Formulated in the 1970s by British atmospheric
chemist James Lovelock; proposes that Earth’s atmosphere,
hydrosphere, geosphere, and biosphere interact as a
self-regulating system that maintains conditions necessary for
life to survive. In this view the Earth acts as if it is a giant
self-regulating organism in which life creates changes in one
system to accommodate changes in another, in order to keep
conditions on Earth within the narrow limits that allow life
to continue.
The average temperature on the Earth has been maintained
between 50°F and 86°F (10°C and 30°C) for the past
3.5 billion years, despite the fact that the solar energy received
by the Earth has increased by 40–330 percent since the
Hadean. The temperature balance has been regulated by
changes in the abundance of atmospheric greenhouse gases,
controlled largely by volcanic degassing and the reduction of
CO2 by photosynthetic life. A slight increase or decrease in
CO2 and other greenhouse gases could cause runaway greenhouse
or icehouse global climates, yet life has been able to
maintain the exact balance necessary to guarantee its survival.
The presence of certain gases such as ammonia at critical
levels in the atmosphere for maintaining soil pH near 8,
the optimal level for sustaining life, is critical for maintaining
atmospheric oxygen levels. This critical balance is unusual,
as methane is essentially absent from the atmospheres of
Venus and Mars where life does not exist. The salinity of the
oceans has been maintained at around 3.4 percent, in the
narrow range required for marine life, reflecting a critical
balance between terrestrial weathering, evaporation, and
precipitation.
The exact mechanisms that cause the Earth to maintain
these critical balances necessary for life are not well known.
However, as solar luminosity increases, the additional energy
received by the Earth is balanced by the amount of energy
radiated back to space. This can be accomplished by changes
in the surface reflectance (albedo) through changes in the
amount of ice cover, types of plants, and cloud cover. It is
increasingly recognized that changes in one Earth system produce
corresponding changes in other systems in self-regulation
processes known as homeostasis. Critical for Gaia are
the links between organisms and the physical environment,
such that many proponents of the theory regard the planet as
one giant superorganism.
See also ATMOSPHERE; CLIMATE CHANGE; GREENHOUSE
EFFECT; SUPERCONTINENT CYCLE; and feature essay
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