The Gaia Hypothesis

In 1965, J.E. Lovelock published the first scientific paper suggesting the Gaia hypothesis. The Gaia hypothesis states that the temperature and composition of the Earth's surface are actively controlled by life on the planet. It suggests that if changes in the gas composition, temperature or oxidation state of the Earth are caused by extraterrestial, biological, geological, or other disturbances, life responds to these changes by modifying the abiotic environment through growth and metabolism. In simplier terms, biological responses tend to regulate the state of the Earth's environment in their favor.

The evidence for Gaia is as follows:

If not continually replaced by biotic activities gases like methane and hydrogen would become non-existant in the atmosphere in a few decades.
Carbon dioxide (C02) in the Earth's atmosphere is far less abundant than chemistry alone would allow. If life was deleted carbon dioxide would become 30 times more abundant. Large quantities of carbon dioxide are currently locked up by living organisms.
The sun's energy output has increased by 30 % in the past 3.5 billion years. Yet, historical climate data indicates that the temperature of the Earth has only fluctuated by about 5° Celsius from the current average global temperature of 15° Celsius. Computer climate models suggest that a 30 % reduction in solar radiation would create a global average temperature of between -10 and -52° Celsius all things being equal. These results indicate that levels of atmospheric carbon dioxide must have been much higher in the past when the sun was less powerful. Extra atmospheric carbon dioxide would have created a greater greenhouse effect and warmer temperatures. These results also indicate that some mechanism must have removed carbon dioxide from the atmosphere as the sun's output of radiation increased over the Earth's geologic history. This mechanism is the conversion of atmospheric carbon dioxide into fossilized organic matter (natural gas, oil, coal, limestone, and peat). In other words, Gaia!

This theory is important to Physical Geography and other Earth Sciences for the following reasons:

The Gaia theory suggests that the abiotic and biotic environment is made up of many complex interrelationships;
Many of these complex interrelationships are quite delicate and may be altered by human activity to a breaking point; and
The theory suggests that humans must learn to respect Gaia by reducing their intentional modification of the Earth's abiotic and biotic components.