Geophysiology

Geophysiology, the scientific field founded by James Lovelock, is the study of the combined system formed by Life on Earth and its environment: the atmosphere, hydrosphere, and crust. Closely related to Biogeochemistry, founded by Vladimir Vernadsky (one of Lovelock's role models), Geophysiology may differ slightly in that it goes beyond the basic insight that life affects its environment (the weakest form of the Gaia Hypothesis) and insists on treating the combination of life and environment as a unified entity that regulates its climate, acidity, and nutrient cycles the same way an individual organism does.

Gaia is energetically open, taking in massive amounts of solar energy and radiating waste heat to space. But it is almost completely materially closed, taking in only tiny amounts of mass from meteorites and losing even tinier amounts as hydrogen escapes from the top of the atmosphere. Most of the interesting aspects of Geophysiology revolve around the management of nutrient cycles using negative feedback to maintain homeostasis in various properties of the environment, although Gaian positive feedback spirals have also precipitated major changes in Earth systems, such as the oxygen crisis.

History of the hypothesis
James Lovelock began developing the Gaia Hypothesis when he was working at the NASA Jet Propulsion Laboratory in 1961, providing input into the design of experiments to detect life on Mars to be included on the Viking Mars landers. Lovelock and a visiting philosopher, Dian Hitchcock, published two papers asserting that life on any planet must alter the composition of the atmosphere and put it in a state of chemical disequilibrium. This hypothesis, if it had been wholeheartedly accepted, would have rendered the Viking experiments unnecessary, given that Mars's atmosphere is very close to chemical equilibrium, in marked contrast with that of Earth. Hence, Lovelock is convinced that there is no life on Mars.

Early critics of the Gaia Hypothesis claimed that it was teleological, requiring life on Earth to have an overriding purpose that manifested itself in deliberately planned changes to the environment. To answer this objection, Lovelock invented a simple model called Daisyworld that showed how flowers blindly adapting to and unintentionally modifying their environment could set up a Gaian homeostasis in a planet's climate.

Global temperature and the carbon cycle
In absence of life, Earth's atmosphere would be much thinner but would contain massively more carbon dioxide, a greenhouse gas that would keep the planet much hotter than it currently is. Very early in Earth's history, during the Archaean Eon, cyanobacteria gained the ability to draw down so much carbon dioxide that they may have risked allowing the planet to freeze, since the Sun was about 25% cooler at the time. Lovelock hypothesizes that methanogens may have saved Earth from this fate and established one of the earliest Gaian equilibria by emitting large amounts of methane into the atmosphere. This extra methane would have been removed from the air at the beginning of the Proterozoic Eon, as part of the shift to an oxygen-dominated atmosphere.

Heat input from the Sun has increased steadily over Earth's lifespan, requiring a concomitant decrease in atmospheric concentrations of carbon-based greenhouse gases to maintain homeostasis within a range hospitable for life. It's generally agreed that life as we know it will be unable to survive this slow warming trend beyond about a billion years in the future. Lovelock asserts that the oscillation between Ice Ages and interglacial periods over the last few million years are early signs that the Gaian homeostasis is beginning to break down.

The oxygen cycle
Throughout the Archaean, despite the presence of photosythesizers, oxygen remained a trace gas in the atmosphere. But once it became a major atmospheric component, life needed to regulate it to prevent the oxygen concentration from becoming toxic or, later, becoming so high that it would lead to the destruction of the forests by fire.

Ocean salinity and the calcium cycle
One of Lovelock's boldest claims is that life engineered the system of coastal lagoons that removes some of the excess salt from the oceans through evaporation. In absence of life, it's possible that the oceans would have grown more saline continuously over the Earth's lifespan as rivers moved salts from the land to the oceans. Such a buildup of salt would have made the oceans uninhabitable.