Editing Gaia hypothesis

{{Short description|Scientific hypothesis about Earth}}
{{Other uses of|Gaia}}
[[File:The Earth seen from Apollo 17.jpg|thumb|upright=1.2|The study of planetary habitability is partly based upon extrapolation from knowledge of the [[Earth]]'s conditions, as the Earth is the only planet currently known to harbour life (''[[The Blue Marble]]'', 1972 [[Apollo 17]] photograph).]]
{{Rights of nature}}
The '''Gaia hypothesis''' ({{IPAc-en|ˈ|ɡ|aɪ|.|ə}}), also known as the '''Gaia theory''', '''Gaia paradigm''', or the '''Gaia principle''', proposes that living [[organism]]s interact with their [[Inorganic compound|inorganic]] surroundings on [[Earth]] to form a [[Synergy|synergistic]] and [[Homeostasis|self-regulating]] [[complex system]] that helps to maintain and perpetuate the conditions for [[life]] on the planet.

The Gaia hypothesis was formulated by the chemist [[James Lovelock]]<ref name="J1972" /> and co-developed by the microbiologist [[Lynn Margulis]] in the 1970s.<ref name="lovelock1974">{{Cite journal |last1=Lovelock |first1=J. E. |last2=Margulis |first2=L. |date=1974 |title=Atmospheric homeostasis by and for the biosphere: the gaia hypothesis |journal=Tellus A |volume=26 |issue=1–2 |pages=2–10 |doi=10.3402/tellusa.v26i1-2.9731 |doi-access=free |s2cid=129803613 |language=en |bibcode=1974Tell...26....2L }}</ref> Following the suggestion by his neighbour, novelist [[William Golding]], Lovelock named the hypothesis after [[Gaia]], the primordial deity who personified the Earth in [[Greek mythology]]. In 2006, the [[Geological Society of London]] awarded Lovelock the [[Wollaston Medal]] in part for his work on the Gaia hypothesis.<ref>{{cite web|title=Wollaston Award Lovelock|url=https://www.geolsoc.org.uk/About/History/Awards-Citations-Replies-2001-Onwards/2006-Awards-Citations-Replies|access-date=19 October 2015}}</ref>

Topics related to the hypothesis include how the [[biosphere]] and the [[evolution]] of organisms affect the stability of [[global temperature]], [[salinity]] of [[seawater]], [[atmospheric oxygen]] levels, the maintenance of a [[hydrosphere]] of liquid water and other environmental variables that affect the [[habitability of Earth]].

The Gaia hypothesis was initially criticized for being [[teleological]] and against the principles of [[natural selection]], but later refinements aligned the Gaia hypothesis with ideas from fields such as [[Earth system science]], [[biogeochemistry]] and [[systems ecology]].{{sfn|Turney|2003}}<ref name="Schwartzman2002">{{cite book |author=Schwartzman, David |title=Life, Temperature, and the Earth: The Self-Organizing Biosphere |publisher=Columbia University Press |date=2002 |isbn=978-0-231-10213-1 }}</ref><ref>[[John Gribbin|Gribbin, John]] (1990), "Hothouse earth: The greenhouse effect and Gaia" (Weidenfeld & Nicolson)</ref> Even so, the Gaia hypothesis continues to attract criticism, and today many scientists consider it to be only weakly supported by, or at odds with, the available evidence.<ref name="kirchner2002">{{Citation|last=Kirchner|first=James W.|author-link=James Kirchner|title=Toward a future for Gaia theory|date=2002|journal=[[Climatic Change (journal)|Climatic Change]]|volume=52|issue=4|pages=391–408|doi=10.1023/a:1014237331082|s2cid=15776141}}</ref><ref name="volk2002">{{Citation|last=Volk|first=Tyler|author-link=Tyler Volk|title=The Gaia hypothesis: fact, theory, and wishful thinking|date=2002|journal=Climatic Change|volume=52|issue=4|pages=423–430|doi=10.1023/a:1014218227825|s2cid=32856540}}</ref><ref name="beerling2007">{{cite book |last=Beerling |first=David |author-link=David Beerling|date=2007 |title=The Emerald Planet: How plants changed Earth's history |url=http://ukcatalogue.oup.com/product/9780192806024.do |location=Oxford|publisher=Oxford University Press |isbn= 978-0-19-280602-4 }}</ref>{{sfn|Tyrrell|2013}}

==Overview==
Gaian hypotheses suggest that organisms [[Co-evolution|co-evolve]] with their environment: that is, they "influence their [[abiotic]] environment, and that environment in turn influences the [[Biota (ecology)|biota]] by [[Darwinism|Darwinian process]]". Lovelock (1995) gave evidence of this in his second book, ''Ages of Gaia'', showing the evolution from the world of the early [[Bacteria|thermo-acido-philic]] and [[methanogenic bacteria]] towards the oxygen-enriched [[atmosphere]] today that supports more [[Phanerozoic|complex life]].

A reduced version of the hypothesis has been called "influential Gaia"<ref name="Lapenis-2002">{{Cite journal|last=Lapenis|first=Andrei G.|year=2002|title=Directed Evolution of the Biosphere: Biogeochemical Selection or Gaia?|journal=The Professional Geographer|volume=54 |issue=3|pages=379–391|via=[Peer Reviewed Journal]|doi=10.1111/0033-0124.00337|bibcode=2002ProfG..54..379L |s2cid=10796292}}</ref> in the 2002 paper "Directed Evolution of the Biosphere: Biogeochemical Selection or Gaia?" by Andrei G. Lapenis, which states the [[Biota (ecology)|biota]] influence certain aspects of the abiotic world, e.g. [[temperature]] and atmosphere.  This is not the work of an individual but a collective of Russian scientific research that was combined into this peer-reviewed publication. It states the coevolution of life and the environment through "micro-forces"<ref name="Lapenis-2002" /> and biogeochemical processes. An example is how the activity of [[Photosynthesis|photosynthetic]] bacteria during Precambrian times completely modified the [[Earth's atmosphere|Earth atmosphere]] to turn it aerobic, and thus supports the evolution of life (in particular [[eukaryotic]] life).

Since barriers existed throughout the twentieth century between Russia and the rest of the world, it is only relatively recently that the early Russian scientists who introduced concepts overlapping the Gaia paradigm have become better known to the Western scientific community.<ref name="Lapenis-2002" /> These scientists include [[Piotr Kropotkin|Piotr Alekseevich Kropotkin]] (1842–1921) (although he spent much of his professional life outside Russia), [[:ru:Ризположенский, Рафаил Васильевич|Rafail Vasil’evich Rizpolozhensky]] (1862 – {{Circa|1922}}), [[Vladimir Ivanovich Vernadsky]] (1863–1945), and [[:ru:Костицын, Владимир Александрович|Vladimir Alexandrovich Kostitzin]] (1886–1963).

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 may have counterbalancing effects on environmental change. Opponents of this view sometimes reference examples of events that 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 at the end of the [[Archean|Archaean]] and the beginning of the [[Proterozoic]] periods.

Less accepted versions of the hypothesis claim that changes in the biosphere are brought about through the [[Superorganism|coordination of living organisms]] and maintain those conditions through [[homeostasis]]. In some versions of [[Gaia philosophy]], all lifeforms are considered 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 [[Coevolution|coevolving]] diversity of living organisms.

The Gaia paradigm was an influence on the [[deep ecology]] movement.<ref>{{cite book |editor1-first=David Landis |editor1-last=Barnhill |editor2-first=Roger S. |editor2-last=Gottlieb |title=Deep Ecology and World Religions: New Essays on Sacred Ground |publisher=SUNY Press |year=2010 |page=32}}</ref>

==Details==
The Gaia hypothesis posits that the Earth is a self-regulating [[complex system]] involving the [[biosphere]], the [[Earth's atmosphere|atmosphere]], the [[hydrosphere]]s and the [[pedosphere]], tightly coupled as an evolving system. The hypothesis contends that this system as a whole, called Gaia, seeks a physical and chemical environment optimal for contemporary life.{{sfn|Lovelock|2009|p=255}}

Gaia evolves through a [[Cybernetic#In biology|cybernetic]] [[feedback]] system operated by the [[biota (ecology)|biota]], leading to broad stabilization of the conditions of habitability in a full homeostasis. Many processes in the Earth's surface, essential for the conditions of life, depend on the interaction of living forms, especially [[microorganisms]], with inorganic elements. These processes establish a global control system that regulates Earth's [[Sea surface temperature|surface temperature]], [[atmosphere composition]] and [[ocean]] [[salinity]], powered by the global thermodynamic disequilibrium state of the Earth system.<ref>Kleidon, Axel. ''How does the earth system generate and maintain thermodynamic disequilibrium and what does it imply for the future of the planet?''. Article submitted to the ''Philosophical Transactions of the Royal Society'' on Thu, 10 Mar 2011</ref><!-- Article submitted to Royal Society is not a valid reference. This must be replaced by actual article citation if accepted, or an alternative reference -->

The existence of a planetary homeostasis influenced by living forms had been observed previously in the field of [[biogeochemistry]], and it is being investigated also in other fields like [[Earth system science]]. The originality of the Gaia hypothesis relies on the assessment that such homeostatic balance is actively pursued with the goal of keeping the optimal conditions for life, even when terrestrial or external events menace them.{{sfn|Lovelock|2009|p=179}}

