During the early 1960s, while Ilya Prigogine realized the crucial link between nonequilib- rium systems and nonlinearity (seeSection 8.3.1), and Humberto Maturana puzzled over the organization of living systems (see Section 12.1.2), the atmospheric chemist James Lovelock had an illuminating insight that led him to formulate a model that is perhaps the most surprising and most beautiful expression of self-organization – the idea that the planet Earth as a whole is a living, self-organizing system.
The origins of Lovelock’s daring hypothesis lie in the early days of the NASA space program. While the idea of the Earth being alive is very ancient and speculative theories about the planet as a living system had been formulated several times, as we mentioned in the Introduction, the space flights during the early 1960s enabled human beings for the first time to look at our planet from outer space and perceive it as an integrated whole. This perception of the Earth in all its beauty – a blue and white globe floating in the deep darkness of space – moved the astronauts deeply and, as several have since declared, was a profound spiritual experience that forever changed their relationship to the Earth (see Kelley, 1988). The magnificent photographs of the whole Earth that they brought back provided the most powerful symbol for the global ecology movement.
At that time, NASA invited James Lovelock to the Jet Propulsion Laboratories in Pasadena, California, to help design instruments for the detection of life on Mars (see Lovelock,1979). In contemplating this problem, Lovelock found that the fact that all living organisms take in energy and matter and discard waste products was the most general characteristic of life he could identify. Lovelock assumed that life on any planet would use the atmosphere and oceans as fluid media for raw materials and waste products. Therefore, he speculated, one might be able, somehow, to detect the existence of life by analyzing the chemical composition of a planet’s atmosphere.
The terrestrial atmosphere contains gases like oxygen and methane, which are very likely to react with each other but coexist in high proportions, resulting in a mixture of gases far from chemical equilibrium. Lovelock realized that this special state must be due to the presence of life on Earth. Plants produce oxygen constantly and other organisms produce other gases, so that the atmospheric gases are being replenished continually while they undergo chemical reactions. In other words, Lovelock recognized the Earth’s atmosphere as an open system, far from equilibrium, characterized by a constant flow of energy and matter – the telltale sign of life identified by Prigogine around the same time.
The process of self-regulation is the key to Lovelock’s idea. He knew from astrophysics that the heat of the Sun has increased by 25% since life began on Earth and that, in spite of this increase, the Earth’s surface temperature has remained constant, at a level comfortable
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for life, during those 4 billion years. What if the Earth were able to regulate its temperature, he asked, as well as other planetary conditions – the composition of its atmosphere, the salinity of its oceans, and so on – just as living organisms are able to self-regulate and keep their body temperature and other variables constant? Lovelock (1991) realized that this hypothesis amounted to a radical break with conventional science. Rather than seeing the Earth as a dead planet, composed of inanimate rocks, oceans, and atmosphere, he proposed to consider it as a complex system, “comprising all of life and all of its environment tightly coupled so as to form a self-regulating entity” (Lovelock1991, p. 12).
In 1969 Lovelock presented his hypothesis of the Earth as a self-regulating system for the first time at a scientific meeting in Princeton. Shortly after that, a novelist friend, recognizing that Lovelock’s idea represents the renaissance of a powerful ancient myth, suggested the name “Gaia hypothesis” in honor of the Greek goddess of the Earth. Lovelock gladly accepted the suggestion and in 1972 published the first extensive version of his idea in a paper titled “Gaia as seen through the atmosphere” (Lovelock,1972).
At that time, the microbiologist Lynn Margulis was studying the very processes Lovelock needed to understand – the production and removal of gases by various organisms, including especially the myriad bacteria in the Earth’s soil.
The scientific backgrounds and areas of expertise of James Lovelock and Lynn Margulis turned out to be a perfect match. Margulis had no problems answering Lovelock’s many questions about the biological origins of atmospheric gases, while Lovelock contributed concepts from chemistry, thermodynamics, and cybernetics to the emerging Gaia theory. Thus the two scientists were able to gradually identify a complex network of feedback loops which – so they hypothesized – bring about the self-regulation of the planetary system (Lovelock and Margulis,1974).
The outstanding feature of these feedback loops is that they link together living and nonliving systems. We can no longer think of rocks, animals, and plants as being separate. Gaia theory shows that there is a tight interlocking between the planet’s living parts – plants, microorganisms, and animals – and its nonliving parts – rocks, oceans, and the atmosphere. The feedback cycles interlinking these living and noinliving systems regulate the Earth’s climate, the salinity of its oceans, and other important planetary conditions. In view of the threats of climate change and other global environmental predicaments, the understanding of the Gaia system is now not only a subject of great intellectual fascination, but has also become a matter of great urgency (as we shall discuss inChapters 16and17).
