The global temperature on Earth is regulated by the global carbon cycle. The main negative feedback is provided by the weathering processes mediated by the biosphere. In the past the Earth system was characterized by lowering temperatures caused by continental growth and declining outgassing at mid-ocean ridges. In the future, however, temperatures will rise due to the increase in solar luminosity.
The Cambrian explosion can be explained by extrinsic environmental causes, that is, a gradual cooling of the Earth. The Cambrian explosion was so rapid because of a positive feedback between the spread of biosphere, increased silicate weathering, and a consequent cooling of the climate. The environment itself has been actively changed by the biosphere maintaining the temperature conditions for its existence. Therefore, this explanation of the Cambrian explosion is in line with the Gaia theory of the Earth as a self-regulating system.
Prokaryotes, eukaryotes, and complex multicellular life forms will become extinct in reverse sequence of their appearance. Nonlinear interactions in the biosphere-geosphere system caused bistability during the Neo- and Mesoproterozoic era. There is no bistability in the future solutions for complex life. Therefore, complex organisms will not become extinct by an implosion (in comparison to the Cambrian explosion). Eukaryotes and complex life become extinct because of too high surface temperatures in the future. The time of extinction is mainly determined by the upper temperature tolerance limit of these life forms. The ultimate life span of the biosphere is defined by the extinction of prokaryotes in about 1.6 Gyr because of CO2 starvation. Only in a small fraction (1.3-1.7 Gyr) of its habitability time (6.2 Gyr) can our home planet harbor advanced life forms.
See also: Weathering.
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