (F) during the evolution of Daisyworld.
4. Contemporary geophysiology and the humid tropics
Gaia theory suggests that we inhabit and are part of a quasi-living entity that has the capacity for global homeostasis. This is the basis of geophysiology, and if this theory is correct, then we cannot model the consequences of perturbations, such as those that we are now causing by our own actions, as if the world were a passive system like the spaceship earth.
It has been said by politically-inclined critics that the Gaia hypothesis is a fabrication, an argument developed to allow industry to pollute at will, since mother Gaia will clean up the mess. It is true that a system in homeostasis is forgiving about disturbance, but only when it is healthy and well within the bounds of its capacity to regulate. When such a system is stressed to near the limits of its capacity to regulate, even a small jolt may cause it to jump to a new stable state or even to fail entirely. In these circumstances, pollution, changes in land use, or changes in the ecology of the continental shelves could be the recipes for disaster global in scale.
It could be that the regulation of the earth's climate is not far from one of these limits. Thus, if some part of climate regulation is connected with the natural level of CO2, then clearly we are close to the limits of its regulation. Carbon diioxide cannot be reduced much below the level observed in the last glaciation, near to 180 ppmv, without seriously limiting the rate of growth of the more abundant C3-type plants. If we perturb the earth's radiation balance by adding more CO2 and other greenhouse gases to the atmosphere, or reduce the earth's capacity to regulate by decreasing the area of forests, or both of these together, then we could be surprised by a sudden jump of both CO2 and temperature to a new and much-warmer steady state, or by the initiation of periodic fluctuations between that state and our present or a colder climate. A biogeochemist or a climatologist could argue that even if a system like Gaia exists, its response to environmental change would be extremely slow compared with current human concerns. Reasonable though this criticism may seem, it begs the question, for we as animals are a part of Gaia and can respond to human concerns, but also it is wrong to assume that a system that includes processes with slow response times cannot act quickly.
The anomalously low concentration of CO2 on earth in comparison with that of the other terrestrial planets, and especially the fact that the mean surface temperature of the earth is on the cool side of the optimum for the biota, suggest strongly that the biota are regulating the climate by pumping CO2 from the air. The common feature of most of our pollution and of our exploitation of the land surface seems to be unintentionally to thwart this natural process. How then do questions of global regulation bear on our special interest in the humid tropics? I think that it reinforces in several ways the general conviction from conventional modeling that large-scale changes of land use in the tropics will not be limited in their effects to those regions only (Dickinson, 1986) and geophysiology reminds us that the climatic effects of forest clearance are likely to be additive to those of CO2 and other greenhouse gases. Even the most intricate climate models of the present type cannot predict the consequences of these changes unless the biota are included in the models in a way that recognizes their very active presence and their preference for a narrow range of environmental variables. Putting the biota in a box with inputs and outputs, as in a biogeochemical model, does not do this. By analogy, the most-detailed knowledge of the biochemistry of oxidative metabolism in humans says nothing about how we sustain our personal thermostasis in the hot or cold environments we encounter.
Most climatologists agree that forests tend to increase the cloudiness of the atmosphere above them and that the clouds alter the climate of the forest regions, both in terms of temperature and rainfall (e.g., Molion, Chapter 15 and Solati. Chapter 16). The geophysiologist would see, in addition, that the active process of evapotranspiration by the forest trees could be coupled to the climatology so as to maintain the region in a state ofhomeostasis within that range of climate preferred by the trees.
There is as yet no answer to the question: What is the area of land of a region of the humid tropics that can be developed as open farm land or as silviculture without significantly perturbing both the regional and the global environment? It is a question like: What is the proportion of skin area that can be burned without a significant systems failure? This second question has been answered by the direct observation of the consequences of accidental burns; so far as I am aware it has not been modeled. Perhaps detailed geophysiological modeling can answer the environmental question. Certainly the simple models illustrated here were well behaved, but if human physiology is a guide, empirical conclusions drawn from a close study of the local climatic consequences of regional changes of land use are more likely to yield the information we seek.
In some ways the ecosystem of, for example, a forest in the humid tropics is like a human colony in Antarctica or on the moon. It is only self-supporting to a limited extent, and its continued existence depends upon the transport of nutrients and other essentials from the world. At the same time ecosystems and colonies try to minimize their losses by conserving water, heat, and nutrients; to this extent they are self regulating. The tropical rain forest, likewise, keeps wet by modifying its environment so as to favor rainfall. Traditional ecology has tended to consider ecosystems in isolation. Geophysiology reminds us that all ecosystems are interconnected. By analogy, in an animal, the liver has some capacity for the regulation of its internal environment, and its liver cells can be grown in the isolation of tissue culture. But neither the animal nor its liver can live alone; they depend upon their interconnection.
We do not know if there are vital ecosystems on the earth, although it would be difficult to imagine life continuing without the anoxic ecosystems of the sediments. The forests of the humid tropics do not add significantly to the world's oxygen budget nor to the exchange of essential elements through the atmosphere. Their intensive biosynthesis is recycled within their boundaries. Where they may be significant on a global scale is in their effect on climate through evapotranspiration and the effect of their presence on the regional albedo. The transfer of nutrients and the products of weathering down tropical rivers are obviously part of their interconnection and may have a global significance.
If evapotranspiration or the movements of materials down tropical rivers to the oceans are vital to the present homeostasis, then the replacement of forests with agricultural or grassland surrogates would not only deny those regions to their surviving inhabitants but also might threaten the rest of the system as well. We do not yet know whether the tropical forest systems are vital to the present planetary ecology. They might be like the temperate forests that seem to be expendable without serious harm; temperate forests have suffered extensive destruction during glaciations as well as during the recent expansion of agriculture.
We are, so far as geophysiology is concerned, very much in the natural-history phase of information gathering. It would seem, therefore, that the traditional ecological approach of examining the forest ecosystem in isolation is as important to our understanding as is the consideration of its interdependence within the whole system.
Insight into the potential value of physiology for the understanding of global problems can come from reading the book by Riggs (1970), Control Theory and Physiological Feedback Mechanisms, particularly those sections concerned with temperature regulation and with systems failure. The recent paper by Holling (1986) relates the physiological approach to contemporary problems.
If it turns out that Gaia theory provides a fair description of earth's operating system, then most assuredly we have been visiting the wrong specialists for the diagnosis and cure of our global ills. We need answers to such questions as: How stable is the present system? What will perturb it? Can the effects of perturbation be reversed? And can the world maintain its present climate and composition without the humid tropics in their present form? These are all questions within the province of geophysiology.
This paper was prepared for the United Nations University, Tokyo, Japan. I am deeply grateful to the University and to its program officer, Dr. Walter Shearer, for unstinted encouragement and support.
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