Gaia theory looks at life in a systemic way, bringing together geology, microbiology, atmospheric chemistry, and other disciplines whose practitioners are not used to commu- nicating with each other. Lovelock and Margulis challenged the conventional view that those are separate disciplines, that the forces of geology set the conditions for life on Earth, and that the plants and animals were mere passengers who by chance found just the right conditions for their evolution. According to Gaia theory, life creates the conditions for its own existence.
At first the opposition of the scientific community to this new view of life was fierce. It is intriguing that of all the theories and models of self-organization, Gaia theory encountered
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by far the strongest resistance. One is tempted to wonder whether this highly irrational reaction by the scientific establishment was triggered by the evocation of Gaia, the powerful archetypal myth.
Scientists claimed that Gaia theory was unscientific because it was teleological – that is, implying the idea of natural processes being shaped by a purpose – although Lovelock and Margulis never made such a claim. The scientific establishment attacked the theory as teleological, because they could not imagine how life on Earth could create and regulate the conditions for its own existence without being conscious and purposeful. “Are there committee meetings of species to negotiate next year’s temperature?”, those critics asked with malicious humor (quoted in Lovelock,1991).
Lovelock responded with an ingenious mathematical model, called “Daisyworld” and published in collaboration with the marine and atmospheric scientist Andrew Watson. The model is a computer simulation of a vastly simplified Gaian system, in which it is absolutely clear that the temperature regulation is an emergent property of the system that arises automatically, without any purposeful action, as a consequence of feedback loops between the planet’s organisms and their environment (Watson and Lovelock,1983).
During subsequent years, Lovelock and his colleagues designed several more sophisti- cated versions of Daisyworld, which generated lively discussions among biologists, geo- physicists, and geochemists (see Harding, 2006; Schneider et al.,2004). In fact, Daisyworld has become the mathematical basis for many other simulations of Gaia in the multidisci- plinary fields of Earth system science and biogeochemistry. In particular, such models have been applied increasingly to studies of climate change at the prestigious Hadley Centre for Climate Prediction and Research in the UK and at other similar institutions.
Because of the central role of the Daisyworld simulation in Gaia theory, we have asked one of Lovelock’s collaborators, the ecologist Stephan Harding, to discuss the model in some detail in a guest essay (see p. 166), and to show how it has contributed to the transformation of the Gaia idea from a controversial hypothesis into a respected theory.
To be considered as truly alive, the Gaia system must be shown to satisfy the various criteria of life that we discuss in this book. In this regard, it is useful to recall that there are three different levels of organization we have to consider in the complex systems of life. The first is self-organization, the capability of assuming an organized structure thanks to the inner rules of the system. The second level is autopoiesis, when the self-organization is such that it can regenerate from within all its own components (this is the necessary conditions for life itself). Finally, there is the level of the living organism, when autopoiesis becomes associated with cognition, and we have therefore both the necessary and sufficient conditions for life. In the literature, including the literature on Gaia theory, these three levels are not always clearly distinguished, and occasionally we find some confusion between them, as, for example, when self-organization is assumed to be equivalent to life.
These are fascinating issues, for the details of which we have to wait untilChapter 16, since we first need to gain a more comprehensive understanding of cognition (Chapter 12) and also of ecosystems (Chapter 16), which are the living systems most similar to the system of the planet as a whole, known to ecologists as the Earth system.
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Guest essay
Daisyworld Stephan Harding
Schumacher College, Dartington, Devon, UK
Daisyworld is a computer model of a planet, warmed by a sun with steadily increasing heat radiation, and with only two species growing on it – black daisies and white daisies. Seeds of these daisies are scattered throughout the planet, which is moist and fertile everywhere, but daisies will grow only within a certain temperature range (between 5°C and 40 °C, with optimal growth at temperatures near 22°C).
Lovelock programmed his computer with the mathematical equations, well known from thermodynamics, that correspond to all these conditions; chose a planetary temperature at the freezing point for the starting condition, and then let the model run on the computer. “Will the evolution of the Daisyworld ecosystem lead to the self-regulation of climate?” was the crucial question he asked himself.
The results were spectacular. As the model planet warms up (in later two-dimensional versions of the model), at some point the equator becomes warm enough for plant life. The black daisies appear first because they warm themselves by absorbing solar energy better than the white daisies and are therefore more fit for survival and reproduction. Thus in its first phase of evolution Daisyworld shows a ring of black daisies scattered around the equator (Figure 8.13).
