|Style over substance|
The Gaia hypothesis is the idea that the Earth's biosphere acts to maintain a homeostasis of the planet's physical conditions (for example, atmospheric composition and temperature). Because of the teleological nature of the theory and the tendency of some of its proponents to dress their beliefs up in scientific jargon, it is typically classified as a failed hypothesis at best, pseudoscience at worst.
It is known to
hippies its adherents as the "Gaia Theory" or even "Gaian Science". They assert that the physical components and systems of the Earth itself are linked together in a system that allows the Earth to maintain a "preferred" homeostasis. Originally proposed by Dr. James Lovelock, it was given the name of Gaia after the Greek word for "Earth." Believers in this hypothesis are, therefore, called "Gaianists."
Lovelock asserted that the biosphere and the physical components of the Earth (atmosphere, cryosphere, hydrosphere and lithosphere) are closely integrated to form a complex interacting system that maintains the climatic and biogeochemical conditions on Earth in a preferred homeostasis. He claims that, because of this complex system, the Earth reacts to change in a manner similar to a living organism.
The "Gaia hypothesis" also shows up in a lot of anti-environmentalist writings which basically conflate all environmentalists with Gaianists and portray it as a secular religion, despite how fringe the hypothesis is today. In fact when pundits/politicians even bring it up today it's much more likely because a wingnut is ranting against it then because someone is actually adhering to the hypothesis.
 Relationship to historical climate change
The most-used example of the hypothesis is as an explanation of the historical climate shifts of the Earth. It proposes that the biological makeup of Earth's population has changed relative to the brightness and heat of the Sun. The younger, cooler Earth had a heavily carbon-based atmosphere, which produced a warming greenhouse effect, but as the Sun grew brighter, the excess carbon dioxide was converted to massive amounts of vegetation across the globe (what is known as the Carboniferous epoch). This plant matter then was buried through geological processes, creating the quantities of oil and coal found underground. The sequestration of this carbon cooled the Earth.
In the modern era, these stockpiles of carbon have been released by human actions, reigniting the greenhouse effect, warming the Earth again, and disrupting the "preferred equilibrium". The Gaiasphere's reaction, according to one
stupid movie hypothesis, will be the stoppage of the Gulf Stream, causing considerable cooling of the North Atlantic and mainland Europe. The resultant ice age should increase Earth's albedo (brightness), thereby reflecting more solar energy and causing worldwide cooling: and thus the balance would be restored.
The main problem with the above scenario is the lack of any known (or even proposed) mechanism by which biological change could be directed in order to achieve a particular result. Granted, Earth is an incredibly complex mess of interconnected systems, but the equilibrium they achieve is in no way deliberate. When something disrupts the balance (and, really, it's always in slight flux), things change, and a new balance is achieved. The system only looks tuned for life because we're a product of said system.
Complex systems routinely involve negative feedback loops that maintain equilibrium in the system, resembling homeostasis, without any directing force being necessary. To this extent, the Gaia hypothesis does not offer anything new; the idea that the biosphere is composed of highly interconnected, interdependent, and non-isolable systems, operating in dynamic equilibrium, is non-controversial in the hard science (as opposed to the woo) of ecology.
To go some way towards countering criticism of his first book's lack of a plausible mechanism, Lovelock's proposed the model of Daisyworld, a simplified ecosystem made up of only two elements, black daisies which a