15 Coupled Evolution of Earth’s Atmosphere and Biosphere

Abstract

Earth’s climate has remained conducive to life for more than 3.5 billion years, probably because of higher concentrations of CO2 and CH4 greenhouse gases in the past that compensated for a much lower solar flux. Both CO2 and CH4 are involved in biogeochemical negative feedback loops that help to stabilize climate. The most significant event in Earth’s atmospheric history was an increase in atmospheric O2. The energetic advantage of aerobic metabolism facilitated larger levels of anabolism (the conversion of simple molecules to complex molecules), which gave rise to multicellular life, anatomical differentiation, and diverse biological complexity. The need for abundant O2 to facilitate complex life on any planet is probably universal given the thermodynamic uniqueness of oxygen in the periodic table. Earth’s atmosphere has evolved to a state that is chemically and physically distinctive compared to the atmospheres on the other planets. Chemically, Earth’s atmosphere is continuously maintained at a low entropy state by the biosphere. Physically, Earth is the sunniest planet in terms of energy flux on its surface, while the atmosphere has the most unpredictable weather systems and, paradoxically, the slowest atmospheric jets of any planet with an atmosphere.