1. Terra as a self-regulating living entity
Gaia Theory
Terra as a self-regulating living entity

2. Gaia Hypothesis (Theory)
First proposed by Sir James Lovelock in 1975
"a complex entity involving the Earth's biosphere, atmosphere, oceans, and soil; the totality constituting a feedback or cybernetic system which seeks an optimal physical and chemical environment for life on this planet."
Take note of the idea of cybernetics or the ability of a system to regulate itself with an algorithm.
Lovelock trained in medicine before he started work on his theory.
Lovelock’s important paper on the theory can be read at

3. Key points
feedback or cybernetic system which seeks an optimal physical and chemical environment for life on this planet
The Earth has a self regulating system that ensures homeostasis
Is this a teleology?
Teleology- a philosophical position that every phenomenon has a purpose or design. In science we do not adopt an assumption of teleology because of the falsifiability of scientific conclusions

4. Lovelock’s arguments
The global surface temperature of the Earth has remained constant, despite an increase in the energy provided by the Sun
Atmospheric composition remains constant, even though it should be unstable
Ocean salinity is constant (3.4%)
The sun is a middle-aged star. It is estimated to be 4.57 billion years old. Stars usually have a life span of 10 billion years at most. The Sun is 25-30% more hot at present compared when it was young.
The presence of oxygen makes the atmosphere potentially unstable. However this gas is continuously replenished by photosynthesis of plants.
The origin of the oceans is important in the history of the planet. The saltiness of the sea (salinity) is a result of the weathering of minerals. Will the oceans continue to become saltier? Not likely. In fact the sea has had about the same salt content for many hundred of millions if not billions of years. The salt content has reached a steady state. Dissolved salts are being removed from seawater to form new minerals at the bottom of the ocean as fast as rivers and hydrothermal processes are providing new salts.
We can summarize this discussion. Wherever water comes into contact with the rocks of Earth’s crust, either on land or in the ocean or within the oceanic crust, some of the minerals in the rock dissolve and are carried by the water to the ocean. The salt content of seawater does not change because new minerals are forming on the sea floor at the same rate as salt is added. Thus, the salt content of the sea is at steady state.

5. Empirical evidence
The Sun has become 25% hotter since life began on earth. Earth’s temperature has remained constant
Earth’s atmospheric constituents are chemically unstable due to the presence of a large fraction of oxygen: Earth’s atmosphere make-up is constant
The sun is a middle-aged star. It is estimated to be 4.57 billion years old. Stars usually have a life span of 10 billion years at most. The Sun is 25-30% more hot at present compared when it was young.
Photosynthesis ensures oxygen is replenished

6. Empirical evidence
The salinity and composition of the Earth’s oceans have remained constant in 4 billion years of evolution
Given the physical and chemical conditions of Earth, we should have a runaway greenhouse effect. We haven’t.
Why does the Earth have no greenhouse effect?
An increase in temperature from greenhouse gases leading to increased water vapour which is a greenhouse gas causing further warming is a positive feedback. This cannot be a runaway effect or the runaway effect would have occurred long ago. Positive feedback effects are common and can always exist while runaway effects are much rarer and cannot be operating at all times.
If the effects from the second iteration of the loop of effects is larger than the effects of the first iteration of the loop this will lead to a self perpetuating effect. If this occurs and the feedback only ends after producing a major temperature increase, it is called a runaway greenhouse effect.

7. Daisyworld Model Tests the predictions of Gaia theory
A planet orbits the Sun and there are two populations of daisy, a black and white one.
The planet has no oceans, has sufficient water and a constant atmosphere
See this link for more information

8. Daisyworld
Consider a hypothetical earth-like planet, but without any ocean and water isn’t a limiting factor. There is no polar ice. There are two populations of daisies, a white and black one. The albedo of Daisyworld is the same as of Earth.
The planet is not evenly heated. The equatorial regions get most of the heat. Supposing you play God and in this virtual planet you reduce the incoming solar radiation?
According to the model, the black daisies will be naturally selected. Their populations will increase while the white ones decline. Since black absorbs heat, the planet is warmed.
What would happen if you as God, increases the Sun’s radiation output?

9. Daisyworld
Planet of gray colored surface with two types of plants – black daisy and white daisy.
Barren surface and surface covered with black or white daisies have different albedos.
By competition, the percentage of area covered with black or white daisies varies.
This varies total albedo, thus affecting global temperature.
The color of daisies indicates the amount of surface reflection which, in turn, affects the Earth’s temperature.
When temperature is too cold, daisy seeds cannot germinate.
When temperature is too hot, all daisies die.
The growth rates depend only on the local temperature.
Local temperature in part affected by local albedo

10. The Daisyworld System

11. Daisyworld Results
From The Ages of Gaia, J. Lovelock

12. Growth of daisies
Under the model temperature stabilizes at 25 C, which is the most comfy temp for humans and other organisms!

13. Daisyworld
This summarizes our original ideas. The important result of the computer model is that with the presence of life, temperature is maintained at a constant level that makes life possible.

14. Daisyworld
As in any computer model, the system can be made more complex.

15. Model results If the planet cools, more black daisies grow.
If the planet heats, more white daisies grows.
Planet ends up having a constant environment
Addition of more species results in a more stable and constant planetary environment
If the model is made more complex, the system becomes more constant in its predictions.

16. Criticisms of Gaia theory
Teleological: How can living components of the biosphere decide to regulate the planet’s environment?
Non-repeatability
The hypothesis is untestable
Under our current philosophical idea of “control” only a being with a brain or a will can control something. How can the Earth “control” or regulate its environment?
Unless we discover another Earth or an Earthlike planet, the theory is non-repeatable and non-testable except in a computer model.

17. Gaia theory today
With more information, we know that Earth has rather peculiar planetary conditions
Gaia theory is central to the new science of Earth Systems Science or Geophysiology
To learn more about Geophysiology go to