Gaia hypothesis
The Gaia hypothesis is an ecological hypothesis that proposes that living and nonliving parts of the earth are viewed as a complex interacting system that can be thought of as a single organism. Named after the Greek earth goddess, this hypothesis postulates that all living things have a regulatory effect on the Earth's environment that promotes life overall.

# History
The Gaia hypothesis was first scientifically formulated in the 1960s by the independent research scientist Dr. James Lovelock, as a consequence of his work for NASA on methods of detecting life on Mars. He initially published the Gaia Hypothesis in journal articles in the early 1970s followed by a popularising 1979 book Gaia: A new look at life on Earth.  He named this self-regulating living system after the Greek goddess Gaia, using a suggestion from the novelist William Golding, who was living in the same village as Lovelock at the time (Bowerchalke, Wiltshire, UK). The Gaia Hypothesis has since been supported by a number of scientific experiments and provided a number of useful predictions, and hence is properly referred to as the Gaia Theory.
Since 1971, the noted microbiologist Dr. Lynn Margulis has been Lovelock's most important collaborator in developing Gaian concepts.
Until 1975 the hypothesis was almost totally ignored.  An article in the New Scientist of February 15, 1975, and a popular book length version of the theory, published as The Quest for Gaia, began to attract scientific and critical attention to the hypothesis. The theory was then attacked by many mainstream biologists.  Championed by certain environmentalists and climate scientists, it was vociferously rejected by many others, both within scientific circles and outside them.

# Theories
Lovelock draws comparison with the resistance to the introduction of the idea of plate tectonics within geology, noting that it took about 30 years before it became universally accepted as true.
Today the Gaia theory is more commonly referred to as Earth System Science, and is a class of scientific models of the geo-biosphere in which life as a whole fosters and maintains suitable conditions for itself by helping to create an environment on Earth suitable for its continuity.
Gaia "theories" have non-technical predecessors in the ideas of many cultures. Today, "Gaia theory" is sometimes used among scientists on the basis that the earlier Gaia hypothesis has withstood rigorous scientific testing.  It is also used by non-scientists to refer to hypotheses of a self-regulating Earth that are non-technical but take inspiration from scientific models. At the second Chapman Conference of the American Geophysical Union, at Valencia in Spain, for instance, Lynn Margulis in her closing address "Modes of Confirmation of the Gaia Hypothesis" conceded that despite being elevated to "Gaia theory" in the 1980s, there was still confusion about what Gaia was in reality.  Among many scientists, "Gaia" still carries connotations of lack of scientific rigor and quasi-mystical thinking about the planet Earth. Lovelock's own reframing of the hypothesis as "Geophysiology" and the growing acceptance of "Earth System Science" has silenced many of these critics.

