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Lovelock on Gaia 18.12 concepts 769 AM 12/12/03 5:40 pm Page 769 concepts The living Earth Gaia evidence, these earlier ideas remained anec- James Lovelock dotal. In 1925 Alfred Lotka conjectured that Organisms and their environment magine a science-based civilization far it would be easier to model the evolution of evolve as a single, self-regulating distant in the Galaxy that had built an organisms and their material environment system. Iinterferometer of such resolving power coupled as a single entity than either of them that it could analyse the chemical composi- separately. Gaia had its origins in these earlier tion of our atmosphere. Simply from this thoughts, from the evidence gathered by the track temperature change, not regulate it. analysis, they could confidently conclude biogeochemists Alfred Redfield and Evelyn But the restraining function connecting that Earth, alone among the planets of the Hutchinson and from the mind-wrenching growth with temperature is not negotiable; Solar System, had a carbon-based life and top-down view provided by NASA. chemistry,not biology,sets its constants. an industrial civilization. They would have Although welcomed by atmospheric At this stage, the Gaia theory was missing seen methane and oxygen coexisting in the scientists, Earth scientists were cautious. plausible control mechanisms. The first dis- upper atmosphere, and their chemists Biologists, especially Ford Doolittle and covered was a biological process that redressed would have known that these gases are Richard Dawkins, argued strongly that global the imbalance of the nutritious elements continually consumed and replaced. The self-regulation could never have evolved, as sulphur and iodine — these are abundant in odds of this happening by chance inorganic the organism was the unit of selection,not the the oceans, but deficient on the land surface. chemistry are very long indeed. Such pers- biosphere. In time I realized that they were It was widely assumed that hydrogen sulphide istent deep atmospheric disequilibrium right — but still I thought, something keeps and sea salt aerosol drifted from the ocean to reveals the low entropy characteristic of life. the Earth habitable. In 1981 I composed a the land. In 1971 I discovered that methyl They would conclude that ours was a live model of dark- and light-coloured plants iodide and dimethyl sulphide were ubiquitous planet — and the presence of CFCs in the that competed for growth on a planet in in the Atlantic surface waters, and from my atmosphere would suggest an industry progressively increasing sunlight. My inten- measurements Peter Liss calculated their unwise enough to have allowed their escape. tion was not to make a blueprint for the fluxes in 1974. He argued that these biogenic As part of NASA’s planetary exploration Earth, but a model to show that Gaia is gases were the main carriers of the natural team in 1965, thoughts such as these led me consistent with natural selection. This elemental cycles of sulphur and iodine. to propose atmospheric analysis for detect- ‘Daisyworld’ regulated its temperature close Then in 1982, the geochemists James ing life on Mars.I also wondered what could be to that fittest for plant growth and — unusu- Walker,P.B.Hayes and Jim Kasting suggested keeping Earth’s chemically unstable atmos- ally for an evolutionary model made from that the weathering of calcium silicate rock phere constant and so appropriate for life,and coupled differential equations — it was could regulate carbon dioxide and climate. what kept the climate tolerable despite a 30% stable, insensitive to initial conditions and Greater warmth leads to more rainfall and increase in solar luminosity since the Earth resistant to perturbation. Daisyworld is a faster removal of carbon dioxide from formed. Together, these thoughts led me to darwinian,but the evolution of the organisms the atmosphere by rock weathering, which the hypothesis that living organisms regulate and the evolution of temperature proceed as provides a negative feedback on tempera- the atmosphere in their own interest, and the a single, coupled process. The model was ture. This plausible mechanism is by itself too novelist William Golding suggested Gaia as much criticized,but so far has resisted falsifi- small to account for the observed rate of its name. Although the concept of a live Earth cation. It was easy to show that Daisyworld weathering.Organisms on the rocks and in the is ancient, Newton was the first scientist to tolerates ‘cheats’ — daisies that grow but soil bring it to life as a Gaian mechanism; their compare the Earth to an animal or a vegetable. offer nothing towards self-regulation. Other growth varies with temperature and their Hutton, Huxley and Vernadsky expressed critics claimed that daisies would adapt to presence amplifies the rate of weathering. similar views but, lacking quantitative changing temperature and therefore simply In 1986, there was the awesome discovery by Robert Charlson,James Lovelock,Meinrat Andreae