Gaia Theory
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Copyright © April, 1999
The Gaia Hypothesis A distinction should be made between the "biosphere" and "Gaia". The "biosphere" is defined as "the geographical region where living organisms exist", while "Gaia" is defined as "the superorganism composed of all life tightly coupled with the atmosphere, oceans and surface rocks" (Lovelock 1995). Lynn Margulis has rejected the statements "the Earth is alive" or "Earth is an organism" since the outset (Bunyard 1996, p 55), although she recognizes that Gaia is very complex. Gaia is a system where life evolves and influences the environment; and vice versa. Due to an increase in the knowledge and understanding of planetary processes, the Earth is no longer considered to be alive, even by the theory's advocates (Kerr, 1988). Gaia is essentially a "cybernetic system" that has developed mechanisms for the maintenance of conditions conducive to life, by life.
Criticisms of Gaia Theory Another criticism has been against the notion that the Earth is "alive" (against the analogy between planetary physiology and organismic physiology), because natural selection cannot act at the planetary level. Doolittle argued that "global homeostasis cannot be for the same reasons that the organs of an animal promote physiological homeostasis" (Barlow 1991). For the Earth's apparent homeostatic adaptations to have developed, a set of "rival Gaia's" would have had to have been selected against. "Biospheres which did not develop efficient homeostatic regulation of their planetary atmospheres tended to go extinct" (Dawkins 1982, pp 234-236). The notion of "interplanetary selection" is of course, ludicrous. The argument however, was directed against using "organismic physiology" as an analogy for "planetary physiology". Dawkins essentially argues that organisms have homeostatic adaptations because of evolutionary processes, such as natural selection. Planets cannot develop such "adaptations" unless a "good of the group" argument is used. Lovelock acknowledges these arguments, and agrees that "Gaia lacked a firm theoretical basis" initally (Lovelock 1988, p 32). The Earth is no longer considered to be alive, even by the theory's advocates (Kerr, 1988). Rather, the Earth is a complex and regulatable system that has evolved, due to the collective influence of life. The main criticism of "Gaia" has been that the theory is teleological -- i.e. there is some purpose for Gaia, or some foresight into her planning. Evolution has no foresight or planning. Natural selection is 'blind' to the future (Dawkins 1986). Lovelock has said that he never proposed a teleological theory (Lovelock 1990), and even constructed a computer model to refute the argument (see below). The original proposal had been less theoretically sound than it was poetic, and this was probably the reason for the misconception. Evidence lacked for control mechanisms, and as such, the theory was regarded as belonging more to the theological realm, than the scientific realm. Another argument against Gaia theory deals with the processes behind evolution. Gaia theory seemed to imply that a "global altruism" was required -- individuals would function for the good of the group. Strict Darwinism on the other hand, refutes the "good of the group" argument. Evolution occurs in small, but cumulative and gradual steps (Dawkins 1986). Individuals compete with others to grow and reproduce, and they do so to preserve and pass their genes on to future generations. Natural selection acts on the individual or rather, on the phenotypic effects of an individual's genes -- not on the group as a whole (Dawkins 1989). Individuals that form groups do so for the benefit of their selfish genes (e.g. there is safety in numbers; safety for genes). An individual may act altruistically, but does so for its own gene preservation, not for group preservation. Thus, the evolution of a global altruism is not easily reconciled. Gaia theory, however, is not an anti-Darwinist notion. Gaia and natural selection are complementary. Gaia, as a planetary phenomenon of self-regulation, emerges from natural selection (Lenton 1998). One concern with the theory is how or when did Gaia actually come into existence? Life has existed on Earth for at least 3.8 BY. There have been several mass extinctions throughout Earth's history due to external and internal events, but when did the regulation become a planetary phenomenon? The question is perhaps, unanswerable. Some believe that Gaia was merely an "accident" of evolution, and that the theory isn't needed since conventional evolutionary thought already provides sufficient explanations. Is the theory at least testable? A comparison of Earth's neighbouring planets may give some insight into how Gaia self-regulates. Computer simulations may provide a basis for theoretical testing. Studying biogeochemical cycles has provided a basis for the mechanisms of self-regulation. A question arises as to whether Gaia can reproduce. Lovelock argues that "life is a planetary scale phenomenon" and as such, has no need to reproduce (Joesph 1990). On the other hand, Lovelock describes the terraforming of Mars in his book The Greening of Mars (1984) -- a self-sustaining biosphere is generated in stages by imparting life to Mars. In this case, Earth would have "reproduced" through human actions. Terraforming Mars or even the moon, could provide a practical basis for testing the theory's merits. Lovelock is not necessarily concerned with whether his theory is "right" -- a good theory is one that generates new questions and gives some direction to research.