# Lovelock's initial hypothesis
Lovelock defined Gaia as:
His initial hypothesis was that the biomass modifies the conditions on the planet to make conditions on the planet more hospitable  the Gaia Hypothesis properly defined this "hospitality" as a full homeostasis. Lovelock's initial hypothesis, accused of being teleological by his critics, was that the atmosphere is kept in homeostasis by and for the biosphere.
Lovelock suggested that life on Earth provides a cybernetic, homeostatic feedback system operated automatically and unconsciously by the biota, leading to broad stabilization of global temperature and chemical composition.
With his initial hypothesis, Lovelock claimed the existence of a global control system of surface temperature, atmosphere composition and ocean salinity. His arguments were:
- The global surface temperature of the Earth has remained constant, despite an increase in the energy provided by the Sun.
- 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% to 30%; however the surface temperature of the planet has remained remarkably constant when measured on a global scale. Furthermore, he argued, the atmospheric composition of the Earth is constant. The Earth's atmosphere currently consists of 79% nitrogen, 20.7% oxygen and 0.03% carbon dioxide. Oxygen is the second most reactive element after fluorine, and should combine with gases and minerals of the Earth's atmosphere and crust. Traces of methane (at an amount of 100,000 tonnes produced per annum) should not exist, as methane is combustible in an oxygen atmosphere. This composition should be unstable, 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 and do not generally tolerate values above 5%. Ocean salinity constancy was a long-standing mystery, because river salts should have raised the ocean salinity much higher than observed.  Recently it was suggested that salinity may also be strongly influenced by seawater circulation through hot basaltic rocks, and emerging as hot water vents on ocean spreading ridges. However, the composition of sea water is far from equilibrium, and it is difficult to explain this fact without the influence of organic processes.
The only significant natural source of atmospheric carbon dioxide (CO2) is volcanic activity, while the only significant removal is through the precipitation of carbonate rocks. In water, CO2 is dissolved as a "carbonic acid," which may be combined with dissolved calcium to form solid calcium carbonate (limestone). Both precipitation and solution are influenced by the bacteria and plant roots in soils, where they improve gaseous circulation, or in coral reefs, where calcium carbonate is deposited as a solid on the sea floor. Calcium carbonate can also be washed from continents to the sea where it is used by living organisms to manufacture carbonaceous tests and shells. Once dead, the living organisms' shells fall to the bottom of the oceans where they generate deposits of chalk and limestone. 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 produce CO2 in the atmosphere, CO2 participates in rock weathering as carbonic acid, 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 and other Gaia scientists like Stephan Harding, 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.

## Controversial concepts
Lovelock, especially in his older texts, indulged in language that has later caused fiery debates. For instance many of his biological critics such as Stephen J. Gould and Richard Dawkins attacked his statement in the first paragraph of his first Gaia book (1979), that "the quest for Gaia is an attempt to find the largest living creature on Earth."
Lynn Margulis, the coauthor of Gaia hypotheses, is more careful to avoid controversial figures of speech than is Lovelock. In 1979 she wrote, in particular, that only homeorhetic and not homeostatic balances are involved: 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. Also she wrote that there is no special tendency of biospheres to preserve their current inhabitants, and certainly not to make them comfortable. Accordingly, the Earth is  a kind of community of trust which can exist at many discrete levels of integration. This is true for all multicellular organisms which do not live or die all at once: not all cells in the body die instantaneously, nor are homeostatic "set points" constant through the life of an organism.

# Critical analysis

## Basis
This theory is based on the idea that the biomass self-regulates the conditions on the planet to make its physical environment (in particular temperature and chemistry of the atmosphere) on the planet more hospitable to the species which constitute its "life". The Gaia Hypothesis proper defined this "hospitality" as a full homeostasis. A model that is often used to illustrate the original Gaia Hypothesis is the so-called Daisyworld simulation.
Whether this sort of system is present on Earth is still open to debate. Some relatively simple homeostatic mechanisms are generally accepted. For example, when atmospheric carbon dioxide levels rise, the biomass of photosynthetic organisms increases  and thus removes more carbon dioxide from the atmosphere, but the extent to which these mechanisms stabilize and modify the Earth's overall climate are not yet known. Less clear is the reason why such traits evolve in a system in order to produce such effects.  Lovelock accepts a process of systemic Darwinian evolution for such mechanisms, creatures that evolve that improve their environment for their survival will do better than those which damage their environment.  But many Darwinists have difficulty accepting such mechanisms can exist.

## Criticism
After initially being largely ignored by scientists, (from 1969 till 1977), thereafter for a period, the initial Gaia hypothesis was ridiculed by some scientists. On the basis of its name alone, the Gaia hypothesis was derided as some kind of neo-Pagan New Age religion. Many scientists in particular also criticised the approach taken in his popular book "Gaia, a New look at Life on Earth" for being teleological; a belief that all things have a predetermined purpose.  Lovelock seems to have accepted this criticism of some of his statements, and has worked hard to remove the taint of teleological purpose from his theories, 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).
In 1981, W. Ford Doolittle, in the CoEvolution Quarterly article "Is Nature Motherly" argued that there was nothing in the genome of individual organisms which could provide the feedback mechanisms Gaia theory proposed, and that therefore the Gaia hypothesis was an unscientific theory of a maternal type without any explanatory mechanism.  In 1982 Richard Dawkins in his book The Extended Phenotype argued that organisms could not act in concert as this would require foresight and planning from them. Like Doolittle he rejected the possibility that feedback loops could stabilize the system.  Dawkins claimed "there was no way for evolution by natural selection to lead to altruism on a Global scale".
Stephen Jay Gould criticised Gaia as merely a metaphorical description of Earth processes.  He wanted to know the actual mechanism by which self-regulating homeostasis was regulated.