===Regulation of global surface temperature===
{{See also|Paleoclimatology}}
[[File:All palaeotemps.png|thumb|upright=1.5|Rob Rohde's palaeotemperature graphs]]

Since life started on Earth, the energy provided by the [[Sun]] has increased by 25–30%;<ref name="Owen1979">{{cite journal | author = Owen, T. | author2 = Cess, R.D. | author3 = Ramanathan, V. | date = 1979 | title = Earth: An enhanced carbon dioxide greenhouse to compensate for reduced solar luminosity | journal = [[Nature (journal)|Nature]] | volume = 277 | pages = 640–2 | doi = 10.1038/277640a0 | issue=5698 | bibcode = 1979Natur.277..640O | s2cid = 4326889 }}</ref> however, the surface temperature of the planet has remained within the levels of habitability, reaching quite regular low and high margins.  Lovelock has also hypothesised that methanogens produced elevated levels of methane in the early atmosphere, giving a situation similar to that found in petrochemical smog, similar in some respects to the atmosphere on [[Titan (moon)|Titan]].<ref>{{cite book |last=Lovelock |first=James |date=1995 |title=The Ages of Gaia: A Biography of Our Living Earth |isbn=978-0-393-31239-3 |publisher=W. W. Norton & Co. |location=New York}}</ref> This, he suggests, helped to screen out ultraviolet light until the formation of the ozone layer, maintaining a degree of homeostasis.  However, the [[Snowball Earth]]<ref>Hoffman, P.F. 2001. [http://www.snowballearth.org ''Snowball Earth theory'']</ref> research has suggested that "oxygen shocks" and reduced methane levels led, during the [[Huronian]], [[Sturtian]] and [[Marinoan]]/[[Cryogenian|Varanger]] Ice Ages, to a world that very nearly became a solid "snowball". These epochs are evidence against the ability of the pre [[Phanerozoic]] biosphere to fully self-regulate.

Processing of the greenhouse gas CO<sub>2</sub>, explained below, plays a critical role in the maintenance of the Earth temperature within the limits of habitability.

The [[CLAW hypothesis]], inspired by the Gaia hypothesis, proposes a [[feedback|feedback loop]] that operates between [[ocean]] [[ecosystem]]s and the [[Earth]]'s [[climate]].<ref name="CLAW87">{{cite journal |doi=10.1038/326655a0 |author1=[[Robert Jay Charlson|Charlson, R. J.]] |author2=[[James Lovelock|Lovelock, J. E]] |author3=Andreae, M. O. |author4=Warren, S. G. |title=Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate |journal=Nature |volume=326 |issue=6114 |pages=655–661 |date=1987 |bibcode=1987Natur.326..655C |s2cid=4321239 }}</ref>  The [[hypothesis]] specifically proposes that particular [[phytoplankton]] that produce [[dimethyl sulfide]] are responsive to variations in [[climate forcing]], and that these responses lead to a [[negative feedback]] loop that acts to stabilise the [[temperature]] of the [[Earth's atmosphere]].

Currently the increase in human population and the environmental impact of its activities, such as the multiplication of [[greenhouse gases]] may cause negative feedbacks in the environment to become [[positive feedback]]. Lovelock has stated that this could bring an [[James Lovelock#The revenge of Gaia|extremely accelerated global warming]],{{sfn|Lovelock|2009|p={{pn|date=July 2024}}}} but he has since stated the effects will likely occur more slowly.<ref>{{cite news |first=Ian |last=Johnston |title='Gaia' scientist James Lovelock: I was 'alarmist' about climate change |newspaper=NBC News |url=http://worldnews.nbcnews.com/_news/2012/04/23/11144098-gaia-scientist-james-lovelock-i-was-alarmist-about-climate-change?lite |date=23 April 2012 |access-date=22 August 2012 |archive-url=https://web.archive.org/web/20120913163635/http://worldnews.nbcnews.com/_news/2012/04/23/11144098-gaia-scientist-james-lovelock-i-was-alarmist-about-climate-change?lite |archive-date=13 September 2012 }}</ref>

====Daisyworld simulations====
{{Main|Daisyworld}}
[[File:StandardDaisyWorldRun2color.gif|thumb|280px|Plots from a standard black and white [[Daisyworld]] simulation]]

In response to the criticism that the Gaia hypothesis seemingly required unrealistic [[group selection]] and [[Cooperation (evolution)|cooperation]] between organisms, James Lovelock and [[Andrew Watson (scientist)|Andrew Watson]] developed a mathematical model, [[Daisyworld]], in which [[Competition (biology)|ecological competition]] underpinned planetary temperature regulation.<ref name="daisyworld">{{cite journal
|date = 1983
|title = Biological homeostasis of the global environment: the parable of Daisyworld
|journal = Tellus
|volume = 35B
|pages = 286–9
|bibcode = 1983TellB..35..284W
|doi = 10.1111/j.1600-0889.1983.tb00031.x
|last1 = Watson | first1= A.J. | last2= Lovelock | first2= J.E
|issue = 4
}}</ref>

Daisyworld examines the [[Earth's energy budget|energy budget]] of a [[planet]] populated by two different types of plants, black [[Asteraceae|daisies]] and white daisies, which are assumed to occupy a significant portion of the surface. The colour of the daisies influences the [[albedo]] of the planet such that black daisies absorb more light and warm the planet, while white daisies reflect more light and cool the planet.  The black daisies are assumed to grow and reproduce best at a lower temperature, while the white daisies are assumed to thrive best at a higher temperature.  As the temperature rises closer to the value the white daisies like, the white daisies outreproduce the black daisies, leading to a larger percentage of white surface, and more sunlight is reflected, reducing the heat input and eventually cooling the planet. Conversely, as the temperature falls, the black daisies outreproduce the white daisies, absorbing more sunlight and warming the planet. The temperature will thus converge to the value at which the reproductive rates of the plants are equal.

Lovelock and Watson showed that, over a limited range of conditions, this [[negative feedback]] due to competition can stabilize the planet's temperature at a value which supports life, if the energy output of the Sun changes, while a planet without life would show wide temperature changes.  The percentage of white and black daisies will continually change to keep the temperature at the value at which the plants' reproductive rates are equal, allowing both life forms to thrive.

It has been suggested that the results were predictable because Lovelock and Watson selected examples that produced the responses they desired.<ref>{{cite journal | doi = 10.1023/A:1023494111532 | date = 2003 | last1 = Kirchner | first1 = James W. | journal = Climatic Change | volume = 58 |issue=1–2| pages = 21–45 |title=The Gaia Hypothesis: Conjectures and Refutations | s2cid = 1153044 }}</ref>

===Regulation of oceanic salinity===
Ocean [[salinity]] has been constant at about 3.5% for a very long time.<ref name="Segar-2012">{{Cite book|title=The Introduction to Ocean Sciences|last=Segar|first=Douglas|publisher=Library of Congress|year=2012|isbn=978-0-9857859-0-1|url=http://www.reefimages.com/oceans/SegarOcean3Chap05.pdf|pages=Chapter 5 3rd Edition|access-date=2017-02-05|archive-date=2016-03-25|archive-url=https://web.archive.org/web/20160325060308/http://www.reefimages.com/oceans/SegarOcean3Chap05.pdf|url-status=dead}}</ref> Salinity stability in oceanic environments is important as most cells require a rather constant salinity and do not generally tolerate values above 5%. The constant ocean salinity was a long-standing mystery, because no process counterbalancing the salt influx from rivers was known. Recently it was suggested<ref name="Gorham19912">{{cite journal|last=Gorham|first=Eville|date=1 January 1991|title=Biogeochemistry: its origins and development |journal=Biogeochemistry|publisher=Kluwer Academic|volume=13|issue=3|pages=199–239|doi=10.1007/BF00002942|s2cid=128563314|issn=1573-515X}}</ref> that salinity may also be strongly influenced by [[seawater]] circulation through hot [[basalt]]ic rocks, and emerging as hot water vents on [[mid-ocean ridge]]s. However, the composition of seawater is far from equilibrium, and it is difficult to explain this fact without the influence of organic processes. One suggested explanation lies in the formation of salt plains throughout Earth's history. It is hypothesized that these are created by bacterial colonies that fix ions and heavy metals during their life processes.<ref name="Segar-2012" />

In the biogeochemical processes of Earth, sources and sinks are the movement of elements. The composition of salt ions within our oceans and seas is: sodium (Na<sup>+</sup>), chlorine (Cl<sup>−</sup>), sulfate (SO<sub>4</sub><sup>2−</sup>), magnesium (Mg<sup>2+</sup>), calcium (Ca<sup>2+</sup>) and potassium (K<sup>+</sup>). The elements that comprise salinity do not readily change and are a conservative property of seawater.<ref name="Segar-2012" /> There are many mechanisms that change salinity from a particulate form to a dissolved form and back. Considering the metallic composition of iron sources across a multifaceted grid of thermomagnetic design, not only would the movement of elements hypothetically help restructure the movement of ions, electrons, and the like, but would also potentially and inexplicably assist in balancing the magnetic bodies of the Earth's geomagnetic field. The known sources of sodium i.e. salts are when weathering, erosion, and dissolution of rocks are transported into rivers and deposited into the oceans.

