Life wields a potent influence on the composition of the atmosphere composition, producing a chemical disequilibrium, as seen in the high concentration of reactive atmospheric oxygen. Photosynthesis sustains this chemical disequilibrium by releasing oxygen and removing carbon dioxide from the air. It occurred to James Lovelock (1965), an atmospheric chemist, that such a non-equilibrium atmospheric state would be a guide to the presence of life on other planets (see also Hitchcock and Lovelock 1967). This line of thought led Lovelock, in collaboration with microbiologist Lynn Margulis, to design the Gaia hypothesis (Lovelock 1972, 1979; Lovelock and Margulis 1974), the novelist William Golding suggesting the name. A key component of the Gaia hypothesis is the assertion that the biosphere maintains atmospheric homeostasis, primarily though negative feedback processes, and in so doing sustains environmental conditions conducive to life. This simple idea has proved extremely controversial and has stimulated scientific debate.
The central premise of the Gaia hypothesis comes in two versions, which give rise to the strong Gaia hypothesis and the weak Gaia hypothesis (Kirchner 1991). In the strong Gaia hypothesis, the biosphere is able to change the environment to suit life; in the weak Gaia hypothesis, the biosphere is able hold the environment within limits fit for life. Weak Gaia is a middle-of-the-road view between the Hadean and strong Gaian hypotheses. It predicts that life wields a substantial influence over some features of the abiotic world, mainly by playing a pivotal role in biogeochemical cycles. Life's influence is sufficient to have produced highly anomalous environmental conditions in comparison with the flanking terrestrial planets, Venus and Mars. Notable anomalies include the presence of highly reactive gases (including oxygen, hydrogen, and methane) coexisting for long times in the atmosphere, the stability of the Earth's temperature in the face of increasing solar luminosity, and the relative alkalinity of the oceans. By interacting with the surface materials of the planet, life has sustained these unusual conditions of temperature, chemical composition, and alkalinity for much of geological time. For this reason, to 'understand the Earth's surface we must under stand the biota and its properties; we can no longer rely on physical sciences for a description of the planet' (Margulis and Hinkle 1991, 11). The weak Gaia hypothesis does not call upon anything other than mechanistic processes to explain terrestrial evolution, but it does contend that the biosphere built and maintains the abiotic portion of the ecosphere.
Strong Gaia is, to some, the unashamedly teleological idea that the Earth is a superor-ganism controlling the terrestrial environment to suit its own ends. In his earlier writings, Lovelock seemed to favour strong Gaia. He believed that it is useful to regard the planet Earth, not as an inanimate globe of rock, liquid, and gas driven by geological processes, but as a sort of biological superorganism, a single life-form, a living planetary body that adjusts and regulates the conditions in its surroundings to suit its needs (e.g. Lovelock 1991). For Lovelock, Gaia includes the biosphere and the rest of the Earth. He explains that just 'as the shell is part of a snail, so the rocks, the air, and the oceans are part of Gaia' (Lovelock 1988, 10). It is unsure if life is able to exert an influence deep inside the Earth, if the biosphere is merely the epidermis of a living global creature. However, the evolving biosphere has maintained an intercourse with the Earth's interior through plate-tectonic processes, the magmatic system being the medium and the mechanism of communication (Shaw 1994, 246). In a recent book, Lovelock (2000) stepped back a little from his original 'somewhat outrageous statements'. He explained that, to make himself heard, he had to act like a neglected child who behaves badly in order to gain attention, and simply used the metaphor of a living Earth to make humourless biologists think that he really thought the Earth was alive and reproduces, whereas in fact he did not.
Gaian scientists claim that traditional biology and geology offer ineffective methods with which to study the planetary organism. The right tool for the job, they contend, is geo-physiology - the science of bodily process writ large and applied to the entire planet, or at least that outer shell encompassing the biosphere. The differences of approach and emphasis are fundamental - if the strong Gaia hypothesis should be correct, and the Earth really is an integrated superorganism, then the biosphere will regulate and maintain itself through a complex system of homeostatic mechanisms, just as the human body adjusts to the vicissitudes of its surroundings. Consequently, the biosphere may be a far more robust and resilient beast than has often been suggested. For instance, homeostatic mechanisms may exist for healing the hole in the ozone layer or preventing the global thermometer from blowing its top.
To Lovelock (1991), the Gaia hypothesis, in all forms, suggests three important things. First life is a global, not local, phenomenon. It is not possible for sparse life to inhabit a planet - there must be a global film of living things because organisms must regulate the conditions on their planet to overcome the ineluctable forces of physical and chemical evolution that would render it uninhabitable. Second, the Gaia hypothesis adds to Darwin's vision by negating the need to separate species evolution from environmental evolution. The evolution of the living and non-living worlds are so tightly knit as to be a single indivisible process. A coherent coupling between organisms and the material environment, and not just survival of the fittest, is a measure of evolutionary success. Third, the Gaia hypothesis may provide a way to view the planet in mathematical terms that 'joyfully accepts the nonlinearity of nature without being overwhelmed by the limitations imposed by the chaos of complex dynamics' (Lovelock 1991, 10). Geophysiologists, a new and interdisciplinary breed of Earth and life scientist who probes the complex interdependent cycles that run through the geosphere and ecosphere, are currently investigating these ideas.
It is salutary to rehearse Lovelock's demonstration of 'Gaia in action' in Daisyworld, a simple mathematical model of a hypothetical, Earth-like planet with no ocean. Andrew
Watson and Lovelock (1983) conceived and constructed Daisyworld to show that, without their being any designed purpose to life, a self-regulating biosphere can emerge from interactions between life and its physical environment. The model considers what happens to the biosphere as the Sun grows more luminous - brighter - over billions of years and temperatures rise (Figure 9.2). Two species of daisy - black daisies and white daisies - populate the surface of Daisyworld and form its biosphere. The black daisies warm their local environment because, having low albedos (reflectivities), they absorb sunlight, while the white daises cool their local environment because they have high albedos and reflect sunlight. Once daisies become established, they regulate the planetary surface temperature by competing for space. At the outset, when the Sun is relatively dim and conditions are cool, the seeds of black and white daisies occur over the planetary surface. Conditions near the Equator are warm enough to stimulate germination, but black daisies have the edge over the white
Figure 9.2 Predictions of planetary temperature in the Daisyworld model.
Figure 9.2 Predictions of planetary temperature in the Daisyworld model.
daisies because, with a lower albedo, they absorb more heat (sunlight). As the Sun grows brighter, temperatures rise, the black daisies spread polewards, and the white daisies start to thrive in the warmer temperatures and keep the surface temperature as much as 60°C lower than would be the case on a bare planetary surface. The ever-brightening Sun raises temperatures even more, and eventually the black daisies suffer stress from overheating and survive only at the poles. The white daisies, which with higher albedos reflect more solar radiation, now occupy the rest of the planet. Eventually, it becomes to hot for even the white daisies and the biosphere is destroyed. However, the model demonstrates that, for a long time, the biosphere stabilizes the planetary temperature while the Sun grows brighter. At the outset, black daisies increase surface temperatures, but as the heat-reflecting white daises come to out-compete their black cousins, the surface temperatures remain roughly constant.
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