Earth's carbon cycle natural thermostat

19:42, Oct 11 2012

The Gaia Hypothesis was proposed by James Lovelock in 1969 and states that all living matter on Earth (called the biosphere) functions as a single organism which Lovelock called Gaia after the Greek goddess of the Earth.

It is controversial because the hypothesis goes on to state that this organism, the biosphere of Earth, actively self-regulates the temperature and chemistry in the atmosphere to produce the conditions suited to supporting life. An example is the weathering cycle which operates as follows.

When volcanoes erupt, carbon dioxide is blasted into the atmosphere. This causes the average global temperature to rise because carbon dioxide is a greenhouse gas. Some of this carbon dioxide reacts with water vapour present in the atmosphere to form carbonic acid which rains on to the silicate rock of the Earth.

Silicate rock reacts with carbonic acid to produce calcium ions and carbonates which are then transported to the oceans by rivers. Marine animals use the calcium ions and carbonates to make shells and claws. When these animals die the shells and claws descend to the ocean bed where they are gradually buried and compressed via the action of ocean currents and tides. Calcium ions and carbonates compacted in this way, form calcium carbonate (limestone).

Over millions of years the limestone is transported deep below the surface by tectonic plate action where under enormous pressures and temperatures the carbonate is transformed back into carbon dioxide. The magma energised by this highly volatile carbon dioxide punches a path towards the surface, eventually blasting the carbon dioxide back into the atmosphere via a volcano.

This cycle acts as a natural thermostat because the chemical reaction that draws carbon dioxide out of the atmosphere by reacting it with water vapour is highly temperature dependent. If the global temperature is cold then the reaction is slow so the removal of carbon dioxide from the atmosphere is slow.


But the volcanoes continue to blast carbon dioxide into the atmosphere, so the carbon dioxide concentration in the atmosphere increases. This causes the average global temperature to rise. Higher temperatures mean the chemical reactions responsible for withdrawing carbon dioxide, speed up and so more carbon dioxide is withdrawn from the atmosphere; that causes the average global temperature to cool.

This addition and withdrawal of carbon dioxide from the atmosphere acts as a natural thermostat and serves to keep the average global temperature in the range tolerable to complex life.

Lovelock argues that processes such as this prove his Gaia Hypothesis. In 1983 he and a colleague developed a computer simulation which they called Daisyworld.

Daisyworld is a hypothetical planet that orbits its sun which is gradually getting hotter as the simulation proceeds.

Daisyworld is inhabited by two species: black daisies which absorb light well and white daisies that reflect light well. Black daisies do better in cool conditions because they can make better use of what solar radiation there is, whereas when things get hot, white daisies thrive because they can reflect heat and keep cool.

If black daisies dominate then the planet darkens and so absorbs more solar radiation, hence becomes hotter. Hot conditions causes the dark daisy population to decline (they overheat), but such conditions favour the white daisies, which are good reflectors of heat, so their numbers increase.

When white daisies begin to dominate, more of the solar radiation is reflected and the planet cools. Lovelock showed that even though the sun was gradually warming, the planet's temperature was controlled by the competition between the two daisy species. The equilibrium temperature range turned out to be the optimal temperature for daisy growth.

An interpretation of the Gaia Hypothesis is that it is self-interest that drives the compromise between dark/white populations and not some ulterior group selection.

A scientific explanation of this phenomenon is that life co-evolves with its environment, meaning that living organisms influence their inanimate environment but also that the inanimate environment influences the organisms. The environment is not static but changes through processes such as continental drift which influence the global climate and produce new habitats.

In response, life changes through Darwinian processes which in turn can change the environment. For example, oxygen was first introduced into the Earth's atmosphere 3.5 billion years ago by an organisms called cyanobacteria.

The veracity of the Gaia Hypothesis is still questioned but the idea of the Earth as an organism is an intriguing one.

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