## A question of definition
An argument against the idea that Gaia is a "living" organism is the fact that the planet has not been, and is not, able to reproduce. One of the criteria of the empirical definition of life is its ability to replicate and pass on their genetic information to succeeding generations. Lovelock, however, defines life as a self-preserving, self-similar system of feedback loops like Humberto Maturana's autopoiesis; as a self-similar system, life could be a cell as well as an organ embedded into a larger organism as well as an individual in a larger inter-dependent social context. The biggest context of interacting inter-dependent living entities is the Earth. The problematic empirical definition is getting "fuzzy on the edges": Why are highly specialized bacteria like E. coli that are unable to thrive outside their habitat considered "life", while mitochondria, which have evolved independently from the rest of the cell, not? Maturana and Lovelock changed this with the autopoiesis deductive definition which to them explains the phenomenon of life better; some aspects of the empirical definition, however, no longer apply. Reproduction becomes optional: bee swarms reproduce, while the biosphere has no need to. Lovelock himself states in the original Gaia book that even that is not true; given the possibilities, the biosphere may multiply in the future by colonizing other planets, as humankind may be the primer by which Gaia will reproduce. Humanity's exploration of space, its interest in colonizing and even terraforming other planets, lends some plausibility to the idea that Gaia might in effect be able to reproduce.  The astronomer Carl Sagan also remarked that from a cosmic viewpoint, the space probes since 1959 have the character of a planet preparing to go to seed.

# Daisy World simulations
Lovelock responded to criticisms with the mathematical Daisyworld model (1983) to prove the possibility that feedback mechanisms might exist in nature, and that control of the global biomass could occur without consciousness being involved.
Using computer simulations of a hypothetical Daisyworld (with no atmosphere, taking into account only the different albedos of a black and white daisy type) and a mathematical approach, Lovelock and Andrew Watson proved that the controlled stability of the climate was an automatic consequence of the feedback mechanisms that would foster one kind of daisy over another.  Thus the Gaian stabilities were not being teleological. The Gaia hypothesis had always asserted that Gaia was homeostatic, i.e. that the biota influence the abiotic world in a way that involves homeostatic feedback.  Later tests, using grey daisies in addition to black and white ones, from which mutations could lead to evolution of different colours, and the introduction of daisy-eating rabbits and rabbit-eating foxes only seemed to increase the stability of the Daisyworld simulations.   More recently other research, modelling the real biochemical cycles of Earth, and using various "guilds" of life (eg. photosynthesisers, decomposers, herbivores and primary and secondary carnivores) has also been shown to produce Daisyworld-like regulation and stability, which helps to explain planetary biological diversity.
This enables nutrient recycling within a regulatory framework derived by natural selection amongst species, where one being's harmful waste becomes low energy food for members of another guild. This research on the Redfield ratio of Nitrogen to Phosphorus shows that local biotic processes can regulate global systems (See Keith Downing & Peter Zvirinsky, The Stimulated Evolution of Biochemical Guilds: Reconciling Gaia Theory with Natural Selection).