The [[Mediterranean Sea]] as being Gaia's kidney is found ([http://scimar.icm.csic.es/scimar/index.php/secId/6/IdArt/209/ here]) by [[Kenneth Hsu|Kenneth J. Hsu]], a correspondence author in 2001. Hsu suggests the "[[desiccation]]" of the Mediterranean is evidence of a functioning Gaia "kidney". In this and earlier suggested cases, it is plate movements and physics, not biology, which performs the regulation. Earlier "kidney functions" were performed during the "[[Deposition (geology)|deposition]] of the [[Cretaceous]] ([[Atlantic Ocean|South Atlantic]]), [[Jurassic]] ([[Gulf of Mexico]]), [[Permian–Triassic extinction event|Permo-Triassic]] ([[Europe]]), [[Devonian]] ([[Canada]]), and [[Cambrian]]/[[Precambrian]] ([[Gondwana]]) saline giants."<ref>{{Cite web|url=http://scimar.icm.csic.es/scimar/index.php/secId/6/IdArt/209/|title=Scientia Marina: List of Issues|website=scimar.icm.csic.es|language=en|access-date=2017-02-04}}</ref>

===Regulation of oxygen in the atmosphere===
{{See also|Geological history of oxygen}}
[[File:Vostok 420ky 4curves insolation.jpg|thumb|280px|Levels of gases in the atmosphere in 420,000 years of ice core data from [[Vostok Station|Vostok, Antarctica research station]]. Current period is at the left. <!-- Unsourced material based on GIMP FX version of this chart. The current version here is correct, original. This verbiage must be removed: Note that current CO<sub>2</sub> levels are more than 390 ppm, far higher than at any time in the last 400,000 years -->]]

The Gaia hypothesis states that the Earth's [[Atmospheric chemistry#Atmospheric composition|atmospheric composition]] is kept at a dynamically steady state by the presence of life.{{sfn|Lovelock|2009|p=163}} The atmospheric composition provides the conditions that contemporary life has adapted to. All the atmospheric gases other than [[noble gas]]es present in the atmosphere are either made by organisms or processed by them.

The stability of the atmosphere in Earth is not a consequence of [[chemical equilibrium]]. [[Oxygen]] is a reactive compound, and should eventually combine with gases and minerals of the Earth's atmosphere and crust.  Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the [[Great Oxygenation Event]].<ref name=Anabar2007>{{Cite journal| last4 = Arnold| last6 = Creaser| last3 = Lyons| first1 = A. | first2 = Y.| last9 = Scott| last2 = Duan | first3 = T. | first4 = G.| last8 = Gordon | first5 = B. | first10 = J. | first6 = R.| last10 = Garvin | first7 = A.| last11 = Buick | first8 = G. | first11 = R. | first9 = C.| title = A whiff of oxygen before the great oxidation event?| journal = Science| volume = 317| issue = 5846| year = 2007| last7 = Kaufman| pages = 1903–1906| last5 = Kendall| pmid = 17901330| last1 = Anbar | doi = 10.1126/science.1140325|bibcode = 2007Sci...317.1903A | s2cid = 25260892}}</ref> Since the start of the [[Cambrian]] period, atmospheric oxygen concentrations have fluctuated between 15% and 40% of atmospheric volume.<ref name=Berner1999>{{Cite journal
| pmid = 10500106
| date=Sep 1999 | last1 = Berner | first1 = R. A.
| title = Atmospheric oxygen over Phanerozoic time
| volume = 96
| issue = 20
| pages = 10955–10957
| issn = 0027-8424
| journal = Proceedings of the National Academy of Sciences of the United States of America
| doi = 10.1073/pnas.96.20.10955
| pmc = 34224
|bibcode = 1999PNAS...9610955B | doi-access=free }}</ref> Traces of [[Atmospheric methane|methane]] (at an amount of 100,000 tonnes produced per year)<ref name="Cicerone1988">{{cite journal |last1=Cicerone |author2-link=Ronald Oremland|first1=R.J. |last2=Oremland |first2=R.S. |date=1988 |title=Biogeochemical aspects of atmospheric methane |journal=Global Biogeochemical Cycles |volume=2 |issue=4 |pages=299–327 |url=http://webfiles.uci.edu/setrumbo/public/Methane_papers/Cicerone_Global%20Biogeochem%20Cy_1988.pdf |doi=10.1029/GB002i004p00299 |bibcode=1988GBioC...2..299C|s2cid=56396847 }}</ref> should not exist, as methane is combustible in an oxygen atmosphere.

Dry air in the [[atmosphere of Earth]] contains roughly (by volume) 78.09% [[nitrogen]], 20.95% oxygen, 0.93% [[argon]], 0.039% [[Carbon dioxide in the Earth's atmosphere|carbon dioxide]], and small amounts of other gases including [[methane]]. Lovelock originally speculated that concentrations of oxygen above about 25% would increase the frequency of wildfires and conflagration of forests. This mechanism, however, would not raise oxygen levels if they became too low. If plants can be shown to robustly over-produce O<sub>2</sub> then perhaps only the high oxygen forest fires regulator is necessary. Recent work on the findings of fire-caused charcoal in Carboniferous and Cretaceous coal measures, in geologic periods when O<sub>2</sub> did exceed 25%, has supported Lovelock's contention.{{citation needed|date=June 2012}}

===Processing of CO<sub>2</sub>===
{{See also|Carbon cycle}}
Gaia scientists see the participation of living organisms in the [[carbon cycle]] as one of the complex processes that maintain conditions suitable for life. The only significant natural source of [[Carbon dioxide in Earth's atmosphere|atmospheric carbon dioxide]] ([[Carbon dioxide|CO<sub>2</sub>]]) is [[volcanic activity]], while the only significant removal is through the precipitation of [[carbonate rocks]].<ref name="Karhu1996">{{cite journal | author = Karhu, J.A. | author2 = Holland, H.D. | date = 1 October 1996 | title = Carbon isotopes and the rise of atmospheric oxygen | journal = [[Geology (journal)|Geology]] | volume = 24 | issue = 10 | pages = 867–870 | doi = 10.1130/0091-7613(1996)024<0867:CIATRO>2.3.CO;2|bibcode = 1996Geo....24..867K }}</ref> Carbon precipitation, solution and [[Carbon fixation|fixation]] 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 is used by living organisms to manufacture carbonaceous tests and shells. Once dead, the living organisms' shells fall. Some arrive at the bottom of shallow seas where the heat and pressure of burial, and/or the forces of plate tectonics, eventually convert them to deposits of chalk and limestone. Much of the falling dead shells, however, redissolve into the ocean below the carbon compensation depth.

One of these organisms is ''[[Emiliania huxleyi]]'', an abundant [[coccolithophore]] [[algae]] which may have a role in the formation of [[cloud]]s.{{sfn|Harding|2006|p=65}} CO<sub>2</sub> excess is compensated by an increase of coccolithophorid life, increasing the amount of CO<sub>2</sub> locked in the ocean floor. Coccolithophorids, if the CLAW Hypothesis turns out to be supported (see "Regulation of Global Surface Temperature" above), could help increase the cloud cover, hence control the surface temperature, help cool the whole planet and favor precipitation necessary for terrestrial plants.{{citation needed|date=July 2015}} Lately the atmospheric CO<sub>2</sub> concentration has increased and there is some evidence that concentrations of ocean [[algal bloom]]s are also increasing.<ref>{{Cite web | date = 12 September 2007 | title = Interagency Report Says Harmful Algal Blooms Increasing | url = http://www.publicaffairs.noaa.gov/releases2007/sep07/noaa07-r435.html | url-status = dead | archive-url = https://web.archive.org/web/20080209234239/http://www.publicaffairs.noaa.gov/releases2007/sep07/noaa07-r435.html | archive-date = 9 February 2008 }}</ref>

[[Lichen]] and other organisms accelerate the [[weathering]] of rocks in the surface, while the decomposition of rocks also happens faster in the soil, thanks to the activity of roots, fungi, bacteria and subterranean animals. The flow of carbon dioxide from the atmosphere to the soil is therefore regulated with the help of living organisms. When CO<sub>2</sub> levels rise in the atmosphere the temperature increases and plants grow. This growth brings higher consumption of CO<sub>2</sub> by the plants, who process it into the soil, removing it from the atmosphere.

==History==

===Precedents===
[[File:NASA-Apollo8-Dec24-Earthrise.jpg|thumb|''[[Earthrise]]'' taken from [[Apollo 8]] by astronaut [[William Anders]], December 24, 1968]]

The idea of the Earth as an integrated whole, a living being, has a long tradition. The [[Gaia (mythology)|mythical Gaia]] was the primal Greek goddess personifying the [[Earth]], the Greek version of "[[Mother Nature]]" (from Ge = Earth, and Aia = [[PIE]] grandmother), or the [[Earth Mother]]. James Lovelock gave this name to his hypothesis after a suggestion from the novelist [[William Golding]], who was living in the same village as Lovelock at the time ([[Bowerchalke]], [[Wiltshire]], UK). Golding's advice was based on Gea, an alternative spelling for the name of the Greek goddess, which is used as prefix in geology, geophysics and geochemistry.{{sfn|Lovelock|2009|pp=195-197}} Golding later made reference to Gaia in his [[Nobel prize]] acceptance speech.

