Gaia theory, proposed by James Lovelock, broadly proposes that life on the planet could be considered to operate as one organism (named Gaia). Like other organisms, Gaia regulates conditions to keep them favourable to her own existence. Atmospheric gas concentrations, temperature and ocean salinity are all tightly regulated and kept within the bounds that support the living beings of Gaia.

This is more remarkable than it may seem at first glance. If we take atmospheric gases as an example, the concentration of oxygen in the air is kept at approximately 21%. So what you may say. But oxygen is a highly reactive gas which rapidly combines with other elements, making it utterly remarkable for the level to remain constant at 21% - which it has for around 300 million years!

This provides evidence of a complex regulatory system maintaining what in the human organism is called homeostasis. The mechanisms by which this kind of regulation occurs are not fully understood as there are so many different interactions and relationships on the planet that combine to bring about such stability. However, some of the interactions have been identified, and looking at one such group of interactions may help build an appreciation of the kind of dynamics that maintain life as we know it.

The relationships governing the regulation of the carbon cycle seem a good example to choose, as Gaia's balance of carbon is becoming dangerously disturbed and carbon dioxide levels are affecting global temperature. There is more than one web of relationships that regulates carbon dioxide levels in the atmosphere, the one represented in the diagram below is known as the long term carbon cycle, as it takes a long time for carbon to travel through the processes involved.

Taken from Stephan Harding's book Animate Earth p115

Arrows with solid lines represent direct couplings. If we take the arrow from temperature to plant growth as an example, this means that if the temperature rises, so does the amount of plant growth. Conversely, if the temperature decreases, so does the amount of plant growth. Arrows with dashed lines indicate an inverse coupling.

This time we'll use the example of the arrow from silicate rock weathering to carbon dioxide in the atmosphere.

Silicate rock weathering is a process in which granite and basalt rocks react chemically with carbon dioxide and water. One of the outcomes of this reaction is the removal of carbon dioxide from the air. Plants enhance this process greatly.

Plants break up rocks with their roots, increasing the surface area available to react with the water and carbon dioxide. They also release carbon dioxide from their roots, adding to the ingredients necessary for rock weathering (Other living beings also assist silicate rock weathering. Worms, lice, nematodes, millipedes etc. all break up soil and allow water to penetrate. Microbes secrete substances that break up rocks and also produce carbon dioxide).

The inverse coupling in the diagram below indicates that if the amount of rock weathering increases then the amount of carbon dioxide will decrease. Conversely, if the amount of rock weathering decreases, the amount of carbon dioxide will increase. There are actually many processes represented on this one diagram, taking just one feedback loop as an example we can look at:

If you imagine carbon dioxide being released into the atmosphere and run this feedback circuit in your head, you'll see that it triggers processes that bring carbon dioxide levels back down again. This is known as a negative feedback loop (negative to change) and is one of the ways the gases in the atmosphere are kept at constant levels.

However, taking this further, if too much carbon dioxide enters the atmosphere and the temperature rises too much, it will not provide an environment favourable to plant growth and plants will start to die, changing the temperature/plant link to an inverse coupling as follows:

If you follow this circuit in your mind you will see that it is a vicious circle and will lead to temperature spiralling out of control. This is known as a positive feedback loop.

Luckily such feedback loops as we just explored do not exist in isolation, but the more positive loops that manifest within a system, the more pressure is put on the negative ones. There may come a time where the negative loops left in the system cannot resist changes on the Gaian level anymore. Once this happens, large scale change might occur quite quickly, with a similar quality to the phase transition or sudden change in order described for the ant colonies above.

As human activities are currently emitting excessive levels of carbon dioxide into the atmosphere, it is fast becoming a very real possibility that the system will not be able to regulate carbon dioxide well enough anymore to maintain liveable global temperatures.

Within the time humans have been on the planet, Gaia has been in a relatively stable state (appearing homeostatic). However, history reveals that, ultimately, Gaia is more homeorhetic than homeostatic. Homeorhesis is a term coined by C H Waddington, and relates to transitions from one stable state to another. In Waddington's definition a human too is homeorhetic, especially as the person is growing. This term relates to being able to make changes to an organism (like the growing form changing) whilst keeping other things the same (like the concentrations of chemicals in the bloodstream). Waddington describes homeorhesis as preserving a flow.

Actually, Gaia has oscillated between hot and cold climatic states throughout her existence, Lovelock even speculates that glacial, cold states seem the more stable and healthy state of the two, with warmer states (interglacials) being shorter lived and possibly comparable to running a fever in humans. It is in an interglacial period that we find ourselves now.

There have also been numerous previous mass extinctions on earth, changing the players in the living biosphere profoundly.

One of the most startling and sobering of these was what Lynn Margulis and Dorian Sagan have dubbed the oxygen holocaust and describe as a worldwide pollution crisis that occurred about 2000 million years ago. Before this time there was very little oxygen in the atmosphere, until the microbes alive at this time discovered a new way to get the hydrogen that they needed out of water by using a new form of photosynthesis. The oxygen left over from this process was released into the atmosphere.

Oxygen is toxic to many microbes and the very microbes releasing the oxygen were threatened severely by it as it started to build up (Sound familiar? Carbon dioxide threatens the very beings responsible for its emission now).

However, although this has been described as a mass holocaust, there was a remarkable creative evolution amongst the microbes that survived. They learned to utilise the toxin threatening them as a resource (inventing respiration). This created such rich new opportunities that life on earth eventually moved into a new phase of abundance.

Gaia seems perfectly capable of changing the goalposts and establishing a new world order when pushed.

Stephan Harding, Animate Earth, Green Books, 2006 James Lovelock,The Ages of Gaia, Oxford University Press, 1988

James Lovelock, Gaia, Medicine for an Ailing Planet, Gaia Books, 2005

James Lovelock, The Revenge of Gaia, Penguin Books, 2006

Lynn Margulis and Dorian Sagan, Microcosmos, Touchstone, 1986

Lynn Margulis and Michael F. Dolan, Early Life: Evolution on the Precambrian Earth, Jones and Bartlett, 2002

Fritjof Capra, The Web of Life, Flamingo, 1997

C H Waddington, Tools for Thought, Granada Publishing, 1977

Sophia's closing comment does perhaps leave some room for hope! - Joan