Editing Gaia hypothesis (section)

===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>