In the eighteenth century, as [[geology]] consolidated as a modern science, [[James Hutton]] maintained that geological and biological processes are interlinked.<ref name=CapraWeb>{{cite book |author=Capra, Fritjof |title=The web of life: a new scientific understanding of living systems |publisher=Anchor Books |location=Garden City, N.Y |date=1996 |page=[https://archive.org/details/weboflifenewscie00capr/page/23 23] |isbn=978-0-385-47675-1 |url=https://archive.org/details/weboflifenewscie00capr/page/23 }}</ref> Later, the [[naturalist]] and explorer [[Alexander von Humboldt]] recognized the coevolution of living organisms, climate, and Earth's crust.<ref name=CapraWeb /> In the twentieth century, [[Vladimir Vernadsky]] formulated a theory of Earth's development that is now one of the foundations of ecology. Vernadsky was a Ukrainian [[geochemist]] and was one of the first scientists to recognize that the oxygen, nitrogen, and carbon dioxide in the Earth's atmosphere result from biological processes. During the 1920s he published works arguing that living organisms could reshape the planet as surely as any physical force. Vernadsky was a pioneer of the scientific bases for the environmental sciences.<ref>{{cite book |first=S. R. |last=Weart |year=2003 |title=The Discovery of Global Warming |place=Cambridge |publisher=Harvard Press}}</ref> His visionary pronouncements were not widely accepted in the West, and some decades later the Gaia hypothesis received the same type of initial resistance from the scientific community.

Also in the turn to the 20th century [[Aldo Leopold]], pioneer in the development of modern [[environmental ethics]] and in the movement for [[wilderness]] conservation, suggested a living Earth in his biocentric or holistic ethics regarding land.

{{blockquote|It is at least not impossible to regard the earth's parts—soil, mountains, rivers, atmosphere etc,—as organs or parts of organs of a coordinated whole, each part with its definite function. And if we could see this whole, as a whole, through a great period of time, we might perceive not only organs with coordinated functions, but possibly also that process of consumption as replacement which in biology we call metabolism, or growth. In such case we would have all the visible attributes of a living thing, which we do not realize to be such because it is too big, and its life processes too slow.| Stephan Harding | ''Animate Earth''{{sfn|Harding|2006|p=44}}
}}

Another influence for the Gaia hypothesis and the [[environmental movement]] in general came as a side effect of the [[Space Race]] between the Soviet Union and the United States of America. During the 1960s, the first humans in space could see how the Earth looked as a whole. The photograph ''[[Earthrise]]'' taken by astronaut [[William Anders]] in 1968 during the [[Apollo 8]] mission became, through the [[Overview Effect]] an early symbol for the global ecology movement.<ref>[http://digitaljournalist.org/issue0309/lm11.html 100 Photographs that Changed the World by Life - The Digital Journalist]</ref>

===Formulation of the hypothesis===
[[File:James Lovelock in 2005.jpg|thumb|[[James Lovelock]], 2005]]

Lovelock started defining the idea of a self-regulating Earth controlled by the community of living organisms in September 1965, while working at the [[Jet Propulsion Laboratory]] in California on methods of detecting [[life on Mars (planet)|life on Mars]].<ref name="Lovelock1965">{{cite journal | author = Lovelock, J. E. | date = 1965 | title = A physical basis for life detection experiments | journal = [[Nature (journal)|Nature]] | volume = 207 | issue = 7 | pages = 568–570 | doi = 10.1038/207568a0 | pmid=5883628|bibcode = 1965Natur.207..568L | s2cid = 33821197 }}</ref><ref>{{Cite web |url=http://www.jameslovelock.org/page4.html |title=Geophysiology |access-date=2007-05-05 |archive-url=https://web.archive.org/web/20070506073502/http://www.jameslovelock.org/page4.html |archive-date=2007-05-06 |url-status=dead }}</ref>  The first paper to mention it was ''Planetary Atmospheres: Compositional and other Changes Associated with the Presence of Life'', co-authored with C.E. Giffin.<ref>{{cite journal | author1 = Lovelock, J. E. | author2 = Giffin, C.E. | date = 1969 | title = Planetary Atmospheres: Compositional and other changes associated with the presence of Life | journal = Advances in the Astronautical Sciences | volume = 25 | pages = 179–193 | isbn = 978-0-87703-028-7 }}</ref> A main concept was that life could be detected in a planetary scale by the chemical composition of the atmosphere. According to the data gathered by the [[Pic du Midi de Bigorre|Pic du Midi observatory]], planets like Mars or Venus had atmospheres in [[chemical equilibrium]]. This difference with the Earth atmosphere was considered to be a proof that there was no life in these planets.

Lovelock formulated the ''Gaia Hypothesis'' in journal articles in 1972<ref name="J1972">{{cite journal |first=J. E. |last=Lovelock | title = Gaia as seen through the atmosphere | date = 1972 | journal = [[Atmospheric Environment]] | volume = 6 | issue = 8 | pages = 579–580 | doi = 10.1016/0004-6981(72)90076-5 |bibcode = 1972AtmEn...6..579L }}</ref> and 1974,<ref name="lovelock1974" /> followed by a popularizing 1979 book ''Gaia: A new look at life on Earth''. An article in the ''[[New Scientist]]'' of February 6, 1975,<ref>Lovelock, John and Sidney Epton, (February 8, 1975). "The quest for Gaia". [https://books.google.com/books?id=pnV6UYEkU4YC New Scientist], p. 304.</ref> and a popular book length version of the hypothesis, published in 1979 as ''The Quest for Gaia'', began to attract scientific and critical attention.

Lovelock called it first the Earth feedback hypothesis,{{sfn|Lovelock|2001}} and it was a way to explain the fact that combinations of chemicals including [[oxygen]] and [[methane]] persist in stable concentrations in the atmosphere of the Earth. Lovelock suggested detecting such combinations in other planets' atmospheres as a relatively reliable and cheap way to detect life.

[[File:Lynn Margulis.jpg|thumb|upright|left|[[Lynn Margulis]]]]

Later, other relationships such as sea creatures producing sulfur and iodine in approximately the same quantities as required by land creatures emerged and helped bolster the hypothesis.<ref>{{cite journal | first1=W.D. | last1=Hamilton | first2=T.M. | last2=Lenton | title=Spora and Gaia: how microbes fly with their clouds | journal=Ethology Ecology & Evolution | volume=10 | pages=1–16 | date=1998 | issue=1 | url=http://ejour-fup.unifi.it/index.php/eee/article/viewFile/787/733 | format=PDF | doi=10.1080/08927014.1998.9522867 | bibcode=1998EtEcE..10....1H | url-status=dead | archive-url=https://web.archive.org/web/20110723055017/http://ejour-fup.unifi.it/index.php/eee/article/viewFile/787/733 | archive-date=2011-07-23 | url-access=subscription }}</ref>

In 1971 [[microbiologist]] Dr. [[Lynn Margulis]] joined Lovelock in the effort of fleshing out the initial hypothesis into scientifically proven concepts, contributing her knowledge about how microbes affect the atmosphere and the different layers in the surface of the planet.{{sfn|Turney|2003}} The American biologist had also awakened criticism from the scientific community with her advocacy of the theory on the origin of [[eukaryote|eukaryotic]] [[organelle]]s and her contributions to the [[endosymbiotic theory]], nowadays accepted. Margulis dedicated the last of eight chapters in her book, ''The Symbiotic Planet'', to Gaia. However, she objected to the widespread personification of Gaia and stressed that Gaia is "not an organism", but "an emergent property of interaction among organisms". She defined Gaia as "the series of interacting ecosystems that compose a single huge ecosystem at the Earth's surface. Period". The book's most memorable "slogan" was actually quipped by a student of Margulis'.

James Lovelock called his first proposal the ''Gaia hypothesis'' but has also used the term ''Gaia theory''. Lovelock states that the initial formulation was based on observation, but still lacked a scientific explanation. The Gaia hypothesis has since been supported by a number of scientific experiments<ref name="J1990">{{cite journal |first=J. E. |last=Lovelock | title = Hands up for the Gaia hypothesis | date = 1990 | journal = [[Nature (journal)|Nature]] | volume = 344 | issue = 6262 | pages = 100–2 | doi = 10.1038/344100a0|bibcode = 1990Natur.344..100L | s2cid = 4354186 }}</ref> and provided a number of useful predictions.<ref name="Volk2003">{{cite book |author=Volk, Tyler |title=Gaia's Body: Toward a Physiology of Earth |publisher=[[MIT Press]] |location=Cambridge, Massachusetts |date=2003 |isbn=978-0-262-72042-7 }}</ref>

===First Gaia conference===
In 1985, the first public symposium on the Gaia hypothesis, ''Is The Earth a Living Organism?'' was held at [[University of Massachusetts Amherst]], August 1–6.<ref>{{cite news |last=Joseph |first=Lawrence E. |title=Britain's Whole Earth Guru |work=The New York Times Magazine |date=November 23, 1986 |url=https://www.nytimes.com/1986/11/23/magazine/britain-s-whole-earth-guru.html |access-date=1 December 2013}}</ref> The principal sponsor was the [[National Audubon Society]]. Speakers included James Lovelock, [[Lynn Margulis]], [[George Wald]], [[Mary Catherine Bateson]], [[Lewis Thomas]], [[Thomas Berry]], [[David Abram]], [[John Todd (Canadian biologist)|John Todd]], Donald Michael, [[Christopher Bird]], [[Michael A. Cohen|Michael Cohen]], and William Fields. Some 500 people attended.<ref>Bunyard, Peter (1996), ''Gaia in Action: Science of the Living Earth'' (Floris Books)</ref>

===Second Gaia conference===
In 1988, [[climatology|climatologist]] [[Stephen Schneider (scientist)|Stephen Schneider]] organised a conference of the [[American Geophysical Union]]. The first Chapman Conference on Gaia,{{sfn|Turney|2003}} was held in San Diego, California, on March 7, 1988.