# The first Gaia conference
In 1988, to draw attention to the Gaia hypothesis, the climatologist Stephen Schneider organised a conference of the American Geophysical Union's first Chapman Conference on Gaia, held at San Diego in 1989, solely to discuss Gaia.
At the conference James Kirchner criticised the Gaia hypothesis for its imprecision.  He claimed that Lovelock and Margulis had not presented one Gaia hypothesis, but four -
- CoEvolutionary Gaia - that life and the environment had evolved in a coupled way.  Kirchner claimed that this was already accepted scientifically and was not new.
- Homeostatic Gaia - that life maintained the stability of the natural environment, and that this stability enabled life to continue to exist.
- Geophysical Gaia - that the Gaia theory generated interest in geophysical cycles and therefore led to interesting new research in terrestrial geophysical dynamics.
- Optimising Gaia - that Gaia shaped the planet in a way that made it an optimal environment for life as a whole.  Kirchner claimed that this was not testable and therefore was not scientific.
Of Homeostatic Gaia, Kirchner recognised two alternatives.  "Weak Gaia" asserted that life tends to make the environment stable for the flourishing of all life.  "Strong Gaia" according to Kirchner, asserted that life tends to make the environment stable, in order to enable the flourishing of all life.  Strong Gaia, Kirchner claimed, was untestable and therefore not scientific.
Referring to the Daisyworld Simulations, Kirchner responded that these results were predictable because of the intention of the programmers - Lovelock and Watson, who selected examples which would produce the responses they desired.
Lawrence Joseph in his book "Gaia: the birth of an idea" argued that Kirchner's attack was principally against Lovelock's integrity as a scientist. As Jon Tuney demonstrates, it is not Lovelock's style to respond to such personal attacks and Lovelock did not attack Kirchner's views for ten years, until his autobiography "Homage to Gaia", where he spoke of Kirchner's sophistry.  Lovelock and other Gaia-supporting scientists, however, did attempt to disprove the claim that the theory is not scientific because it is impossible to test it by controlled experiment.  For example, against the charge that Gaia was teleological Lovelock and Andrew Watson offered the Daisyworld model (and its modifications, above) as evidence against most of these criticisms.
Lovelock was careful to present a version of the Gaia Hypothesis which had no claim that Gaia intentionally or consciously maintained the complex balance in her environment that life needed to survive. It would appear that the claim that Gaia acts "intentionally" was a metaphoric statement in his popular initial book and was not meant to be taken literally.  This new statement of the Gaia hypothesis was more acceptable to the scientific community.
The accusations of teleologism were largely dropped after this conference.

# An early range of views
Some have found James Kirchner's suggested spectrum, proposed at the First Gaia Chapman Conference, useful in suggesting that the original Gaia hypothesis could be split into a spectrum of hypotheses, ranging from the undeniable (Weak Gaia) to the radical (Strong Gaia).

## Weak Gaia
At one end of this spectrum is the undeniable statement that the organisms on the Earth have altered its composition. A stronger position is that the Earth's biosphere effectively acts as if it is a self-organizing system, which works in such a way as to keep its systems in some kind of "meta-equilibrium" that is broadly conducive to life. The history of evolution, ecology and climate show that the exact characteristics of this equilibrium intermittently have undergone rapid changes, which are believed to have caused extinctions and felled civilizations (see climate change).
Weak Gaian hypotheses 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." Lovelock (1995) gave evidence of this in his second book, showing the evolution from the world of the early thermo-acido-phyllic and methanogenic bacteria towards the oxygen enriched atmosphere today that supports more complex life.
The weakest form of the theory has been called "influential Gaia". It states that biota barely influence certain aspects of the abiotic world, e.g. temperature and atmosphere.
The weak versions 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 eukaryotic life) . However, these theories do not claim the atmosphere modification has been done in coordination and through homeostasis.  Also such critical theories have yet to explain how conditions on Earth have not been changed by the kinds of run-away positive feedbacks that have affected Mars and Venus.
Biologists and earth scientists usually view the factors that stabilize the characteristics of a period as an undirected emergent property or entelechy of the system; as each individual species pursues its own self-interest, for example, their combined actions tend to have counterbalancing effects on environmental change. Opponents of this view sometimes reference examples of life's actions that have resulted in dramatic change rather than stable equilibrium, such as the conversion of the Earth's atmosphere from a reducing environment to an oxygen-rich one. However, proponents sometimes say that those atmospheric composition changes created an environment even more suitable to life.
Some go a step further and hypothesize that all lifeforms are part of one single living planetary being called Gaia. In this view, the atmosphere, the seas and the terrestrial crust would be results of interventions carried out by Gaia through the coevolving diversity of living organisms. While it is arguable that the Earth as a unit does not match the generally accepted biological criteria for life itself (Gaia has not yet reproduced, for instance; it still might spread to other planets through human space colonization and terraforming), many scientists would be comfortable characterising the earth as a single "system".

