Gaia

Gaia Interactions between the geosphere, biosphere, and atmosphere have been studied for at least the past 40 years, but, until the late 1970s, conventional wisdom has been dominated by the view that life exists only because material conditions on Earth happen to be just right. The idea that these interactions can be self-regulating is new. Gaia, the Greek goddess, was the name suggested to James Lovelock by the novelist William Golding to express the idea that life defines the material conditions needed for its survival, and that it makes sure they stay there. They hypothesis first grew from attempts to explain why conditions on Earth differed so markedly from those on its ‘dead’ neighbours Venus and Mars. In collaboration with the microbiologists Lynn Margulis, Lovelock became intrigued by several observations that appeared to suggest that conditions ideal for supporting life had been maintained on Earth against odds as unlikely as ‘surviving unscathed a drive blindfold through rush-hour traffic’:1. For at least the past 2 billion years, the Earth's atmosphere has been maintained in a state of profound chemical disequilibrium, in which incompatible gases such as oxygen and methane coexist. Indeed, it was while working at NASA on the Viking mission to Mars that Lovelock, in collaboration with the philosopher Dian Hitchcock, first hypothesized the unlikeliness of there being life on Mars because of the chemical equilibrium of its atmosphere. By their reckoning, the gases of Mars and Venus are like the exhaust gases from an internal combustion engine in which all the useful energy is spent.2. The Earth appears to have maintained a surface temperature of between 10 °C and 30 °C, ideal for the sustenance of life, throughout the past 3.5 billion years. This is in spite of the fact that Earth now receives between 1.4 and 3.3 times more energy than it did at the time of its formation.3. The apparent indispensability of certain gases that are peculiar to Earth led protagonists of the Gaia hypothesis to ask: ‘What purpose does constituent X serve in the atmosphere?’ Ammonia, for example, is present in trace quantities, yet it is considered to be essential in maintaining soils at a pH of around eight, that is, optimal for sustaining life. Compared to the atmospheres of Mars and Venus, in which carbon dioxide is concentrated at levels of more than 95 per cent, carbon dioxide makes up only 0.03 per cent of the Earth's atmosphere, where it is essential in driving photosynthesis. However, the atmosphere is particularly susceptible to increases in carbon dioxide—too much triggers a devastating ‘greenhouse effect’ in which planetary temperature would soon rise above 30 °C and all life would die. Methane is more or less absent from the Martian and Venusian atmospheres, but its presence in minute but ubiquitous concentrations of 1.7 parts per million on Earth is crucial to the support of life by maintaining atmospheric oxygen levels.4. Considering the current input of salt to the sea from the land, it would take only 80 million years for the oceans to reach their present level of salinity. In fact, the salinity of the world's oceans has been maintained at a more or less constant 3.4 per cent. The importance of ‘managing’ salinity is illustrated by the fact that new organisms are capable of surviving in salinities greater than 6 per cent.In 1981, W. Ford Doolittle published an important critique of Gaia theory in which he wondered ‘how does Gaia know if she is too cold or too hot, and how does she instruct the biosphere to behave accordingly? From his perspective, Doolittle was uncomfortable with the idea that Gaia seemed to require a teleological capacity for foresight and planning in the biota. Strenuous efforts were now required to understand mechanisms by which the planet might self-regulate. The result was Daisyworld, a numerical simulation of an ecosystem comprising, at first, just two species of daisy—black and white.

Daisyworld is a bleak and cloudless planet, with a constant and low concentration of greenhouse gases, that orbits a star not unlike our Sun in which solar luminosity is increasing. The mean surface temperature of Daisyworld is therefore determined by the balance between radiant energy received from the star and energy radiated back to space from the planet, according to the reflectivity of its surface (albedo). Both species of daisy survive at temperatures of between 5 °C and 40 °C, their optimal temperature being 22.5 °C. The model examines changes in the relative abundance of the black and white daisies in response only to changing temperature, itself controlled largely by whether the high-albedo white daisies, or the low-albedo black daisies, dominate.

