In 1979, British atmospheric chemist James E. Lovelock and his collaborator, University of Massachusetts, Amherst, microbiologist Lynn Margulis, postulated that the chemical composition of the Earth's atmosphere and oceans was actively molded and maintained by living organisms on the Earth. Up to this point, scientists assumed that life adapted to the changing terrestrial environment as needed but did not mold the Earth into a lifesupporting planet. Lovelock's hypothesis turned this assumption around and asserted that biological organisms not only had actively transformed the Earth into a life-friendly place but also continued to make the Earth a life-friendly planet. Novelist William Golding suggested that Lovelock call his hypothesis the "Gaia hypothesis" in deference to the Greek goddess who gave birth to the gods.

Lovelock went further, describing the Earth and its associated biosphere as one large, living, selfregulating organism. While many scientists ridiculed this characterization as a kind of neopagan, New Age religion, some environmental activists found in it a powerful metaphor. Speaking of the Earth as alive and even sentient provided a strong emotional appeal for environmental causes. Describing environmental degradation as "wounding" the Earth conveyed a strong moral impetus for ecological concerns.

The Gaia hypothesis proposes that biologically mediated, negative feedback mechanisms contribute to environmental homeostasis and make the environment suitable for life. Perhaps most controversially, the Gaia hypothesis further argues that these negative feedback mechanisms arise by means of neo-Darwinian selection.

The surface temperature of the Earth provides an example of one such mechanism. The global surface temperature has remained relatively constant, even though the energy output of the Sun has increased by 25 to 30 percent since life began on the planet. Temperature regulation results from a decrease in the atmospheric concentration of the greenhouse gas (GHG) carbon dioxide (CO2). CO2 reduction is due to bacteria and plants, which metabolize CO2 into biomass. Furthermore, increased atmospheric CO2 levels increase plant carbon sequestration and removal of CO2 from the atmosphere. Increased carbon utilization also increases the number of marine, carbon-utilizing algae called coccolithophores. When coccolithophores die, they release dimethyl sulfide, which nucleates cloud formation. Increased cloud cover, which was initially induced by increased atmospheric CO2, further cools the Earth. Other lifedependent homeostatic processes are also thought to regulate the composition of Earth's atmosphere and the salinity of the oceans.

Some climate scientists favor the Gaia hypothesis, while others are highly critical of it. Virtually all Earth scientists recognize that living organisms have significant effects on the physical and chemical aspects of the environment. However, biologically mediated feedbacks are not intrinsically homeostatic, since some of them can destabilize the environment and are deleterious to life. For example, increased warming increases respiration rates of soil-based organisms, which increases CO2 release from soils. Increased soil carbon release increases the greenhouse effect and global warming. This positive feedback system destabilizes the terrestrial environment. Furthermore, genomic studies of plants and animals have failed to reveal Gaian global feedback mechanisms. Likewise, natural selection affects individual organisms and is not a global process. Similarly, natural selection does not act with foresight.

With respect to global climate change, the Earth, according to the Gaia hypothesis, should use a series of feedback mechanisms to regulate global temperatures. Thus, the freezing of the poles covers them with ice, which reflects the rays of the Sun and cools the planet. Likewise, increases in atmospheric CO2 cause increased marine algal growth, which removes CO2 and decreases greenhouse warming.

However, human intervention has short-circuited these negative feedback mechanisms. Industrial pollution of the oceans has introduced increased amounts of mercury, which prevents algal growth and destroys the primary means of regulating atmospheric CO2 levels. Increased CO2 levels cause increase greenhouse-based heating of the planet, which in turn melts the polar ice caps and decreases the reflectivity of the Earth's surface, further heating the Earth.

In one sense, the Gaia hypothesis is strongly optimistic, since it strongly asserts that the Earth and at least some of its resident life will adapt and survive. However, this optimism does not include humanity, since the environment of the Earth after such dramatic climate change will probably be highly inhospitable to human life.

Bibliography:

1) Lovelock, James. The Ages of Gaia: A Biography of Our Living Earth. New York: W. W. Norton, 1995.

2) Lovelock, James. Gaia: A New Look at Life on Earth. New ed. New York: Oxford University Press, 2000.

3) Lovelock, James. The Revenge of Gaia: Earth's Climate Crisis and the Fate of Humanity. New York: Basic Books, 2007.

4) Margulis, Lynn. Symbiotic Planet: A New Look at Evolution. New York: Basic Books, 1999.

5) Schneider, Stephen H., et al., eds. Scientists Debate Gaia: The Next Century. Cambridge, Mass.: MIT Press, 2008.

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