During the "philosophical foundations" session of the conference, [[David Abram]] spoke on the influence of metaphor in science, and of the Gaia hypothesis as offering a new and potentially game-changing metaphorics, while [[James Kirchner]] criticised the Gaia hypothesis for its imprecision. Kirchner claimed that Lovelock and Margulis had not presented one Gaia hypothesis, but four:

* [[Coevolution|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.
* [[Geophysics|Geophysical]] Gaia: that the Gaia hypothesis 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, ''to enable'' the flourishing of all life. Strong Gaia, Kirchner claimed, was untestable and therefore not scientific.<ref>{{cite journal | bibcode=1989RvGeo..27..223K | doi = 10.1029/RG027i002p00223 | title=The Gaia hypothesis: Can it be tested? | date=1989 | last1=Kirchner | first1=James W. | journal=Reviews of Geophysics | volume=27 | issue=2 | pages=223–235 }}</ref>

Lovelock and other Gaia-supporting scientists, however, did attempt to disprove the claim that the hypothesis 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.<ref name="daisyworld"/>  Lovelock said that the Daisyworld model "demonstrates that self-regulation of the global environment can emerge from competition amongst types of life altering their local environment in different ways".<ref>{{cite journal | pmid=10968941 | date=2000 | last1=Lenton | first1=TM | last2=Lovelock | first2=JE | s2cid=5486128 | title=Daisyworld is Darwinian: Constraints on adaptation are important for planetary self-regulation | volume=206 | issue=1 | pages=109–14 | doi=10.1006/jtbi.2000.2105 | journal=Journal of Theoretical Biology | bibcode=2000JThBi.206..109L }}</ref>

Lovelock was careful to present a version of the Gaia hypothesis that 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 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. Most accusations of [[teleology|teleologism]] ceased, following this conference.{{citation needed|date=May 2021}}

===Third Gaia conference===
By the time of the 2nd Chapman Conference on the Gaia Hypothesis, held at Valencia, Spain, on 23 June 2000,<ref>{{cite news|last=Simón|first=Federico|title=GEOLOGÍA Enfoque multidisciplinar La hipótesis Gaia madura en Valencia con los últimos avances científicos|journal=El País|date=21 June 2000|url=http://elpais.com/diario/2000/06/21/futuro/961538404_850215.html|access-date=1 December 2013|language=es}}</ref> the situation had changed significantly. Rather than a discussion of the Gaian teleological views, or "types" of Gaia hypotheses, 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:<ref>{{cite web |title=General Information Chapman Conference on the Gaia Hypothesis University of Valencia Valencia, Spain June 19-23, 2000 (Monday through Friday) |url=http://www.agu.org/meetings/chapman/chapman_archive/cc00bcall.html |work=AGU Meetings |access-date=7 January 2017 |author=American Geophysical Union |archive-date=4 June 2012 |archive-url=https://archive.today/20120604135856/http://www.agu.org/meetings/chapman/chapman_archive/cc00bcall.html |url-status=dead }}</ref>

# "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 ''collaborated'' with using process models or global models of the climate system that include the biota and allow for chemical cycling?"

In 1997, [[Tyler Volk]] argued that a Gaian system is almost inevitably produced as a result of an evolution towards far-from-equilibrium homeostatic states that maximise [[entropy production]], and Axel Kleidon (2004) agreed stating: "...homeostatic behavior can emerge from a state of MEP associated with the planetary albedo"; "...the resulting behavior of a symbiotic 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". M. Staley (2002) has similarly proposed "...an alternative form of Gaia theory based on more traditional Darwinian principles... In [this] new approach, environmental regulation is a consequence of population dynamics. 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".

===Fourth Gaia conference===
A fourth international conference on the Gaia hypothesis, sponsored by the Northern Virginia Regional Park Authority and others, was held in October 2006 at the Arlington, Virginia campus of George Mason University.<ref>{{cite web|title=Gaia Theory Conference at George Mason University Law School|url=http://www.arlingtonva.us/departments/Communications/PressReleases/page7530.aspx|access-date=1 December 2013|author=Official Site of Arlington County Virginia|archive-url=https://web.archive.org/web/20131203043657/http://www.arlingtonva.us/departments/Communications/PressReleases/page7530.aspx|archive-date=2013-12-03|url-status=dead}}</ref>

Martin Ogle, Chief Naturalist, for NVRPA, and long-time Gaia hypothesis proponent, organized the event. Lynn Margulis, Distinguished University Professor in the Department of Geosciences, University of Massachusetts-Amherst, and long-time advocate of the Gaia hypothesis, was a keynote speaker. Among many other speakers: Tyler Volk, co-director of the Program in Earth and Environmental Science at New York University; Dr. Donald Aitken, Principal of Donald Aitken Associates; [[Thomas Lovejoy|Dr. Thomas Lovejoy]], President of the Heinz Center for Science, Economics and the Environment; [[Robert Corell]], Senior Fellow, Atmospheric Policy Program, American Meteorological Society and noted environmental ethicist, [[J. Baird Callicott]].

==Criticism==
After initially receiving little attention from scientists (from 1969 until 1977), thereafter for a period the initial Gaia hypothesis was criticized by a number of scientists, including [[Ford Doolittle]],<ref name="Doolittle-1981">{{Cite journal|last=Doolittle|first=W. F.|year=1981|title=Is Nature Really Motherly|journal=The Coevolution Quarterly|volume=Spring|pages=58–63}}</ref> [[Richard Dawkins]]{{sfn|Dawkins|1982}} and [[Stephen Jay Gould]].{{sfn|Turney|2003}} Lovelock has said that because his hypothesis is named after a Greek goddess, and championed by many non-scientists,{{sfn|Lovelock|2001}} the Gaia hypothesis was interpreted as a [[neo-Pagan]] [[religion]]. Many scientists in particular also criticized the approach taken in his popular book ''Gaia, a New Look at Life on Earth'' for being [[teleology|teleological]]—a belief that things are purposeful and aimed towards a goal. Responding to this critique in 1990, Lovelock stated, "Nowhere in our writings do we express the idea that planetary self-regulation is purposeful, or involves foresight or planning by the [[biota (ecology)|biota]]".

[[Stephen Jay Gould]] criticized Gaia as being "a metaphor, not a mechanism."<ref name="Gould 1997">{{cite journal |author=Gould S.J. |title=Kropotkin was no crackpot |journal=Natural History |volume=106 |pages=12–21 |date=June 1997  |url=http://libcom.org/library/kropotkin-was-no-crackpot }}</ref> He wanted to know the actual mechanisms by which self-regulating homeostasis was achieved. In his defense of Gaia, David Abram argues that Gould overlooked the fact that "mechanism", itself, is a metaphor—albeit an exceedingly common and often unrecognized metaphor—one which leads us to consider natural and living systems as though they were machines organized and built from outside (rather than as [[autopoiesis|autopoietic]] or self-organizing phenomena). Mechanical metaphors, according to Abram, lead us to overlook the active or agentic quality of living entities, while the organismic metaphors of the Gaia hypothesis accentuate the active agency of both the biota and the biosphere as a whole.{{sfn|Abram|1991}} With regard to causality in Gaia, Lovelock argues that no single mechanism is responsible, that the connections between the various known mechanisms may never be known, that this is accepted in other fields of biology and ecology as a matter of course, and that specific hostility is reserved for his own hypothesis for other reasons.{{sfn|Lovelock|2001}}

Aside from clarifying his language and understanding of what is meant by a life form, Lovelock himself ascribes most of the criticism to a lack of understanding of non-linear mathematics by his critics, and a linearizing form of [[greedy reductionism]] in which all events have to be immediately ascribed to specific causes before the fact. He also states that most of his critics are biologists but that his hypothesis includes experiments in fields outside biology, and that some self-regulating phenomena may not be mathematically explainable.{{sfn|Lovelock|2001}}

===Natural selection and evolution===
Lovelock has suggested that global biological feedback mechanisms could evolve by [[natural selection]], stating that organisms that improve their environment for their survival do better than those that damage their environment. However, in the early 1980s, [[Ford Doolittle|W. Ford Doolittle]] and [[Richard Dawkins]] separately argued against this aspect of Gaia. Doolittle argued that nothing in the [[genome]] of individual organisms could provide the feedback mechanisms proposed by Lovelock, and therefore the Gaia hypothesis proposed no plausible mechanism and was unscientific.<ref name="Doolittle-1981" /> Dawkins meanwhile stated that for organisms to act in concert would require foresight and planning, which is contrary to the current scientific understanding of evolution.{{sfn|Dawkins|1982}} Like Doolittle, he also rejected the possibility that feedback loops could stabilize the system.

