DEFINITION OF GAIA HYPOTHESIS

For billions of years the Earth has maintained its temperature and

atmospheric composition in a narrow range that has permitted life

to exist on the surface. Many scientists have suggested that this

remarkable trait of the planet is a result of life adapting to conditions

that happen to exist and evolve on the planet. An alternative

idea has emerged that the planet behaves as some kind of self-regulating

organism that invokes a series of positive and negative

feedback mechanisms to maintain conditions within the narrow

window in which life can exist. In this scenario, organisms and

their environment evolve together as a single coupled system, regulating

the atmospheric chemistry and composition to the need of

the system. Dr. James Lovelock, an atmospheric chemist at Green

College in Oxford, U.K., pioneered this second idea, known as the

Gaia hypothesis. However, the idea of a living planet dates back at

least to Sir Isaac Newton.

How does the Gaia hypothesis work? The atmosphere is

chemically unstable, yet it has maintained conditions conducive to

life for billions of years even despite a 30 percent increase in solar

luminosity since the Early Precambrian. The basic tenet of the

hypothesis is that organisms, particularly microorganisms, are

able to regulate the atmospheric chemistry and hence temperature

to keep conditions suitable for their development. Although

this tenet has been widely criticized, some of the regulating mechanisms

have been found to exist, lending credence to the possibility

that Gaia may work. Biogeochemical cycles of nutrients

including iodine and sulfur have been identified, with increases in

the nutrient supply from land to ocean leading to increased biological

production and increased emissions to the atmosphere.

Increased production decreases the flux of nutrients from the

oceans to the land, in turn decreasing the nutrient supply, biological

production, and emissions to the atmosphere.

As climate warms, rainfall increases, and the weathering of

calcium-silicate rocks increases. The free calcium ions released

during weathering combine with atmospheric carbon dioxide to

produce carbonate sediments, effectively removing the greenhouse

gas carbon dioxide from the atmosphere. This reduces global

temperatures in another self-regulating process. An additional

feedback mechanism was discovered between ocean algae and

climate. Ocean algae produce dimethyl sulfide gas, which oxidizes

in the atmosphere to produce nuclei for cloud condensation. The

more dimethyl sulfide that algae produce, the more clouds form,

lowering temperatures and lowering algal production of dimethyl

sulfide in a self-regulating process.

That the Earth and its organisms have maintained conditions

conducive for life for 4 billion years is clear. However, at times the

Earth has experienced global icehouse and global hothouse conditions,

where the conditions extend beyond the normal range. Lovelock

relates these brief intervals of Earth history to fevers in an

organism, and he notes that the planet has always recovered. Life

has evolved dramatically on Earth in the past 4 billion years, but this

is compatible with Gaia. Living organisms can both evolve with and

adapt to their environment, responding to changing climates by

regulating or buffering changes to keep conditions within limits that

are tolerable to life on the planet as a whole. However, there are

certainly limits, and the planet has never experienced organisms

such as humans that continually emit huge quantities of harmful

industrial gases into the atmosphere. It is possible that the planet,

or Gaia, will respond by making conditions on Earth uninhabitable

for humans, saving the other species on the planet. As time goes

on, in about a billion years the Sun will expand and eventually burn

all the water and atmosphere off the planet, making it virtually uninhabitable.

By then humans may have solved the problem of where

to move to and developed the means to move global populations to

a new planet.

this will lead to a further increase in temperature. Many computer-

based climate models are attempting to predict how

much global temperatures will rise as a consequence of our

anthropogenic influences, and what effects this temperature

rise will have on melting of the ice sheets (which could be

catastrophic), sea-level rise (perhaps tens of meters or more),

and runaway greenhouse temperature rise (which is possible).

Climate changes are difficult to measure, partly because

the instrumental and observational records go back only a

couple of hundred years in Europe. From these records, global

temperatures have risen by about one degree since 1890, most

notably in 1890–1940, and again since 1970. This variation,

however, is small compared with some of the other variations

induced by natural causes, and some scientists argue that it is

difficult to separate anthropogenic effects from the background

natural variations. Rainfall patterns have also changed

in the past 50 years, with declining rainfall totals over low latitudes

in the Northern Hemisphere, especially in the Sahel,

which has experienced major droughts and famine. However,

high-latitude precipitation has increased in the same time period.

These patterns all relate to a general warming and shifting

of the global climate zones to the north.

See also DESERT; GLACIER; GREENHOUSE EFFECT; ICE AGE;

MILANKOVITCH CYCLE; SEA-LEVEL RISE.

clinometer An instrument used in surveying for measuring

angles of elevation, slope, or incline, also known as an inclinometer.

Clinometers are measuring devices used to measure

the angle of a line of sight above or below the horizontal. The

height of an object can be determined both by using the clinometer

and by measuring the distance to the object. By calculating

a vertical angle and a distance, users can perform

simple trigonometry to figure out the height of an object.

Clinometers are fairly simple to use. A clear line of sight

is needed to measure the angle of an object, and it is common

practice to make more than one reading and take the average

for a more precise result.

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