Atmospheric composition remains constant, even though it should be unstable
Ocean salinity is constant
Since life started on Earth, the energy provided by the sun has increased by 25%; however the surface temperature of the planet has remained constant when measured on a global scale. Furthermore, he argued, the atmospheric composition of the Earth is constant. The Earth's atmosphere consists of 79% nitrogen, 20.7% oxygen and 0.03% carbon dioxide. This composition should be unstable, according to Lovelock, and its stability can only have been maintained with removal or production by living organisms.
Ocean salinity has been constant at about 3.4% for a very long time. Salinity stability is important as most cells require a rather constant salinity degree and do not tolerate much values above 5%. Salinity is partially controlled by evaporation processes, which mostly take place in lagoons.The only significant natural source of atmospheric carbon dioxide (CO2) is volcanic activity, while the only significant removal is through the weathering of some rocks. During weathering, a reaction causes the formation of calcium carbonate. This chemical reaction is enhanced by the bacteria and plant roots in soils, where they improve gaseous circulation. The calcium carbonate can be washed to the sea where it is used by living organisms with carboneous tests and shells. Once dead, the living organisms' shells fall to the bottom of the oceans where they generate deposits of chalk and limestone. In short, a rock was weathered, the resulting carbon dioxide processed by a living organism, and returned to a rock through sedimentation process. Part of the organisms with carboneous shells are the coccolithophores (algae), which also happen to participate in the formation of clouds. When they die, they release a sulfurous gas (DMS), (CH3)2S, which act as particles on which water vapor condenses to make clouds.
Lovelock sees this as one of the complex processes that maintain conditions suitable for life. The volcanoes make the CO2 enter the atmosphere, CO2 participates in limestone weathering, itself accelerated by temperature and soil life, the dissolved CO2 is then used by the algae and released on the ocean floor. CO2 excess can be compensated by an increase of coccolithophoride life, increasing the amount of CO2 locked in the ocean floor. Coccolithophorides increase the cloud cover, hence control the surface temperature, help cool the whole planet and favor precipitations which are necessary for terrestrial plants. For Lovelock, coccolithophorides are one stage in a regulatory feedback loop. Lately the atmospheric CO2 concentration has increased and there is some evidence that concentrations of ocean algal blooms are also increasing.
Criticism
The initial Gaia Hypothesis was highly criticized by many scientists for being teleological, a belief that all things have a predetermined purpose.
It was very critically received, in particular by Richard Dawkins and Ford Doolittle. These latter argue organisms could not act in concertation as this would require foresight and planning from them. They rejected the possibility feedback loops could stabilize the system. In 1982, Dawkins claimed "there was no way for evolution by natural selection to lead to altruism on a Global scale". They find it impossible to see how the feedback loops which Lovelock says stabilise the Gaian system could have evolved. They argue that, as Gaia can't reproduce herself, she cannot be alive in any meaningful sense. They also claim that the theory is not scientific because it is impossible to test it by controlled experiment. Lovelock offered the Daisyworld model as mathematical evidence to refute most of these criticisms.
The initial hypothesis was rather imprecise, and Lovelock later refuted accusation of teleologism by stating "Nowhere in our writings do we express the idea that planetary self-regulation is purposeful, or involves foresight or planning by the biota." �?(Lovelock, J. E. 1990)
DaisyWorld simulations
Lovelock responded to criticisms with the mathematical Daisyworld model (1983), first to prove the existence of feedback mechanisms, second to demonstrate it was possible that control of the global biomass could occur without consciousness being involved.
Gaia conference
In 1988, the climatologist Stephen Schneider organised a conference of the American Geophysical Union solely to discuss Gaia. The accusations of teleologism were dropped after that meeting.
Lovelock presented a new version of the Gaia Hypothesis, which was abandoning any attempt to argue that Gaia intentionally or consciously maintained the complex balance in her environment that life needed to survive. This new hypothesis was more acceptable by the scientific community. He supported his new hypothesis with the metaphor of Daisyworld. Using computer simulations of the Daisyworld parameters (no atmosphere, taking into account different albedos for each daisy type) and a mathematical approach, Lovelock proved that the controlled stability of the climate by life was not being teleological. The new Gaia hypothesis stated that Gaia was homeostatic, i.e. that the biota influence the abiotic world in a way that involves homeostatic feedback.
During the Gaia conference, James Kirchner, a physicist and philosopher took the opportunity of the meeting to explain that there are not one Gaia hypothesis, but several ones. He then described five of these: Influential Gaia, Coevolutionary Gaia, Homeostatic Gaia, Teleological Gaia, Optimizing Gaia.