## Strong Gaia
A version called "Optimizing Gaia" asserts that biota manipulate their physical environment for the purpose of creating biologically favorable, or even optimal, conditions for themselves. "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). Further, "...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."
The most extreme form of Gaia hypothesis is a non-scientific idea that the entire Earth is a single unified organism that is consciously manipulating the climate in order to make conditions more conducive to life. This is a mystical view for which there is no rigorous evidence to support.  It would appear to be a result of the fact that many people do not understand the concept of homeostasis. Many non-scientists instinctively see homeostasis as an activity that requires conscious control, although this is not so.
Another strong hypothesis is the one called "Omega Gaia". Teilhard de Chardin claimed that the Earth is evolving through stages of cosmogenesis, affecting the geosphere, biogenesis of the biosphere, and noogenesis of the noosphere, culminating in the Omega Point. Another form of the strong Gaia hypothesis is proposed by Guy Murchie who extends the quality of a wholistic lifeform to galaxies. "After all, we are made of star dust. Life is inherent in nature." Murchie describes geologic phenomena such as sand dunes, glaciers, fires, etc. as living organisms, as well as the life of metals and crystals. "The question is not whether there is life outside our planet, but whether it is possible to have "nonlife". "
Much more speculative versions of Gaia hypothesis, including all versions in which it is held that the Earth is actually conscious or part of some universe-wide evolution (See Selfish Biocosm hypothesis), which is able to make scientifically testable predictions, although it would still be held to be at best tangential to science, by most thinkers. These are discussed in the Gaia philosophy article. Also with a slender connection, if any, to science is the Gaia Movement, a collection of different organisations operating in different countries, but all sharing a concern for how humans might live more sustainably within the "living system".

# Recent developments
Gaia Theory has developed considerably. Margulis dedicated the last eight chapters of her book, The Symbiotic Planet, to Gaia. She resented the widespread personification of Gaia and stressed that Gaia is "not an organism", but "an emergent property of interaction among organisms". She defined Gaia "the series of interacting ecosystems that compose a single huge ecosystem at the Earth's surface. Period." Yet still she argues, "the surface of the planet behaves as a physiological system in certain limited ways". Margulis seems to agree with Lovelock in that, in what comes to these physiological processes, the earth's surface is "best regarded as alive". The book's most memorable "slogan" was actually quipped by a student of Margulis': "Gaia is just symbiosis as seen from space". This neatly connects Gaia theory to Margulis' own theory of endosymbiosis.
Both Lovelock's and Margulis's understanding of Gaia are now largely considered valid scientific hypotheses, though controversies continue.

