Overall Habitability: Connections with Geological and Astronomical Events and Processes

This book investigates the billion-year takeover of planet Earth by its organisms and ecosystems. This chapter considers the Earth’s overall habitability, that is, the proportion of the surface of the planet that is habitable. It examines variations in the extent of the Earth’s habitable environments over a wide range of periods: diurnal, annual, within centuries, and from tens of thousands to hundreds of millions of years. These variations include the recurrence of large and very large volcanic eruptions, the Ice Age of the Quaternary period (last 2.6 million years), the three snowball Earth episodes, long periods of high and low temperature during the last half billion years, and the alternation of greenhouse and icehouse Earth since 4.5 billion years. The chapter also examines the five mass extinctions documented by the fossil record of the last 540 million years, which are witnesses of major crashes in habitability. Biological innovations have also caused major changes in overall habitability. These innovations include oxygen-producing photosynthesis, calcification, silicification, methanogenesis, oxygen respiration, and multicellularity. The overall habitability of Earth is affected by astronomical and environmental factors that include the Earth’s rotation and orbit around the Sun, the Milankovich orbital cycles, changes in the positions of continents, the impacts of large meteoroids, large volcanic eruptions, and climate changes. In addition, it is presently affected by anthropogenic changes of atmospheric gases. The chapter ends with a summary of key points concerning the interactions between the Solar System, Earth, its overall habitability, and organisms.