Daisyworld

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Progress of a Daisyworld-Simulation with many breeds and a medium scale iteration level

Daisyworld, a computer simulation, is a hypothetical world orbiting a sun whose radiant energy is slowly increasing. It is meant to mimic important elements of the Earth-Sun system, and was introduced by James Lovelock and Andrew Watson in a paper published in 1983 [1] to illustrate the plausibility of the Gaia hypothesis, which later became Gaia Theory. In the original 1983 version, Daisyworld is seeded with two varieties of daisy as its only life forms: black daisies and white daisies. White petaled daisies reflect light, while black petaled daisies absorb light. The simulation tracks the two daisy populations and the surface temperature of Daisyworld as the sun's rays grow more powerful. The surface temperature of daisyworld remains almost constant over a broad range of solar output.

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[edit] Original 1983 Simulation Synopsis

At the beginning of the simulation, the sun's rays are weak and Daisyworld is too cold to support any life. Its surface is barren, and gray. As the luminosity of the sun's rays increases, germination of black daisies becomes possible. Because black daisies absorb more of the sun's radiant energy, they are able to increase their individual temperatures to healthy levels on the still cool surface of Daisyworld. As a result they thrive and the population soon grows large enough to increase the average surface temperature of Daisyworld.

As the surface heats up, it becomes more habitable for white daisies, whose competing population grows to rival the black daisy population. As the two populations reach equilibrium, so too does the surface temperature of Daisyworld, which settles on a value most comfortable for both populations.

In this first phase of the simulation we see that black daisies have warmed Daisyworld so that it is habitable over a wider range of solar luminosity than would have been possible on a barren, gray planet. This allowed growth of the white daisy population, and the two populations of daisies are now working together to regulate the surface temperature.

The second phase of the simulation documents what happens as the sun's luminosity continues to increase, heating the surface of Daisyworld beyond a comfortable range for the daisies. This temperature increase causes white daisies, who are better able to stay cool because of their high albedo or ability to reflect sunlight, to gain a selective advantage over the black daisies. White daisies begin replacing black daisies, which has a cooling effect on Daisyworld. The result is that Daisyworld's surface temperature remains habitable - in fact almost constant - even as the luminosity of the sun continues to increase.

In the third phase of the simulation, the sun's rays have grown so powerful that soon even the white daisies can no longer survive. At a certain luminosity their population crashes, and the barren, gray surface of Daisyworld, no longer able to reflect the sun's rays, rapidly heats up.

At this point in the simulation solar luminosity is programmed to decline, retracing its original path to its initial value. Even as it declines to levels that previously supported vast populations of daisies in the third phase, no daisies are able to grow because the surface of barren, gray Daisyworld is still far too hot. Eventually, the sun's rays decrease in power to a more comfortable level which allows white daisies to grow, who begin cooling the planet.

[edit] Relevance to Earth

Because Daisyworld is so simplistic, having for example, no atmosphere, no animals, only one species of plant life, no consideration of evolution and only the most basic population growth and death models, it should not be directly compared to Earth. This was stated very clearly by the original authors. Even so, it is believed[who?] to provide a number of useful predictions of how Earth's biosphere may respond to, for example, human interference. Later adaptations of Daisyworld (discussed below), which added many layers of complexity, still showed the same basic trends of the original model.

One prediction of the simulation is that the biosphere works to regulate the climate, making it habitable over a wide range of solar luminosity. Many examples of these regulatory systems have been found on Earth[citation needed], and of those found, most act to cool the global temperature by sequestering carbon[citation needed]. This suggests that Earth is in the third phase of Daisyworld (see Original 1983 Simulation Synopsis), in which white daisies are struggling to keep the surface cool.

Towards the end of the third phase the simulation shows that the regulatory ability of Daisyworld is ultimately finite. As the system begins to fail the temperature of the planet slowly increases and, soon after, this triggers a mass extinction of the daisy population and a rapid spike in surface temperature. On Earth, an anthropogenic-induced mass extinction of plants and animals is believed to be underway [2], which is expected to reduce the planet's ability to sequester carbon and cool the climate. At the same time, significant amounts of carbon dioxide are being released into the atmosphere. Earth's regulatory system is now beginning to show signs of these stresses[citation needed]. Within the past century the average surface temperature has been gradually increasing, and in more recent decades, this increase has begun to accelerate [3]. These symptoms are similar to those that precede the collapse of life on Daisyworld.

[edit] Modifications to the Original Simulation

Later extensions of the Daisyworld simulation included rabbits, foxes and other species. One of the more surprising findings of these simulations is that the larger the number of species, the greater the improving effects on the entire planet (i.e., the temperature regulation was improved). These findings lent support to the idea that biodiversity is valuable.

Daisyworld has also attracted a substantial amount of criticism[who?] since 1983. Much of the criticism centers around the fact that although it is often used as an analogy for Earth, the original simulations leaves out many important details of the true Earth system. For example, the system requires an ad-hoc death rate (γ) to sustain homeostasis, and it does not take into account the difference between species-level phenomena and individual level phenomena. Detractors[who?] of the simulation believed inclusion of these details would cause it to become unstable, and therefore, false. Many of these issues are addressed in a more recent paper by Timothy Lenton and James Lovelock in 2001 [4]. In this paper it is shown that inclusion of these factors actually improves Daisyworld's ability to regulate its climate.

[edit] References and implementations

A version of the Daisyworld simulation, with several shades of gray daisies, was included in the Maxis video game SimEarth. Orson Scott Card's novel Xenocide also makes several references to Daisyworld.

[edit] See also

[edit] External references and links

  1. ^ Watson, A.J.; J.E. Lovelock (1983). "Biological homeostasis of the global environment: the parable of Daisyworld". Tellus B (International Meteorological Institute) 35 (4): 286–9. 
  2. ^ Thomas, J.A.; M.G. Telfer, D.B. Roy, C.D. Preston, J.J.D. Greenwood, J. Asher, R. Fox, R.T. Clarke, J.H. Lawton (2004). "Comparative Losses of British Butterflies, Birds and Plants and the Global Extinction Crisis". Science 303 (5665): 1879-1881. 
  3. ^ "IPCC Fourth Assessment Report, Chapter 3". 2007-02-05. 237. http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter3.pdf. Retrieved on 2009-03-14. 
  4. ^ Lenton, T.M.; J.E. Lovelock (2001). "Daisyworld revisited: quantifying biological effects on planetary self-regulation". Tellus Series B - Chemical and Physical Meterology 53 (3): 288–305. doi:10.1034/j.1600-0889.2001.01191.x. 
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