### The second Gaia conference
By the time of the 2nd Chapman Conference on the Gaia Hypothesis, held at Valencia, Spain, on 23 June 2000, the situation had developed significantly in accordance with the developing science of Bio-geophysiology. Rather than a discussion of the Gaian teleological views, or "types" of Gaia Theory, the focus was upon the specific mechanisms by which basic short term homeostasis was maintained within a framework of significant evolutionary long term structural change.
The major questions were:
- "How has the global biogeochemical/climate system called Gaia changed in time? What is its history? Can Gaia maintain stability of the system at one time scale but still undergo vectorial change at longer time scales? How can the geologic record be used to examine these questions?"
- "What is the structure of Gaia? Are the feedbacks sufficiently strong to influence the evolution of climate? Are there parts of the system determined pragmatically by whatever disciplinary study is being undertaken at any given time or are there a set of parts that should be taken as most true for understanding Gaia as containing evolving organisms over time? What are the feedbacks among these different parts of the Gaian system, and what does the near closure of matter mean for the structure of Gaia as a global ecosystem and for the productivity of life?"
- "How do models of Gaian processes and phenomena relate to reality and how do they help address and understand Gaia? How do results from Daisyworld transfer to the real world? What are the main candidates for "daisies"? Does it matter for Gaia theory whether we find daisies or not? How should we be searching for daisies, and should we intensify the search? How can Gaian mechanisms be investigated using process models or global models of the climate system which include the biota and allow for chemical cycling?"
Tyler Volk (1997) has suggested that once life evolves, a Gaian system is almost inevitably produced as a result of an evolution towards far-from-equilibrium homeostatic states that maximise entropy production (MEP). Kleidon (2004) agrees with Volk's hypothesis, stating: "...homeostatic behavior can emerge from a state of MEP associated with the planetary albedo"; "...the resulting behavior of a biotic Earth at a state of MEP may well lead to near-homeostatic behavior of the Earth system on long time scales, as stated by the Gaia hypothesis." Staley (2002) has similarly proposed "...an alternative form of Gaia theory based on more traditional Darwinian principles... In  new approach, environmental regulation is a consequence of population dynamics, not Darwinian selection. The role of selection is to favor organisms that are best adapted to prevailing environmental conditions. However, the environment is not a static backdrop for evolution, but is heavily influenced by the presence of living organisms. The resulting co-evolving dynamical process eventually leads to the convergence of equilibrium and optimal conditions."

# Gaia hypothesis in ecology
After much initial criticism, a modified Gaia hypothesis is now considered within ecological science basically consistent with the planet earth being the ultimate object of ecological study. Ecologists generally consider the biosphere as an ecosystem and the Gaia hypothesis, though a simplification of that original proposed, to be consistent with a modern vision of global ecology, relaying the concepts of biosphere and biodiversity. The Gaia hypothesis has been called geophysiology or Earth System Science, which takes into account the interactions between biota, the oceans, the geosphere, and the atmosphere.  To promote research and discussion in these fields an organisation, "Gaia Society for Research and Education in Earth System Science" was started.
An example of the change in acceptability of Gaia theories is the Amsterdam declaration of the scientific communities of four international global change research programmes - the International Geosphere-Biosphere Programme (IGBP), the International Human Dimensions Programme on Global Environmental Change (IHDP), the World Climate Research Programme (WCRP) and the international biodiversity programme DIVERSITAS - recognise that, in addition to the threat of significant climate change, there is growing concern over the ever-increasing human modification of other aspects of the global environment and the consequent implications for human well-being.
They state
"Research carried out over the past decade under the auspices of the four programmes to address these concerns has shown that:
- The Earth System behaves as a single, self-regulating system comprised of physical, chemical, biological and human components. The interactions and feedbacks between the component parts are complex and exhibit multi-scale temporal and spatial variability. The understanding of the natural dynamics of the Earth System has advanced greatly in recent years and provides a sound basis for evaluating the effects and consequences of human-driven change.
- Human activities are significantly influencing Earth's environment in many ways in addition to greenhouse gas emissions and climate change. Anthropogenic changes to Earth's land surface, oceans, coasts and atmosphere and to biological diversity, the water cycle and biogeochemical cycles are clearly identifiable beyond natural variability. They are equal to some of the great forces of nature in their extent and impact. Many are accelerating. Global change is real and is happening now.
- Global change cannot be understood in terms of a simple cause-effect paradigm. Human-driven changes cause multiple effects that cascade through the Earth System in complex ways. These effects interact with each other and with local- and regional-scale changes in multidimensional patterns that are difficult to understand and even more difficult to predict. Surprises abound.
- Earth System dynamics are characterised by critical thresholds and abrupt changes. Human activities could inadvertently trigger such changes with severe consequences for Earth's environment and inhabitants. The Earth System has operated in different states over the last half million years, with abrupt transitions (a decade or less) sometimes occurring between them. Human activities have the potential to switch the Earth System to alternative modes of operation that may prove irreversible and less hospitable to humans and other life. The probability of a human-driven abrupt change in Earth's environment has yet to be quantified but is not negligible.
- In terms of some key environmental parameters, the Earth System has moved well outside the range of the natural variability exhibited over the last half million years at least. The nature of changes now occurring simultaneously in the Earth System, their magnitudes and rates of change are unprecedented. The Earth is currently operating in a no-analogue state."
Sir Crispin Tickell in the 46th Annual Bennett Lecture for the 50th Anniversary of Geology at the University of Leicester in his recent talk "Earth Systems Science: Are We Pushing Gaia Too Hard?" stated "as a theory, Gaia is now winning." 
He continued "The same goes for the earth systems science which is now the concern of the Geological Society of London (with which the Gaia Society recently merged). Whatever the label, earth systems science, or Gaia, has now become a major subject of enquiry and research, and no longer has to justify itself."
These findings would seem to be fully in accord with the Gaia theory.  Despite this endorsement, the late Bill Hamilton, one of the founders of modern Darwinism, whilst conceding the empirical basis of the planetary homeostatic processes on which Gaia is based, states that it is a theory still awaiting its Copernicus.

