Scientists Debate Gaia: The Next Century. Stephen H. Schneider, James R. Miller, Eileen Crist, and Penelope J. Boston, eds. MIT Press, Cambridge, MA, 2004. 377 pp., illus. $50.00 (ISBN 0262194988 cloth).
Since its appearance, the Gaia hypothesis has attracted criticism, skepticism, and outright hostility, for a number of reasons. First, the name “Gaia” (meaning goddess of the earth) implied to some that Earth was alive and also goal-directed (i.e., teleological). The alternative “weak Gaia” hypothesis, which held that Earth was self-regulating (i.e., homeostatic through feedback mechanisms), was more palatable to a larger number of scientists. However, many scientists still believed this version of Gaia was merely a metaphor that explained relatively little that was (presumably) not already understood. Finally, there was no consistent statement or definition of the Gaia hypothesis.
Nevertheless, the Gaia hypothesis is widely credited with having stimulated the development of the science of Earth systems. The Gaia hypothesis challenged the then prevalent reductionist approach of most science, because it required multidisciplinary approaches to understanding Earth's biological and geophysical realms and their interactions. The science of Earth systems received a further boost with the growing realization in the late 20th century that Earth's environment has affected civilization in the past, and that anthropogenic activities are now affecting art and civilization. The current surge of studies aimed at detecting life on other planets also stems in part from the Gaia hypothesis.
So what is the current status of Gaia? Scientists Debate Gaia: The Next Century attempts to answer the question by building on an earlier (1988) Gaia symposium. For one thing, in the authors' view, Gaia has been upgraded from hypothesis to theory. Gaia is certainly a theory at some stage of development, because it is an innovative way of viewing Earth itself (see the introduction, by James Lovelock). Despite the blossoming of Earth systems science, however, there is still no clear definition of the Gaia theory. One author (Lenton, chapter 1) equates Gaia to the “Earth system with abundant life.” This definition implies that Earth cannot function as a system without life. Perhaps this is true for Earth as we know it, but physicochemical systems can exhibit emergent properties such as feedback and self-regulation, and there is no reason to think that such systems could not function on other planets without life, given suitable conditions. Otherwise, how could simple nonliving systems have given rise to life on Earth? Another definition restricts Gaia to the biosphere alone (Volk, chapter 2). Why then do we need the term “Gaia”? “Biosphere” is much more widely accepted and used in the literature.
Second, Gaia theory states that Earth is in equilibrium (steady state) as a result of feedback and self-regulation (homeostasis). The concept of Earth in homeostasis is based on the notion of the uniformity of nature. Early in the 19th century, Charles Lyell (on the basis of observations of modern processes and their rates of change) formulated what came to be called the principle of uniformitarianism. This principle is usually blithely stated in textbooks as “the present is the key to the past,” but it meant far more than this to Lyell. According to him, Earth's behavior was cyclic, so that Earth exhibited no net change (Gould 1987). In Lyell's view, for example, as mountains were slowly uplifted in one place, they were slowly eroded elsewhere, resulting in no net change (i.e., in equilibrium). Geologists are proud to claim this principle as their own, because they infer the existence of past processes and events on the basis of modern analogs. However, uniformitarianism really underlies all scientific inquiry. Indeed, the concept of equilibrium oozes all through modern science. It allows us, for example, to study rates of change for systems that are presumably in steady state. Such studies indicate that Earth's systems do indeed exhibit equilibrium behavior, because they typically move back toward their original state when disturbed. Nevertheless, the feedback mechanisms may not be as tightly coupled as Gaia theory would seem to predict, because feedbacks have had to evolve, and time lags occur between stimulus and response (Föllmi et al., chapter 7; Kump, chapter 8).
