Gaia Theory
Terra as a self-regulating living entity
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."
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?
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%)
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
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.
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
Daisyworld
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
The Daisyworld
System
Global
Temperature
O
S/O
S/O
S
White Daisies
O
Black Daisies
Global Albedo
S
S
O
O
Available Area
Daisyworld Results
From The Ages of Gaia, J. Lovelock
Growth of daisies
Daisyworld
Daisyworld
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
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
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