# The Revenge of Gaia
In James Lovelock's latest book, "The Revenge of Gaia", he argues that the lack of respect humans have had for Gaia, through the damage done to rainforests and the reduction in planetary biodiversity, is testing Gaia's capacity to minimize the effects of the addition of greenhouse gases in the atmosphere.  This eliminates the planet's homeostatic negative feedback potential and increases the likelihood of positive feedbacks associated with runaway global warming.  Similarly the warming of the oceans is extending the oceanic thermocline layer of tropical oceans into the Arctic and Antarctic waters, preventing the rise of oceanic nutrients into the surface waters and eliminating the algal blooms of phytoplankton on which oceanic foodchains depend.  As phytoplankton and forests are the main ways in which Gaia draws down greenhouse gases, particularly carbon dioxide, taking it out of the atmosphere, the elimination of this environmental buffering will see, according to Lovelock, most of the earth becoming uninhabitable for humans and other life-forms by the middle of next century, with a massive extension of tropical deserts.  Given these conditions, Lovelock expects civilization will be hard pressed to survive.

# Influences of the Gaia hypothesis

## Scientific literature
Fritjof Capra, in his fourth book, The Web of Life, used Gaia theory to explain the complications and interconnections in the marvelous web of life.
Peter Russell, in his book The Global Brain, pondered the role that humanity might play in Gaia, pointing out parallels with both nervous systems and cancer.
Stephan Harding, a student of Lovelock has written a book, Animate Earth: Science, Intuition, and Gaia. Replacing the cold, objectifying language of science with a way of speaking of our planet as a sentient, living being, Harding presents the science of Gaia in everyday English.
Tim Flannery, his book "The Weather Makers" talks about "Gaia"

## Art
Alex Grey contrasted the revenge of Gaia against pre-human Gaia in one of his paintings.

## Music
Paul Winter composed a "Missa Gaia", which was performed at the Cathedral of Saint John the Divine in New York in the early 1980s.  The music draws from the sounds of nature including the cry of the Siberian wolf and the song of the Humpback Whale.
An oratorio by American composer Nathan Currier called Gaian Variations was premiered on Earth Day 2004 at Lincoln Center by the Brooklyn Philharmonic, using texts of James Lovelock,
Loren Eiseley and Lewis Thomas.
Spanish folk metal group Mago de Oz has released two sequential concept albums called Gaia (2003) and Gaia II: "La Voz Dormida" (2005).
The Disco Biscuits' song Jigsaw Earth contains references to Gaia's "Jigsaw Earth."
Singer and songwriter James Taylor's 1997 album, Hourglass, includes the song "Gaia".

