What mechanisms have produced a self-regulating Earth system?
Date: 13 January 2020
University of Exeter
PhD in Geography
The Gaia Hypothesis postulates that life and the oceans, crust, and atmosphere of the Earth form a self-regulating planetary-scale system with stabilising properties. Gaia helps to explain the long history of uninterrupted habitability on Earth. Previous Gaian models have uncovered mechanisms for self-regulation in life-environment ...
The Gaia Hypothesis postulates that life and the oceans, crust, and atmosphere of the Earth form a self-regulating planetary-scale system with stabilising properties. Gaia helps to explain the long history of uninterrupted habitability on Earth. Previous Gaian models have uncovered mechanisms for self-regulation in life-environment coupled systems, such as the Earth, and the work in this thesis adds to our understanding on how and when self-regulation can emerge on a planet hosting life, and what conditions help maintain such regulation once established. To place the models presented in this thesis into their proper context this thesis contains background on Earth's history, the history of the Gaia hypothesis and some key Gaian models and known Gaian regulation mechanisms, and a discussion on habitability of exoplanets and our search for alien life. In this thesis I explore a new variant of a pre-existing Gaian model (the flask model) which demonstrates a new regulation mechanism which I call 'single-rein control'. I then adapt this model to explore the hypothesis of 'selection by survival'. This hypothesis suggests that the longer a life harbouring planet survives, the longer it has to acquire further persistence mechanisms. Therefore over time the only planets hosting life still existing will be those that have acquired several self-regulation mechanisms like those present on Earth. The results of this model demonstrate that selection by survival can promote long term persistence of biospheres compared to a null model. In the second part of this thesis I consider how the Gaia hypothesis can inform our search for inhabited exoplanets and I introduce the ExoGaia model, a new model of atmospheric regulation where microbes must 'catch' a window of habitability on their host planet, and quickly form self-regulating feedback loops to prevent the planetary temperature from rising to inhospitable levels. The ExoGaia model demonstrates global regulation and the underlying geochemistry on the planet turns out to be key in determining how robust this regulation is. ExoGaia also demonstrates 'Gaian bottlenecks' where for the same planet life either quickly establishes self-regulating feedback loops and enjoys long term habitability, or fails and becomes extinct, with the host plane quickly reverting to an inhospitable state. This model agrees with the hypothesis that inhabitance and habitability are two sides to the same coin -- that a planet is highly unlike to be in a habitable state, without being inhabited. This thesis argues a case for 'Probable Homeostatic Gaia' -- that not only is the Earth-system homeostatic but that homeostatic regulation is an expected result of a life-environment coupled system. If true, this would would increase our chances of finding other Gaian worlds.
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