Comparative Planetary Systems Venus, at an average distance of 108 million kilometres from the Sun, is comparable in size and mass with the Earth. The Venusian atmosphere consists primarily of CO2: 98% CO2, 1.7% N2, and trace amounts of O2 and CH4. The average surface temperature of Venus is at 477 degrees C, while the surface pressure is 90 atm. Mars (277 million kilometres from the Sun), is much smaller in size and mass than the Earth. The Martian atmosphere also consists primarily of CO2: 95% CO2, 2.7% N2, 0.13% O2, and trace amounts of CH4. The average surface temperature of Mars is at -53 degrees C, and the surface pressure is at 0.0064 atm. The chemistry of both Venus and Mars, can be explained purely by simple chemistry (Lovelock and Margulis, 1979). The conditions on Earth however, cannot be explained solely by abiotic factors. Earth, with an average distance of 150 million kilometers from the Sun, has an atmospheric composition of 0.03% CO2, 79% N2, 21% O2, and trace amounts of CH4 and other gases. Methane is consistently found in parts per million throughout the atmosphere. Methane is also continuously being produced, at about 500 million-tons annually, by living organisms (Lovelock 1995). Oxygen is produced by plants as a waste product -- it is a highly reactive gas and can be explosive at certain concentrations. As seen on Venus and Mars, there are only trace amounts of oxygen in their atmospheres. Most of the oxygen had reacted with carbon compounds (e.g. methane) to form CO2, which is highly abundant in both alien atmospheres. Earth on the other hand, has very low concentrations of CO2 (the reason for this will be described below). Earth has an average surface temperature of 13 degrees C, a surface pressure of 1 atm, and has vast amounts of liquid H2O on its surface. Venus has no surface water -- all of its water is in the atmosphere. Mars may have had liquid water at some point (indicated by geological formations, such as canals and splosh craters), but now only has ice near its poles (Lovelock 1965). When compared to Venus and Mars, Earth is far from chemical equilibrium. As predicted from the Gaia hypothesis, life may be the cause of this disequilibrium. The characteristics of a lifeless Earth have been calculated based on planetary positioning. Earth's average temperature should range between 240 and 340 degrees C (Joseph 1990, p 147). Thus, Earth has a great potential in becoming Venus-like. Yet, on a geological time-scale, global conditions appear to have been relatively constant and favorable for life. There have been a few events of considerable change in the atmospheric composition and climate, during Earth's history. How stable is Gaia? Perturbations of the Gaian system seems to occur in a punctuated fashion (e.g. meteor impacts, glaciations, etc). Mass extinctions have occurred on more than one occasion in Earth's hisory. One such incident was the increase in O2 levels, which resulted in mass extinctions of anaerobes due to oxygen poisoning (Joseph 1990, pp 99-103; Lovelock and Margulis, 1974). Earth's atmosphere is believed to have initially been either reducing or highly reducing. In either case, a trend towards a less reducing (or an oxidizing) atmosphere can be seen. Life originated on Earth at least 3.8 BYA as seen from evidence of microfossils dating back that far. With the development of photosynthesis, O2 levels gradually increased to the point of being toxic to most life on Earth at that time (i.e. anaerobes). A net yield in O2 was accompanied by carbon burial, which reduced the amount of CO2 in the atmosphere. Some species (aerobes) evolved to utilize and/or detoxify the poison, while anaerobes had been strongly selected against (i.e. anaerobes do not survive in high O2 concentrations). Eventually, O2 levels stabilized and reached its present atmospheric concentration of 21%, which has been maintained for the past 600 million years. The point is that life modified its environment on a global scale, and some mechanism must exist that has maintained this anomalous atmospheric composition. Disregarding the human influence on the carbon cyle, the respiration rate (using O2 as a nutrient and releasing CO2 as waste) roughly equals the photosynthetic rate (using CO2 as a nutrient and releasing O2 as waste) on a global scale (Lovelock 1988, pp 127-128). Thus, life may have had a role in creating and maintaining an oxidizing atmosphere, which subsequently allowed for more complex and multicellular organisms to evolve. This event did not require foresight or planning by the biota. It was an emergent property that had its origins in natural selection and feedback loops. The Gaian system is a tight coupling in the evolution of organisms and their environment (Bunyard 1996. p 26). The evolution of organisms cannot be separated from their changing environments. Individuals modify their environment while competing with others. These changes in the environment caused by individuals can then feed back on to those individuals or their descendents.