and Steven Warren of a connection between biogenic dimethyl sulphide gas — the product of ocean algae — its oxidation in the atmosphere to form cloud condensation nuclei, and the subsequent effect of the ASA/BETTMANN/CORBIS N clouds formed on climate. We wondered whether this could be a Gaian regulatory mechanism through the feedback between climate change and algal growth. By the end of the 1980s there was suffi- cient evidence, models and mechanisms, to justify a provisional Gaia theory. Briefly, it states that organisms and their material envi- ronment evolve as a single coupled system, from which emerges the sustained self-regu- lation of climate and chemistry at a habitable state for whatever is the current biota. Like life,Gaia is an emergent phenomenon, comprehensible intuitively, but difficult or impossible to analyse by reduction — not surprisingly it is often misunderstood. Our planet in perspective: Gaia theory explains the constancy of our unstable atmosphere. A simple automatic mechanism, like a NATURE | VOL 426 | 18/25 DECEMBER 2003 | www.nature.com/nature 769 © 2003 Nature Publishing Group 18.12 concepts 769 AM 12/12/03 5:40 pm Page 770 concepts thermostatically controlled oven, requires a Table 1 Some predictions from Gaia sensor to measure the difference between the Prediction (year) Test and result ambient temperature and the set point of Mars lifeless from atmospheric evidence (1968). Viking Mission (1977). Strong confirmation. regulation, and an amplifier to magnify this difference and apply it as negative feedback That elements are transferred from the ocean to Dimethyl sulphide, dimethyl selenide to oppose unwanted change. Living systems the land by biogenic gases (1971). and methyl iodide found (1973, 2000). rarely work in this simple way; they require positive as well as negative feedback for home- Climate regulation through biologically enhanced Microorganisms found greatly to increase rock weathering (1981). the rate of rock weathering. ostasis, and a restraining function replaces the simple manual set point. This function That Gaia is aged (1982). Generally accepted. allows regulation within a physiologically acceptable range, instead of at a single set Climate regulation through cloud albedo control Still under test. value. Andrew Watson and other critics have linked to algal gas emissions (1987). assumed that to be Gaian, a planet must regu- Archaean atmospheric chemistry dominated Still under test but tending to be accepted. late near perfectly — but physiological systems by methane (1988). may perform no better than is needed.No one doubts that humans are in thermostasis, yet Oxygen has not varied from 21DŽ5% for Still under test. our core temperatures range from 35 to 40 ᑻC the past 200 million years (1989). and our extremities from 5 to 45 ᑻC. This Boreal forests regulate their regional climate Now part of global climate modelling. may appear imprecise, but it serves us well. in a Daisyworld manner (1988). For the past ten million years the Earth’s average surface temperature has covered a Biodiversity is a necessary part of planetary Tested by models, but not yet similar range between 11 and 16 ᑻC. This is self-regulation (1992). in the field. not evidence of incompetent regulation — it That the current interglacial is an example of Still controversial. is sufficient to sustain the Earth system. The system failure in a physiological sense (1996). occasional failure of the Earth to regulate efficiently — as in the present interglacial — resembles the physiological condition of a Hamilton wondered, in a joint paper with inorganic chemistry. Moreover, as it grows fever where positive feedback dominates. Lenton, if the need of organisms to disperse older the Earth system weakens, and before Gaia theory does not contradict darwin- was the link that connected ocean algae with long a large planetesimal impact may throw ism, rather it extends it to include evolution- climate. In a 1999 television programme, our planet prematurely into its final hot, dry ary biology and evolutionary geology as a Hamilton said: “Just as the observations of state.A few thermophiles in oasis ecosystems single science. In Gaia theory, organisms Copernicus needed a Newton to explain them, might survive, but we could never recapture change their material environment as well as we need another Newton to explain how dar- the abundant life and lush environment we adapt to it. Selection favours the improvers, winian evolution leads to a habitable planet.” now enjoy. The Earth system is elderly and and the expansion of favourable traits Then the ice began to melt. In 2001, at a we should treat it with respect and care. extends local improvement and can make it conference in Amsterdam — at which four Gaia theory reconciles current thinking global. Inevitably there will be extinctions principal global change research pro- in evolutionary biology with that in evolu- and losers, winners may gain in the short grammes were represented — more than a tionary geology.
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