Geophysiology: the science of Gaia

Geophysiology is still a young science. Our contribution to the NWO Program on Perturbation of Earth Systems may be regarded as an attempt to contribute to the theoretical foundations of geophysiology. Although the idea of Gaia as a benign, organism-like entity (or deity), seems hopelessly romantic, geophysiology is in itself a legitimate field of study, concerning the interactions between biota, the oceans, and the atmosphere.

The two basic tenets of geophysiology

Geophysiology, as envisaged by James Lovelock, is based on the following two tenets:

Biogeochemical impact
The physico-chemical make-up of a life-bearing planet such as ours is profoundly affected by the presence and activities of the planet's biosphere.

Geophysiological homeostasis
Life not only causes certain global physico-chemical variables to adopt a set of values that constitute the hallmark of a life-bearing planet. The presence of life also ensures that these variables return to those values after a global perturbation.

These two tenets are often summed up by saying that the Earth, its biosphere included, behaves like a super-organism, `Gaia'. The first tenet is rather obviously true, at least in a qualitative sense. The second tenet can probably not be maintained in this strong form, for reasons explained below.

How to quantify biogeochemical impact?

Consider such variables as the chemical composition of the atmosphere, the oceans, the Earth's crust, the average surface temperature of the Earth, and so on. On the face of it, it seems likely that all such variables would have been dramatically different if life had not evolved on our planet. In particular, the joint presence of water, carbon-dioxide, and ozone in a planet's spectrum has been proposed as a universal indication of life.

Nonetheless, we have no way of knowing what the Earth would have looked like in terms of those physico-chemical variables if life had not evolved. Without an objective method to calculate the `lifeless equilibrium state' from scratch, the idea of biogeochemical impact must remain qualitative at best. Life should not be defined in terms of a planet's deviation from the `lifeless equilibrium state', as long as such a state cannot be defined without reference to the absence of life, on pain of begging the question.

How strong is geophysiological homeostasis?

Lovelock is quick to point out that the presence of feedback mechanisms does not at all require a `conscious' effort on the part of the organisms. For anyone in need of convincing on that score, the celebrated Daisyworld-model certainly does the job.

However, to quantify the strength of biogeochemical and/or biogeophysical feedback links is no small matter.

A famous example of a putative feedback cycle is the scheme proposed by Shaw, and later by Charlson, Lovelock, Andreae and Warren. According to this model, global temperature homeostasis comes about through a negative feedback cycle of phytoplankton, which produces dimythylsulfide, which gives rise to cloud condensation nuclei, which aid cloud formation and thus diminish incoming solar irradiance and global heating, with an adverse effect on phytoplankton activity. Thus the chain of influences comes full circle, with a net negative feedback effect.

Two points can be raised with respect to this model. Both are more generally applicable, and therefore of some interest.

Strength of the feedback cycle
Mark Lawrence has estimated that the strength of this feedback cycle is between 2 and 5 times too weak to counteract perturbations.

Sign of the feedback cycle
As already noted by Charlson et al., the cycle should have a net negative sign. If phytoplankton responds to increased irradiance with the release of less dimethylsulfide, the negative feedback turns into a positive feedback effect.

The notion of geophysiological homeostasis requires that the strongest feedback cycles have a net negative sign, thus overriding positive feedback cycles. But there is no a priori reason why this should be so.
If you have any strong feelings about Gaia, you might want to tell me, Hugo van den Berg, all about it.

Modelling the biotic elements

The uncertain link in the climate feedback cycle sketched above is the biotic element: the world's pool of phytoplankton. To address uncertainties of this kind, a theory is called for which interrelates the chemical exchange fluxes between biota and their ambient. The concepts of multiple nutrient limitation, redox substrate/light limitation, adaptation, and metabolic versatility play a central role in such a theory.

My aim is to provide an in-depth analysis of these concepts, clarified by means of mathematical modelling. The project `Sulphur and carbon fluxes in microbial mats' deals with microbial mats as a model system for a biotic element in a geochemical context.