Daisyworld The model starts off with a cold planet that warms up to 5 degrees C (due to the increase in solar luminosity) at which point, black daisies begin to grow. Black daisies absorb heat more readily than white daisies, thus black daisies proliferate, covering the surface of Daisyworld and effectively increase the global temperature. This is an example of positive feedback: the black daisies grow at a low inital temperature, which causes the mean temperature to increase, which allows more black daisies to grow, which increase the temperature further. At some point, the temperature becomes too high for black daisies to survive and optimally reproduce, and mutant white daisies begin to out-compete black daisies for space. As white daisies cover more surface area, the global temperature drops to a point at which black daisies out-compete white daisies, in which case, black daisies grow and raise the mean temperature again. This is an example of a negative feedback loop: the system fluctuates around some "optimal" value and opposes any trends to either extreme. The mean temperature of Daisyworld remains relatively constant over time at the optimal temperature for growing, and is regulated by simple competition between the daisies. Thus, regulation on a global scale emerges from simple Darwinian competition, although it has been demonstrated that evolution can occur without natural selection (Saunders 1994). Gaia evolves as a system through feedback loops. Feedback loops are not necessarily dependent on natural selection, however, natural selection can fine tune the regulation mechanism. The model itself has had several criticisms (William 1990, pp 138-145). One problem, is that unstable models can be constructed as easily as stable models ("stable" in this case, meaning self-sustaining). If "cheats" such as grey daisies are introduced into the model, they might disrupt the regulation mechanism and the system could become unstable. Lovelock created a model that included grey daisies, and the system still became self-regulating. Others have created similar models which ended up being unstable. Another problem is that the model did not allow for adaptation: the characteristics of the daisies were already built into the program, rather than arising by evolution. A model containing color mutation has been created to address this problem (Lenton 1998). In this model, daisies are equally likely to mutate in either color direction -- black or white, with gray variants included. Thus "Gaian" and "anti-Gaian" behaviors can arise at the individual level, and planetary self-regulation may still arise. Daisyworld is, of course, an oversimplification of the real world. The model demonstrates that a self-regulating system can emerge from a set of simpler processes without being teleological.
Feedback Loops, Clouds and the Greenhouse Effect The greenhouse effect is a planetary process which may lead to a "global warming." The actual mechanism behind the process is as follows. Solar radiation enters the atmosphere and reaches the surface of the planet where that radiation is either reflected or absorbed. The solar energy is reflected or re-emitted at longer wavelength as infrared radiation from the planetary surface, and then is absorbed by the various gases in the atmosphere. The mean temperature of the planet may increase, if the rate of absorption of radiation by greenhouse gases exceeds the rate of release of energy into space. Whether the radiation is absorbed or reflected, depends on the albedo (or "reflectivity") of the surface. A mechanism that cools the planet, occurs by cloud formation. Clouds tend to have a high albedo, and therefore reflect most incoming light (~45%) before it ever reaches the planet's surface. As such, clouds have a cooling effect. Cloud formation is known to originate from sulphur compounds in the atmosphere. Global cooling has been recorded after large volcanic eruptions (Lovelock 1988, p 146). Volcanoes emit sulphur compounds such as sulfur dioxide and hydrogen sulfide (as well as CO2). These gases are oxidized in the stratosphere, and congregate with water vapor forming sulphuric acid droplets. These droplets persist in the atmosphere, and exist as a white haze that reflects incoming light, thereby cooling the planet. Carbon dioxide is the primary greenhouse gas and is present in the Earth's atmosphere at 340ppm. Venus has an amount of CO2 300 000 times as much, while Mars (with most of its CO2 frozen in the surface) has about 20 times as much as Earth (Lovelock 1988, pp 133-135). On Earth, sources for CO2 consist of volcanic emissions and sinks are present as calcium silicate rocks; life can also be part of the sources or sinks. As mentioned previously, CO2 is removed from the atmosphere by photosynthesis: oxygen is released, while carbon undergoes burial. Life acts as a "pump": carbon is pumped out of the atmosphere and into the ground where it reacts with calcium silicate rocks forming calcium bicarbonate; or into sediments, as calcium bicarbonate shells. Carbon then continues on in the cycle, being transported by ground water and plate tectonics. Life enhances the rate of carbon cycling than can be done by purely abiotic means (e.g. weathering of calcium silicate rocks to react with CO2) and reduces the amount of CO2 in the atmosphere, which has an effect on global temperature. Microorganisms are considered to play a principle role in the global system (Markos 1995; Barlow 1991, p 50; Bunyard 1996, pp 95-101). They may be small individually, but they can reproduce relatively quickly (e.g. doubling time every 20 min), and they occupy nearly every niche on the planet. Collectively microbes have global effects on ocean salinity, atmospheric composition, and indirect effects on climate. As mentioned above, sulfur compounds play a role in cloud formation and consequently global temperature. It has been found that marine algae produce sulphuric compounds, i.e. dimethyl sulphide (DMS) (Lovelock et al. 1972). DMS is a betaine (i.e. an electrically neutral salt) made from the decomposition of dimethylsulfoniopropionate (DMSP) (Lovelock 1988; Lenton 1998). Microbes use betaines for protection against dessication as a response to salt stress, and as an anti-freezing agent. The DMSP conversion to DMS is catalysed by the enzyme DMSP lyase. In the atmosphere, DMS can be converted to sulphate aerosols which nucleate cloud formation. With the Daisyworld model as an analogy, the marine algae represent white daisies: they increase the albedo of the planet, which has a cooling effect on the global temperature. The analogy is perhaps, too simple a description. It has been found that feedbacks on the production of DMS switch over different time periods, and that "no single relationship between temperature and DMS emissions is consistent with all existing data" (Lenton 1998).
Reductionism versus Holism and their Complementarity
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