As the planet warms up further, the equator becomes too hot for the black daisies and they begin to colonize the subtropical zones. At the same time, white daisies appear around the equator. Because they are white, they reflect solar energy and hence cool themselves, allowing them to survive better in hot zones than the black daisies. In the second phase, then, there is a ring of white daisies around the equator, and the subtropical and temperate zones are filled with black daisies, while it is still too cold around the poles for any daisies to grow.
Then the sun gets brighter still and plant life becomes extinct at the equator, where it is now too hot even for the white daisies. In the meantime, white daisies have replaced the black daisies in the temperate zones, and black daisies are beginning to appear around the poles. Thus the third phase shows the equator bare, the temperate zones populated with white daisies, and the zones around the poles filled with black daisies with just the pole caps themselves without any plant life.
In the last and final phase, vast regions around the equator and the subtropical zones are too hot for any daisies to survive, while there are white daisies in the temperate zones and black daisies at the poles. After that, it becomes too hot on the model planet for any daisies to grow and all life becomes extinct.
This is the basic dynamics of the two-dimensional Daisyworld system, which also applies in the most basic initial model with zero dimensions. The crucial property of the model that brings about the emergent self-regulation is that the black daisies, by absorbing solar energy, warm not only themselves but also the planet. Similarly, while the white daisies reflect solar energy and cool themselves, they also cool the planet. Thus solar energy is absorbed and reflected throughout the evolution of Daisyworld, depending on which species of daisies are present.
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(a)
(b)
Figure 8.13 The four evolutionary phases of Daisyworld. Panel (a) shows the evolution of temperature on Daisyworld. (b) The dashed curve shows the rise of temperature with no life present; the solid curve shows how life maintains a constant temperature (Lovelock,1991).
When Lovelock plotted the changes of temperature on the model planet throughout its evolution, he got the striking result that the planetary temperature is kept constant for a vast span of time (Figure 8.13). When the sun is relatively cold, Daisyworld increases its own temperature through solar energy absorption by the black daisies; as the sun gets hotter, the temperature is gradually lowered because of the progressive predominance of energy-reflecting white daisies. Thus Daisyworld, without any foresight or planning, regulates its own
temperature over a vast time range by the dance of the daisies.
What amazed and delighted Lovelock most was that his system of nonlinear equations, modeling the tight coupling between the planet’s nonliving environment and the growth of the two daisy species, produced two startling emergent properties. First, the overall temperature of the model planet remained remarkably constant over a vast period of time in spite of the shifting daisy populations and the ever-brightening sun; second, the temperature settled on a value just below the optimum for daisy growth.
My own work on Daisyworld, conducted with James Lovelock as a guide and mentor, involved designing more complex ecological communities on the model planet to explore how the increase in complexity would affect the stability of the planet’s temperature. We introduced
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many species of daisies with varying pigments, instead of just two; in some models the daisies evolve and change color; in others rabbits eat the daisies and foxes eat the rabbits, and so on (see Harding, 2004).
The net result of these highly complex models was that the small temperature fluctuations that were present in the original Daisyworld simulation flattened out, and self-regulation became more and more stable as the model’s complexity increased. In addition, we put catastrophes into our models that wipe out 30% of the daisies at regular intervals. We found that Daisyworld’s self-regulation is remarkably resilient under these severe disturbances.
These extensive explorations confirmed that feedback loops linking environmental influences to the growth of daisies, which in turn affect the environment, are an essential feature of the Daisyworld model. When we broke some of these cycles, so that there was less influence of the daisies on the environment, the daisy populations began to fluctuate wildly and the whole system went chaotic. But as soon as the feedback loops were restored, linking the daisies back to the environment, the model stabilized and its self-regulation emerged again. These simulations showed me in an impressive way that more complex ecological communities are, in general, more stable, as ecologists had long suspected (see Elton,1958; MacArthur,
1955; Odum,1953).
Another interesting feature of the model that may have disturbing implications for the real Gaia is what happens at the moment when Daisyworld dies of overheating. Just before life disappears, the light daisies cope with small increments of solar energy by increasing their cover on what little bare soil remains. But under a very bright sun, with no more bare soil available, a small increase in solar energy extinguishes life with sudden rapidity. A similar event takes place in more complex versions of the model. Could it be that just an extra increment of pollution or habitat destruction might trigger an equally dramatic shift toward a new and potentially inhospitable climate regime on our real Earth?