Margulis argued in 1999 that "[[Charles Darwin|Darwin]]'s grand vision was not wrong, ''only incomplete.'' In accentuating the direct competition between individuals for resources as the primary selection mechanism, Darwin (and especially his followers) created the impression that the environment was simply a static arena". She wrote that the composition of the Earth's atmosphere, hydrosphere, and lithosphere are regulated around "set points" as in [[homeostasis]], but those set points change with time.{{sfn|Margulis|1998}}

Evolutionary biologist [[W. D. Hamilton]] called the concept of Gaia [[Nicolaus Copernicus|Copernican]], adding that it would take another [[Isaac Newton|Newton]] to explain how Gaian self-regulation takes place through Darwinian [[natural selection]].{{sfn|Lovelock|2009|pp=195-197}}{{better source needed|date=September 2012|reason=it should be possible to find the original place where Hamilton said this}}  More recently Ford Doolittle building on his and Inkpen's ITSNTS (It's The Song Not The Singer) proposal<ref name="ITSNTS">Doolittle WF, Inkpen SA. Processes and patterns of interaction as units of selection: An introduction to ITSNTS thinking. [https://www.pnas.org/content/115/16/4006 PNAS April 17, 2018 115 (16)] 4006-4014</ref> proposed that differential persistence can play a similar role to differential reproduction in evolution by natural selections, thereby providing a possible reconciliation between the theory of natural selection and the Gaia hypothesis.<ref name="Darwinizing Gaia">{{Cite journal|url=https://doi.org/10.1016/j.jtbi.2017.02.015|doi = 10.1016/j.jtbi.2017.02.015|title = Darwinizing Gaia|year = 2017|last1 = Doolittle|first1 = W. Ford|journal = Journal of Theoretical Biology|volume = 434|pages = 11–19|pmid = 28237396|bibcode = 2017JThBi.434...11D|url-access = subscription}}</ref>

===Criticism in the 21st century===
The Gaia hypothesis continues to be broadly skeptically received by the scientific community. For instance, arguments both for and against it were laid out in the journal ''Climatic Change'' in 2002 and 2003. A significant argument raised against it are the many examples where life has had a detrimental or destabilising effect on the environment rather than acting to regulate it.<ref name="kirchner2002"/><ref name="volk2002"/> Several recent books have criticised the Gaia hypothesis, expressing views ranging from "... the Gaia hypothesis lacks unambiguous observational support and has significant theoretical difficulties"<ref>{{cite book |last=Waltham |first=David |date=2014 |title=Lucky Planet: Why Earth is Exceptional – and What that Means for Life in the Universe |url=https://archive.org/details/luckyplanetwhyea0000walt |publisher=Icon Books |isbn=9781848316560 |url-access=registration }}</ref> to "Suspended uncomfortably between tainted metaphor, fact, and false science, I prefer to leave Gaia firmly in the background"<ref name="beerling2007"/> to "The Gaia hypothesis is supported neither by evolutionary theory nor by the empirical evidence of the geological record".<ref>{{cite book |last1=Cockell |first1=Charles |author-link1=Charles Cockell |last2=Corfield |first2=Richard |last3=Dise |first3= Nancy  |last4=Edwards |first4=Neil  |last5=Harris |first5=Nigel  |date=2008 |title= An Introduction to the Earth-Life System |url= http://www.cambridge.org/us/academic/subjects/earth-and-environmental-science/palaeontology-and-life-history/introduction-earth-life-system |location=Cambridge (UK) |publisher= Cambridge University Press |isbn= 9780521729536 }}</ref> The [[CLAW hypothesis]],<ref name="CLAW87" /> initially suggested as a potential example of direct Gaian feedback, has subsequently been found to be less credible as understanding of [[cloud condensation nuclei]] has improved.<ref>{{Citation |last1= Quinn |first1=P.K. |last2= Bates |first2=T.S. |title =The case against climate regulation via oceanic phytoplankton sulphur emissions |journal =Nature |volume=480 |issue=7375 |pages =51–56 |date = 2011 |doi=10.1038/nature10580|bibcode = 2011Natur.480...51Q |pmid=22129724|s2cid=4417436 |url=https://zenodo.org/record/1233319 }}</ref> In 2009 the [[Medea hypothesis]] was proposed: that life has highly detrimental (biocidal) impacts on planetary conditions, in direct opposition to the Gaia hypothesis.<ref>{{cite book |first=Peter |last=Ward |year=2009 |title=The Medea Hypothesis: Is Life on Earth Ultimately Self-Destruction? |publisher=Princeton University Press |isbn=978-0-691-13075-0}}</ref>

In a [[On Gaia: A Critical Investigation of the Relationship between Life and Earth|2013 book-length evaluation of the Gaia hypothesis]] considering modern evidence from across the various relevant disciplines, Toby Tyrrell concluded that: "I believe Gaia is a dead end. Its study has, however, generated many new and thought provoking questions. While rejecting Gaia, we can at the same time appreciate Lovelock's originality and breadth of vision, and recognize that his audacious concept has helped to stimulate many new ideas about the Earth, and to champion a holistic approach to studying it".{{sfn|Tyrrell|2013|p=209}} Elsewhere he presents his conclusion "The Gaia hypothesis is not an accurate picture of how our world works".<ref>{{Citation |last= Tyrrell |first = Toby |title =Gaia: the verdict is… |journal = New Scientist |volume = 220 |issue = 2940 |pages = 30–31 |date= 26 October 2013 |doi=10.1016/s0262-4079(13)62532-4 |ref=none}}</ref> This statement needs to be understood as referring to the "strong" and "moderate" forms of Gaia—that the biota obeys a principle that works to make Earth optimal (strength 5) or favourable for life (strength 4) or that it works as a homeostatic mechanism (strength 3). The latter is the "weakest" form of Gaia that Lovelock has advocated. Tyrrell rejects it. However, he finds that the two weaker forms of Gaia—Coeveolutionary Gaia and Influential Gaia, which assert that there are close links between the evolution of life and the environment and that biology affects the physical and chemical environment—are both credible, but that it is not useful to use the term "Gaia" in this sense and that those two forms were already accepted and explained by the processes of natural selection and adaptation.{{sfn|Tyrrell|2013|p=208}}

==Anthropic principle==
As emphasized by multiple critics, no plausible mechanism exists that would drive the evolution of negative feedback loops leading to planetary self-regulation of the climate.<ref name="volk2002"/><ref name="beerling2007"/> Indeed, multiple incidents in Earth's history (see the [[Medea hypothesis]]) have shown that the Earth and the biosphere can enter self-destructive positive feedback loops that lead to mass extinction events.<ref>{{cite magazine |title=Gaia's evil twin: Is life its own worst enemy? |magazine=[[The New Scientist]] |volume=202 |issue=2713 |date=17 June 2009 |pages=28–31 |type=cover story |url=https://www.newscientist.com/article/mg20227131.400-gaias-evil-twin-is-life-its-own-worst-enemy.html}}</ref>

For example, the [[Snowball Earth]] glaciations appeared to result from the development of [[photosynthesis]] during a period when the [[Faint young Sun paradox|Sun was cooler]] than it is now. These mechanisms will have some effect, but any understanding of glacial-interglacial cycles requires study of the variations in the Earth’s orbit around the Sun, the tilt of its axis of rotation, and the ‘wobble’ in that rotational movement which causes the periodicity in Northern Hemisphere insolation, thereby setting the Earth’s thermal regime. Including studies from the fields of mathematics and Earth science, the fields of geology and geography provide insight into the causes of ice ages. 
Meanwhile, the removal of carbon dioxide from the atmosphere, along with the oxidation of [[atmospheric methane]] by the released oxygen, resulted in a dramatic diminishment of the [[greenhouse effect]].{{refn|group=note|An para-alternative hypothesis  is that the immediate trigger for Snowball Earth may have been a sequence of massive volcanic eruptions that occurred from 717 to 719 million years ago in what is currently the Canadian high arctic. These eruptions presumably lofted massive quantities of sulfur aerosols into the stratosphere, where they reflected incoming solar radiation and had a strong cooling effect.<ref name="Poppick_2019">{{cite web |last1=Poppick |first1=Laura |title=Snowball Earth: The times our planet was covered in ice |url=https://www.astronomy.com/science/snowball-earth-the-times-our-planet-was-covered-in-ice/ |website=Astronomy |date=5 April 2019 |publisher=Kalmbach Media |access-date=6 December 2023}}</ref> }} The resulting expansion of the polar ice sheets decreased the overall fraction of sunlight absorbed by the Earth, resulting in a runaway [[Ice–albedo feedback|ice–albedo positive feedback loop]] ultimately resulting in glaciation over nearly the entire surface of the Earth.<ref name="Harland_1964">{{Cite journal|last=Harland|first=W. B.|date=1964-05-01|title=Critical evidence for a great infra-Cambrian glaciation|journal=Geologische Rundschau|volume=54|issue=1|pages=45–61|doi=10.1007/BF01821169|issn=1432-1149|bibcode=1964GeoRu..54...45H|s2cid=128676272}}</ref> However, volcanic processes at this scale should be understood as relating to the pressure exerted on the Earth’s crust, and released during periods of ice sheet retreat. Breaking out of the Earth from the frozen condition appears to have directly been due to the release of carbon dioxide and methane by volcanos,<ref name=Crowley2001>{{cite journal| author = Crowley, T.J.|author2=Hyde, W.T. |author3=Peltier, W.R. |year = 2001 | title = CO 2 levels required for deglaciation of a 'near-snowball' Earth| journal = [[Geophysical Research Letters]]| volume = 28
| pages = 283–6| doi = 10.1029/2000GL011836| bibcode=2001GeoRL..28..283C| issue = 2| doi-access = }}</ref> although release of methane by microbes trapped underneath the ice could also have played a part.<ref name="Boyd_2010">{{cite journal |last1=Boyd |first1=E.S. |last2=Skidmore |first2=M. |last3=Mitchell |first3=A.C. |last4=Bakermans |first4=C. |last5=Peters |first5=J.W. |title=Methanogenesis in subglacial sediments |journal=Environmental Microbiology Reports |date=2010 |volume=2 |issue=5 |pages=685–692 |doi=10.1111/j.1758-2229.2010.00162.x|pmid=23766256 |bibcode=2010EnvMR...2..685B }}</ref> Lesser contributions to warming would come from the fact that coverage of the Earth by ice sheets largely inhibited photosynthesis and lessened the removal of carbon dioxide from the atmosphere by the weathering of siliceous rocks. However, in the absence of tectonic activity, the snowball condition could have persisted indefinitely.<ref name="Mann_2023">{{cite book |last1=Mann |first1=Michael |title=Our Fragile Moment |date=2023 |publisher=Hachette Book Group |location=New York |isbn=9781541702899}}</ref>{{rp|43–68}}