During the first ‘growing season’, as temperature rises above 5 °C, the white daisies are disadvantaged because, by reflecting sunlight, they make the planet too cool. As Daisyworld's surface temperature creeps above 22.5 °C, the black daisies are increasingly disadvantaged because they absorb too much energy and, consequently, the planet overheats. Between them, the daisies act as a negative feedback mechanism stabilizing their environment. It is only because of the inexorable rise in solar luminosity that the daisies are eventually no longer able to regulate the temperature, and they all die. Increasingly sophisticated simulations of Daisyworlds, some with many more species of daisies, some incorporating ‘rabbits’ that feed on the daisies, themselves predated by ‘foxes’, all demonstrate a powerful tendency for the planet to regulate temperature at a level most conducive to the survival of its biota.

Self-regulation, or homeostasis, has thus become the essential defining property of the modern Gaia theory. Restated, it now proposes that living organisms and their material environment are tightly coupled. The coupled system is a superorganism, and as it evolves there emerges the ability to regulate climate and chemistry. By analogy to the impact that physiology had in bringing together microbiology and biochemistry with medicine, geophysiology has become the new trans-disciplinary environment in which planetary-scale feedback mechanisms are investigated. Geophysiology, an approach to Earth science first advocated by the pioneering geologist James Hutton more than 200 years ago, imposes no teleological demands on the biota. Homeostasis arises as a natural consequence of biota– environment interactions.

Geophysiologists are only just beginning to glimpse mechanisms by which homeostasis might proceed. Physiological analogues abound, such as the homeostatic regulation of blood glucose level by the hormones glucagon, which stimulates glucose production, and insulin, which stimulates glucose utilization. Faced with acutely high blood glucose levels, the body will generate large quantities of insulin that eventually decline to normal levels as the glucose anomaly is dissipated. On a global scale, comparable ‘acute’ crises in the Earth system are exemplified by the relatively sudden decline in atmospheric carbon dioxide levels that followed the formation of the Himalayan mountains. By drawing down and fixing atmospheric carbon dioxide as calcium carbonate, or limestone, the weathering of calcium silicate rocks acts as a huge negative greenhouse effect. The Himalaya are therefore implicated fundamentally in global cooling in the Tertiary period. Furthermore, the uplift of the Tibetan plateau, the largest elevated area of the continents, and the subsequent initiation of the monsoon, has had a profound effect on global climate circulation. The manifold responses of the Earth system to Himalayan mountain-building are yet to be understood fully.

Most geochemists agree on the important role of land vegetation in promoting chemical weathering, thereby leading to the drawing down of atmospheric carbon dioxide and a reduction of the greenhouse effect. However, perhaps the most convincing proof of the control exerted by biota over climate arose from the observation, first made from satellite data, of a possible connection between cloud cover over the oceans and lush, oceanic algal blooms. Nearly all species of oceanic algae produce dimethyl sulphide (DMS) as a by-product of a reaction by which they protect themselves from the saltiness of the sea. Some of the DMS is released into the air where it is oxidized to form microscopic particles of methane sulphonate. These particles constitute the principal cloud condensation nuclei: without them, clouds cannot form. DMS production therefore appears to control cloud cover, and hence the Earth's albedo, in a comparable way to the daisies on Daisyworld.

The importance of DMS and atmospheric carbon dioxide concentration in controlling global climate is supported by analyses of ice cores from Antarctica. Perhaps not surprisingly, they reveal that recent glacial periods coincide with unusually high abundances of methane sulphonate and low carbon dioxide, suggesting that the low glacial temperatures are promoted by high percentage cloud cover (high albedo) and low greenhouse effect. More ominous, however, is the recent analysis of the effects of temperature change on the feedbacks induced by changes in the surface distribution of marine algae and land plants. During the rising temperatures of the interglacials, such as we are now experiencing, the negative feedback mechanisms of both marine algae and land plants are increasingly disabled. As global mean temperature rises above 20 °C, the marine and terrestrial ecosystems are in positive feedback, thereby amplifying further increase in temperature. No one is claiming that the Daisyworld simulations come close to representing the true complexity of the Earth system. Nevertheless, Lovelock and his co-worker, Lee Kump, emphasize that these models do serve to warn of the dangers of the anthropogenic addition of greenhouse gases to the atmosphere, and the destruction of natural ecosystems, at a time when the geophysiological system may be at its least effective, and when the consequences of these actions may be amplified by positive feedback.

Jonathan P. Turner