The best critical analysis of Gaian ideas was done by Jim Kirchner, at a Chapman Conference (American Geophysical Union) in San Diego in 1988 that was devoted to the scientific consideration of Gaia. He separated the jumble of ideas into four clear levels in order of increasing strength of claims from weak to strong (Table 2). These ranged from (1) 'co-evolutionary Gaia'
Table 2 The many types of Gaia according to Kirchnera
Hypothesis type Properties Likely consensus
Influential Biology exerts significant influence over some aspects of the Testable and supported by evidence planetary system
Co-evolutionary Darwinian process in which biota affects nonliving systems, Testable and under active debate in turn they affect biota
Homeostatic System is stabilized by negative feedback loops involving Testable and under active debate biota and physical/chemical systems
Teleological Conditions maintained by the biosphere for its own benefit Testable, refuted by the Daisyworld demonstration
Optimizing The biosphere directly manipulates its environment to provide Skeptically received, possibly not optimum conditions for itself testable, not self-consistent aStrength of statement in order from highest (influential) to lowest (optimizing).
that merely claimed life and Earth had evolved together over time affecting each other; (2) 'homeostatic Gaia' involving self-regulation around set points; (3) 'geophysical Gaia', which overlapped significantly with the physical Earth sciences; and (4) the most extreme claim of 'optimizing Gaia' that life was molding the planet's behavior into a state most favorable toward all life. The latter notion came in for the most criticism as being scientifically untestable and the most radical Gaian idea.
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