## Fiction
At least four works of fiction use the Gaia hypothesis as a central part of the plot.  In two of his science fiction novels, Foundation's Edge (1982) and Foundation and Earth (1984), Isaac Asimov describes the planet Gaia as one on which all things, living and inanimate, are taking part in a planetary consciousness to an appropriate measure. In Asimov's story Gaia strives for an even greater superorganism that it calls Galaxia, and that comprises the whole galaxy.
In Lovelock (1994), a novel by Orson Scott Card & Kathryn H. Kidd, Gaiaology is a fully fledged interdisciplinary science which will soon be put to use by the Earth's first interstellar colony ship.  Assuming the target planet will need terraforming, the job of the ship's Gaiaologist will be to integrate the terrestrial species needed for the colonists' survival with the planet's existing ecology.  The Gaiaologist's "Witness," a form of assistance animal whose job it is to record every waking moment in the life of such a prominent member of society, is the central character of the book, an enhanced Capuchin monkey named after Sir James Lovelock.
The Gaia hypothesis was also used as a central theme in the novel Portent, by James Herbert, in which Lovelock is mentioned by name.
In 2006, author Lee Welles wrote the first children's book about Gaia. Gaia Girls is a series of seven books about four girls who encounter the living Gaia. Gaia grants them special powers that they must use to help her survive the effects of modern humanity.  In November 2006, Gaia Girls - Enter the Earth won the National Outdoor Book Award.
In a story by Alan Moore, "Mogo Doesn't Socialize" (1985), a Green Lantern is a living sentient planet called Mogo.
The children's cartoon Captain Planet includes the character Gaia as the spirit of Earth.
In the film, Final Fantasy: The Spirits Within, Doctor Cid (voiced by Donald Sutherland) has a theory about "Gaia," the spirit of the planet, and how it connects to all living things on the planet. This theory turned out to be true, discovered by Dr. Aki Ross (voiced by Ming-Na).

## Games
Maxis has specifically named the Gaia hypothesis and Lovelock as inspirations for their 1990 game, SimEarth.
The 1992 RPG, E.V.O.: Search for Eden, featured a female goddess named Gaia, who is a personification of Earth.
The 1999 Turn-based strategy PC game, Sid Meier's Alpha Centauri, features a living (and eventually sentient) planet in the Alpha Centauri star system. One of the games factions is named "Gaia's Stepdaughters", a group of environmentalists who believe in living with the planet rather than trying to tame or destroy it. Certain aspects of the game also reflect influence from Gaia theory, such as the planet attempting to defend itself when encroached upon by human industry, etc.
The Game Boy Advance RPG Golden Sun and its sequel Golden Sun: The Lost Age propose a similar theory to the Gaia Hypothesis. The world, Weyard, is shown to be a living being that when cut off from its food source (in this case the energy of alchemy), began to shrink and die. It is the duty of the player to unlock the power of alchemy to save the world from internal decay.
The Super Famicom RPG Illusion of Gaia has Gaia described as a God-like entity who employed the assistance of the protagonist to save the planet from destruction by a comet.
The popular Japanese anime, Eureka Seven, eventually comes to focus on the earth as a living, evolving entity called, "Scab Coral."
In the Playstation 2 video game Wild Arms 3 the scientist group called the 'Council of Seven' produce a similar theory about their own world Filgaia. Eventually they create a specimen to prove it.
In the Compilation of Final Fantasy VII, the planet the story is set on is called Gaia and has a spirit energy of its own (The Lifestream, mako), making the planet itself a living being. Gaia is also the name of the planet on which most of the game of Final Fantasy IX occurs. The later stages of the game show the "creation of  gaia", which show many similarities to the gaia theory.