Strong Gaia theories
Several types of strong theories may be defined.
An even stronger claim states that biota manipulate their physical environment to create optimal conditions. It is sometimes refers to as optimizing Gaia.
"the Earth's atmosphere is more than merely anomalous; it appears to be a contrivance specifically constituted for a set of purposes". (Lovelock and Margulis 1974).
Optimizing Gaia asserts that the biota manipulate their physical environment for the purpose of creating biologically favorable, or even optimal, conditions for themselves.
"...it is unlikely that chance alone accounts for the fact that temperature, pH and the presence of compounds of nutrient elements have been, for immense periods, just those optimal for surface life. Rather, ... energy is expended by the biota to actively maintain these optima". (Lovelock and Margulis 1974)
Another strong theory is the one called Omega Gaia. Teilhard de Chardin claims that the Earth is evolving through stages of geosphere, biosphere, and noosphere, culminating in the Omega Point.
Weak Gaia theories
A version of Gaia theory was developed by Lynn Margulis, a microbiologist, in 1979. Her model is more limited in scope than the one that Lovelock proposed. In particular, that only homeorhetic and not homeostatic balances are involved, and that there is no special tendency of biospheres to preserve their current inhabitants, and certainly not to make them comfortable. Accordingly, the Earth is not a living organism which can live or die all at once, but rather a kind of community of trust which can exist at many discrete levels of integration.
Coauthor of the original Gaia Hypothesis, "Lynn Margulis, tells us that Earth is not homeostatic but homeorhetic: that is, the composition of Earth's atmosphere, hydrosphere and lithosphere are regulated around 'set points' as in homeostasis, but those set points change with time... Gaia is just symbiosis as seen from space." �?from Greenpeace
A system in homeostasis tends to move towards constant values for its parameters, whereas a system in homeorhesis will always exhibit similar dynamic behavior, without necessarily converging to a constant state. There is strong evidence that plants are selected for the microclimate effects which they can have locally to themselves, and good evidence that these patterns also exist on some wider scales, with symbiotic relationships existing for larger scale climate modification.
Other reductionist theories suggest that Gaia is co-evolutive. Co-evolution in this context has been thus defined: "Biota influence their abiotic environment, and that environment in turn influences the biota by Darwinian process."
The weakest form of the theory has been called influential Gaia. It barely states that biota influence certain aspects of the abiotic world, e.g. temperature and atmosphere.
All of these theories are more acceptable from an orthodox science perspective, as they assume non-homeostasis. They state the evolution of life and its environment may affect each other. An example is how the activity of photosynthetic bacteria during Precambrian times have completely modified the Earth atmosphere to turn it aerobic, and as such supporting evolution of life (in particular eucaryotic life) . However, these theories do not claim the atmosphere modification has been done in coordination and though homeostasis.
Semantic debate
The argument is that these symbiotic organisms, being unable to survive apart from each other and their climate and local conditions, form an organism in their own right, under a wider conception of the term organism than is conventionally used. It is a matter for often heated debate whether this is a valid usage of the term, but ultimately it appears to be a semantic dispute. In this sense of the word organism, it is argued under the theory that the entire biomass of the Earth "is a single organism".
Unfortunately, many supporters of the various Gaia theories do not state exactly where they sit on this spectrum; this makes discussion and criticism difficult.
Much effort on behalf of those analyzing the theory currently is an attempt to clarify what these different hypotheses are, and whether they are proposals to "test" or "manipulate" outcomes. Both Lovelock's and Margulis's understanding of Gaia are considered valid scientific theories, and are now a part of biology proper.
Gaia hypothesis in ecology
After initial criticism, Gaia hypothesis is now considered an essential part of ecological science, proposing the planet to be the object of ecological study. Most ecologists agree to assimilate the biosphere to a super ecosystem and consider this hypothesis, though a simplification, is consistent with the modern vision of global ecology, relying the concept of biosphere and biodiversity.
New science
Gaia hypothesis led to the new science called biogeography, or even geophysiology, which take into account the interactions between biota, the oceans and the atmosphere.
Early Modern Parallels
In Lives of a Cell, Lewis Thomas makes an observation very much like Lovelock's Gaia hypothesis:
"I have been trying to think of the earth as a kind of organism, but it is no go. I cannot think of it this way. It is too big, too complex, with too many working parts lacking visible connections. The other night, driving through a hilly, wooded part of southern New England, I wondered about this. If not like an organism, what is it like, what is it most like? Then, satisfactorily for that moment, it came to me: it is most like a single cell."
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