Geologic events with amplifying positive feedbacks (along with some possible biologic participation) led to the greatest mass extinction event on record, the [[Permian–Triassic extinction event]] about 250 million years ago. The precipitating event appears to have been volcanic eruptions in the [[Siberian Traps]], a hilly region of [[flood basalts]] in Siberia. These eruptions released high levels of [[carbon dioxide]] and [[sulfur dioxide]] which elevated world temperatures and acidified the oceans.<ref name="Hulse2022">{{cite journal |last1=Hulse |first1=D |last2=Lau |first2=K.V. |last3=Sebastiaan |first3=J.V. |last4=Arndt |first4=S |last5=Meyer |first5=K.M. |last6=Ridgwell |first6=A |title = End-Permian marine extinction due to temperature-driven nutrient recycling and euxinia |journal=Nat Geosci |date=28 Oct 2021 |volume=14 |issue=11 |pages=862–867 |doi=10.1038/s41561-021-00829-7 |bibcode=2021NatGe..14..862H |s2cid=240076553 |url=https://www.nature.com/articles/s41561-021-00829-7}}</ref> Estimates of the rise in carbon dioxide levels range widely, from as little as a two-fold increase, to as much as a twenty-fold increase.<ref name="Mann_2023"/>{{rp|69–91}} Amplifying feedbacks increased the warming to considerably greater than that to be expected merely from the greenhouse effect of carbon dioxide: these include the ice albedo feedback, the increased evaporation of water vapor (another greenhouse gas) into the atmosphere, the release of methane from the warming of [[methane clathrate|methane hydrate]] deposits buried under the permafrost and beneath continental shelf sediments, and increased wildfires.<ref name="Mann_2023"/>{{rp|69–91}} The rising carbon dioxide acidified the oceans, leading to widespread die-off of creatures with calcium carbonate shells, killing mollusks and crustaceans like crabs and lobsters and destroying coral reefs.<ref>{{cite journal|last=McKinney |first=M. L.|year=1987|title=Taxonomic selectivity and continuous variation in mass and background extinctions of marine taxa|journal=[[Nature (journal)|Nature]]|volume=325|issue=6100|pages=143–145|doi=10.1038/325143a0|bibcode = 1987Natur.325..143M |s2cid=13473769}}</ref> Their demise led to disruption of the entire oceanic food chain.<ref name="Twitchett">{{cite journal |title=Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis |vauthors=Twitchett RJ, Looy CV, Morante R, Visscher H, Wignall PB |journal=[[Geology (journal)|Geology]] |year=2001 |volume=29 |issue=4 |pages=351–354 |doi=10.1130/0091-7613(2001)029<0351:RASCOM>2.0.CO;2 |issn=0091-7613 |bibcode = 2001Geo....29..351T }}</ref> It has been argued that rising temperatures may have led to disruption of the [[chemocline]] separating sulfidic deep waters from oxygenated surface waters, which led to massive release of toxic [[hydrogen sulfide]] (produced by [[Anaerobic organism|anerobic]] bacteria) to the surface ocean and even into atmosphere, contributing to the (primarily methane-driven) collapse of the ozone layer,<ref name="Lamarque_2007">{{cite journal |last1=Lamarque |first1=J.-F. |last2=Kiehl |first2=J. T. |last3=Orlando |first3=J. J. |title=Role of hydrogen sulfide in a Permian-Triassic boundary ozone collapse |journal=Geophysical Research Letters |date=2007 |volume=34 |issue=2 |doi=10.1029/2006GL028384 |doi-access=free |bibcode=2007GeoRL..34.2801L }}</ref> and helping to explain the die-off of terrestrial animal and plant life.<ref name="Kump_2005">{{cite journal |last1=Kump |first1=Lee R. |last2=Pavlov |first2=Alexander |last3=Arthur |first3=Michael A. |title=Massive Release of Hydrogen Sulfide to the Surface Ocean and Atmosphere During Intervals of Oceanic Anoxia |journal=Geology |date=2005 |volume=33 |issue=5 |pages=397–400 |doi=10.1130/G21295.1|bibcode=2005Geo....33..397K }}</ref>

According to the weak [[anthropic principle]], our observation of such stabilizing feedback loops is an observer selection effect.<ref name="Nicholson_2018">{{cite journal |last1=Nicholson |first1=Arwen E. |last2=Wilkinson |first2=Davin M. |last3=Williams |first3=Hywel T. P. |last4=Lenton |first4=Timothy M. |title=Alternative mechanisms for Gaia |journal=Journal of Theoretical Biology  |date=2018 |volume=457 |pages=249–257 |doi=10.1016/j.jtbi.2018.08.032|pmid=30149011 |bibcode=2018JThBi.457..249N |hdl=10871/40424 |hdl-access=free }}</ref><ref name="Fellgett_1988">{{cite journal |last1=Fellgett |first1=P. B. |title=GAIA and the Anthropic Principle |journal=Quarterly Journal of the Royal Astronomical Society |date=1988 |volume=29 |page=85 |bibcode=1988QJRAS..29...85F |url=https://articles.adsabs.harvard.edu//full/1988QJRAS..29...85F/0000085.000.html |access-date=8 December 2023}}</ref><ref name="Doolittle_2020">{{cite web |last1=Doolittle |first1=W. Ford |title=Is the Earth an organism? |url=https://aeon.co/essays/the-gaia-hypothesis-reimagined-by-one-of-its-key-sceptics |website=Aeon |publisher=Aeon Media Group Ltd |access-date=8 December 2023}}</ref> In all the universe, it is only planets with Gaian properties that could have evolved intelligent, self-aware organisms capable of asking such questions.<ref name="Mann_2023"/>{{rp|50}} One can imagine innumerable worlds where life evolved with different biochemistries or where the worlds had different geophysical properties such that the worlds are presently dead due to runaway greenhouse effect, or else are in perpetual Snowball, or else due to one factor or another, life has been inhibited from evolving beyond the microbial level.{{refn|group=note|{{harvp|Dawkins|1982|p=236}}: "The Universe would have to be full of dead planets whose homeostatic regulation systems had failed, with, dotted around, a handful of successful, well-regulated planets, of which the Earth is one."}}

If no means exists for natural selection to operate at the biosphere level, then it would appear that the anthropic principle provides the only explanation for the survival of Earth's biosphere over geologic time. But in recent years, this strictly reductionistic view has been modified by recognition that natural selection can operate at multiple levels of the biological hierarchy — not just at the level of individual organisms.<ref name="Shavit">{{cite web |last1=Shavit |first1=Ayelet |title=Altruism and Group Selection |url=https://iep.utm.edu/altruism-and-group-selection/ |website=Internet Encyclopedia of Philosophy |publisher=University of Tennessee at Martin |access-date=9 December 2023}}</ref>  Traditional Darwinian natural selection requires reproducing entities that display inheritable properties or abilities that result in their having more offspring than their competitors. Successful biospheres clearly cannot reproduce to spawn copies of themselves, and so traditional Darwinian natural selection cannot operate. A mechanism for biosphere-level selection was proposed by Ford Doolittle: Although he had been a strong and early critic of the Gaia hypothesis,<ref name="Doolittle-1981"/> he had by 2015 started to think of ways whereby Gaia might be "Darwinised", seeking means whereby the planet could have evolved biosphere-level adaptations. Doolittle has suggested that ''differential persistence'' — mere survival — could be considered a legitimate mechanism for natural selection. As the Earth passes through various challenges, the phenomenon of differential persistence enables selected entities to achieve fixation by surviving the death of their competitors. Although Earth's biosphere is not competing against other biospheres on other planets, there are many competitors for survival on ''this'' planet. Collectively, Gaia constitutes the single [[clade]] of all living survivors descended from life’s [[last universal common ancestor]] (LUCA).<ref name="Doolittle_2020"/> Various other proposals for biosphere-level selection include sequential selection, entropic hierarchy,<ref name="Arthur_2022">{{cite journal |last1=Arthur |first1=Rudy |last2=Nicholson |first2=Arwen |title=Selection principles for Gaia |journal=Journal of Theoretical Biology |date=2022 |volume=533 |page=110940 |doi=10.1016/j.jtbi.2021.110940 |pmid=34710434 |url=https://www.sciencedirect.com/science/article/abs/pii/S0022519321003593 |access-date=11 December 2023|arxiv=1907.12654 |bibcode=2022JThBi.53310940A }}</ref> and considering Gaia as a [[holobiont]]-like system.<ref name=Wegner_2019>{{cite book |last1=zu Castell |first1=W. |last2=Lüttge |first2=U.   
|last3=Matyssek |first3=R.   |date=2019 |editor-last=Wegner |editor-first=L. |editor-last2=Lüttge |editor-first2=U. |title=Emergence and Modularity in Life Sciences |publisher=Springer, Cham |chapter=Gaia — A Holobiont-like System Emerging From Interaction |doi=10.1007/978-3-030-06128-9_12 |isbn=978-3-030-06128-9 |pages=255–279 | chapter-url=https://link.springer.com/chapter/10.1007/978-3-030-06128-9_12 }}</ref> Ultimately speaking, differential persistence and sequential selection are variants of the anthropic principle,<ref name="Arthur_2022"/> while entropic hierarchy and holobiont arguments may possibly allow understanding the emergence of Gaia without anthropic arguments.<ref name="Arthur_2022"/><ref name=Wegner_2019/>