Moreover, Earth's geologic record is rife with evidence of directional (secular, noncyclic) changes. Directional changes may be so slow that the systems appear to be in equilibrium, when in fact the systems are changing on geologic scales of time. In their early papers, Lovelock and Margulis pointed to the presence of an atmosphere in disequilibrium (i.e., the presence of oxygen) as evidence for life. Early critics countered by pointing out that the presence of oxygen contradicted the Gaia hypothesis of global homeostasis. A similar theme resurfaces several times in the current volume. Volk, for example, argues for a nonteleological view of the Earth as “a waste-world: a system of by-products (and their effects)…. Organisms make metabolic products aimed to ensure their success at living and reproducing, not aimed at transforming or controlling the global environment” (p. 27).
Thus, the processes that occurred during the ancient past have also varied with historical circumstances, or contingency (Gould 1987). One cannot truly understand the behavior of Earth's systems (Gaia) without examining the geologic record, because processes observable on human time scales do not necessarily scale upward to geologic time scales (as Lyell inferred they did), nor do historical constraints remain the same (Martin 1998). (Indeed, the words geology and Gaia share a common etymological derivation.) Chemical elements have always reacted in the same (uniformitarian) manner, to be sure, but the historical constraints under which they have reacted have changed through time. For example, the cooling of the earth led to the formation of continents and the appearance of continental shelves. Continental shelves, in turn, provided widespread (photic) habitat for early photosynthesizing cyanobacteria, the metabolic by-product of which (oxygen) led to eukaryotes. Various branches of photosynthetic eukaryotes eventually led to eukaryotic plankton in the seas and terrestrial forests on land, forming the base of early food chains (Martin 1998).
Paradoxically, then, in helping to maintain equilibrium (homeostasis), Gaia may be slowly driving Earth away from equilibrium because Earth's systems require the flow of matter and energy (including the metabolic products and structures of other creatures) to function (Schwartzman and Volk, chapter 11). Could Gaia therefore be out of equilibrium on geologic scales of time because it obeys the laws of thermodynamics? In the case of the biosphere, natural selection acts to increase the total mass and energy flux through a system, so long as there is unutilized matter and energy made available (Lotka 1925). Through natural selection, species possessing superior energy-capturing and energy-directing devices direct more energy and matter into their biomass than other species, causing the total biomass of the system (ecosystem, biosphere) to increase. Improved energy acquisition and accumulation in one species reduces the total energy available to other species by sequestering nutrients in biomass; to survive, the other species must improve their acquisition of energy. As a result, there has been a “ratchet effect” on the biosphere in terms of energy flow and diversity (Martin 1998). It is precisely this kind of behavior that accounts for the seemingly teleological behavior of Earth's systems and counters the criticisms of evolutionary biologists about what is selected in Gaia (Schneider, chapter 4; Sagan and Whiteside, chapter 15).
Of course, computer models provide a way to effect long-term change on human time scales. Most of the models presented in this volume are based on Lovelock's “Daisyworld,” which was intended only to demonstrate the feasibility of feedback and self-regulation by a system (in Daisyworld, life regulates Earth's surface temperature by regulating the planet's albedo). Daisyworld was never meant to be more than this, but a number of Gaia models are based on Daisyworld because it provides a convenient starting point. Two chapters in this volume point out flaws in the original Daisyworld model and present more realistic models (Weber and Robinson, chapter 20; Nordstrom et al., chapter 21). Although one can test models using sensitivity experiments, the best way to test them is to evaluate them against geologic and climatic data sets. Unfortunately, there are few attempts in this volume to evaluate models against data.
Specific case studies (which might make more scientists willing to view Gaia with less skepticism) are also few in this volume. Besides the chapter by Föllmi and colleagues on phosphorus and carbon cycling, two of the most intriguing are those by Kleidon (chapter 25) on the response of Amazonian forests to aridity in the tropics during glacial intervals, and by Gómez and colleagues (chapter 28) on the bacterial flora of extreme (low-pH) environments of the Tinto River (Spain), which has obvious implications for finding life on other planets.
To sum up, this volume does indeed demonstrate that Gaia is an innovative way of viewing the earth. Like Gaia itself, Gaia theory is also evolving. But, in order to evolve as more than a way of thinking, Gaia theory must continue to test its predictions against historical records on different time scales. It is in this way that Gaia will gain broader acceptance among scientists in the next century.