==See also==
{{Portal|Environment|Earth sciences|Geography}}
{{div col}}
* {{annotated link|Anima mundi}}
* {{annotated link|Biocoenosis}}
* {{annotated link|Earth science}}
* {{annotated link|Environmentalism}}
* {{annotated link|Gaianism}}
* {{annotated link|Global brain}}
* {{annotated link|Holism}}
* {{annotated link|Hylozoism}}
* {{annotated link|Planetary boundaries}}
* ''[[SimEarth]]'' – 1990 video game
* {{annotated link|Spiritual ecology}}
* {{annotated link|Superorganism}}
{{div col end}}

== References ==
=== Notes ===
{{reflist|group=note}}

=== Citations ===
{{Reflist}}

=== Cited sources ===
{{refbegin|colwidth = 30em}}
*{{cite book |title=Scientists On Gaia |editor1-first=Stephen |editor1-last=Schneider |editor2-first=Penelope |editor2-last=Boston |place=Cambridge, Massachusetts |publisher=MIT Press |year=1991 |chapter=The Mechanical and the Organic: On the Impact of Metaphor in Science |first=David |last=Abram |chapter-url=http://www.wildethics.org/essays/the_mechanical_and_the_organic.html |archive-url=https://web.archive.org/web/20120223165936/http://www.wildethics.org/essays/the_mechanical_and_the_organic.html |archive-date=2012-02-23  |via=Wildethics.org}}
*{{Cite book|title=The Extended Phenotype: the Long Reach of the Gene|last=Dawkins|first=Richard|publisher=Oxford University Press|year=1982|isbn=978-0-19-286088-0}}
*{{cite book |last=Harding |first=Stephan |title=Animate Earth |publisher=Chelsea Green Publishing |date=2006 |isbn=978-1-933392-29-5 }}
*{{cite book |last=Lovelock |first=James |year=2001 |title=Homage to Gaia: The Life of an Independent Scientist |isbn=978-0-19-860429-7 |publisher=Oxford University Press |location=Oxford |url-access=registration |url=https://archive.org/details/homagetogaialife0000love}}
*{{cite book |last=Lovelock |first=James |title=The Vanishing Face of Gaia: A Final Warning |publisher=Basic Books |year=2009 | isbn=978-0-465-01549-8  |location=New York}}
*{{cite book |last=Margulis |first=Lynn | title=Symbiotic Planet: A New Look at Evolution | publisher=Weidenfeld & Nicolson |location=London | date=1998 | isbn=978-0-297-81740-6}}
*{{cite book |last=Turney |first=Jon |title=Lovelock and Gaia: Signs of Life |series=Revolutions in Science |publisher=Icon Books |location=UK |date=2003 |isbn=978-1-84046-458-0 |url-access=registration |url=https://archive.org/details/lovelockgaiasign0000turn }}
*{{cite book |last=Tyrrell |first=Toby |date= 2013|title= On Gaia: A Critical Investigation of the Relationship between Life and Earth |location=Princeton |publisher=Princeton University Press |isbn=978-0-691-12158-1 }}
{{Refend}}

==Further reading==
{{refbegin|colwidth = 30em}}
*{{cite book |title=Dharma Gaia: A Harvest of Essays in Buddhism and Ecology |editor-first=A. H. |editor-last=Badiner |publisher=Parallax Press |year=1990 |chapter=The Perceptual Implications of Gaia |first=David |last=Abram |chapter-url=http://www.wildethics.org/essays/the_perceptual_implications_of_gaia.html |archive-url=https://web.archive.org/web/20131031191848/http://www.wildethics.org/essays/the_perceptual_implications_of_gaia.html |archive-date=2013-10-31 |via=Wildethics.org |ref=none}}
*{{cite book |last=Bondì |first=Roberto |title=Blu come un'arancia. Gaia tra mito e scienza |publisher=Prefazione di Enrico Bellone  |location=Torino, Utet |date=2006 |isbn=978-88-02-07259-3 |ref=none}}
*{{cite book |last=Bondì |first=Roberto |title=Solo l'atomo ci può salvare. L'ambientalismo nuclearista di James Lovelock |publisher=Prefazione di Enrico Bellone |location=Torino, Utet |date=2007 |isbn=978-88-02-07704-8 |ref=none}}
*{{cite book |last=Jaworski |first=Helan |title=Le Géon ou la Terre vivante |publisher=Librairie Gallimard  |location=Paris |date=1928 |ref=none}}
*{{cite book |last1=Joseph |first1=Lawrence E. |title=Gaia: The Growth of an Idea |date=1990 |location=New York |publisher=St. Martin's Press |isbn=978-0-31-204318-6 |url=https://archive.org/details/gaia00lawr|url-access=registration |ref=none}}
*{{cite journal |last=Kleidon |first=Axel |title=Beyond Gaia: Thermodynamics of Life and Earth system functioning |journal=Climatic Change |volume=66 |issue=3 |pages=271–319 |date=2004 |doi=10.1023/B:CLIM.0000044616.34867.ec |s2cid=55295082 |ref=none}}
*{{cite book |chapter-url=http://www.jameslovelock.org/gaia-as-seen-through-the-atmosphere/ |chapter=Gaia as seen through the atmosphere |first=James |last=Lovelock |editor1-first=P. |editor1-last=Westbroek |editor2-first=E. W. |editor2-last=deJong |title=Biomineralization and Biological Metal Accumulation |year=1983 |publisher=D. Reidel Publishing Company |pages=15–25 |via=jameslovelock.org |ref=none}}
*{{cite book |last=Lovelock |first=James |date=2000 |title=Gaia: A New Look at Life on Earth |isbn=978-0-19-286218-1 |publisher=Oxford University Press |location=Oxford |ref=none}}
*{{cite news |last=Lovelock |first=James |newspaper=The Independent |url=http://comment.independent.co.uk/commentators/article338830.ece |archive-url=https://web.archive.org/web/20060408121826/http://comment.independent.co.uk/commentators/article338830.ece |archive-date=2006-04-08 |title=The Earth is about to catch a morbid fever |date=16 January 2006 |ref=none}}
*{{cite book |last=Lovelock |first=James | title=The Revenge of Gaia: Why the Earth Is Fighting Back: and How We Can Still Save Humanity | publisher=Allen Lane | date=2007 | isbn=978-0-7139-9914-3 |location=Santa Barbara CA| title-link=The Revenge of Gaia |ref=none}}
*{{cite book |last=Marshall |first=Alan |title=The Unity of Nature: Wholeness and Disintegration in Ecology and Science |publisher=Imperial College Press |location=River Edge, NJ |date=2002 |isbn=978-1-86094-330-0 |ref=none}}
*{{cite book |last=Schneider |first=Stephen Henry |title=Scientists debate Gaia: the next century |publisher=MIT Press |location=Cambridge, Massachusetts |date=2004 |isbn=978-0-262-19498-3 |ref=none}}
*{{cite journal |last=Staley |first=M. |title=Darwinian selection leads to Gaia |journal=J. Theor. Biol. |volume=218 |issue=1 |pages=35–46 |date=September 2002 |pmid=12297068 |doi=10.1006/jtbi.2002.3059 |bibcode=2002JThBi.218...35S |ref=none}}
*{{cite book |last=Thomas |first=Lewis G. |title=The Lives of a Cell; Notes of a Biology Watcher |url=https://archive.org/details/livesofcellnotes00thomrich |url-access=registration |publisher=[[Viking Press]] |location=New York |date=1974 |isbn=978-0-670-43442-8 |ref=none}}
{{refend}}

==External links==
{{Library resources box}}
*Lovelock, James (2006), interviewed in ''How to think about science'', CBC Ideas (radio program), broadcast January 3, 2008.  [http://www.cbc.ca/ideas/episodes/2009/01/02/how-to-think-about-science-part-1---24-listen/ Link]
*[http://news.bbc.co.uk/today/hi/today/newsid_8594000/8594561.stm "Lovelock: 'We can't save the planet{{'"}}] BBC Sci Tech News
*[https://web.archive.org/web/20080517035452/http://www.timesonline.co.uk/tol/news/article726744.ece Interview: Jasper Gerard meets James Lovelock]
* {{webarchive |url=https://web.archive.org/web/20160303221831/http://gaiatheory.co.uk/ |date=3 March 2016 |title=Clips of interview with James Lovelock from 2010}}

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