Life in the Earth System: BIOSPHERE I F 1.1 211 a level to which the inhabitants of Lhasa (Tibet) are accustomed, but which would cause a North American difficulties. Tiredness and lethargy were the consequence. The question now was, what happened to all the oxygen? (Even in BIOSPHERE I the oxygen cycle is not fully understood; Kelly, 1994). Overall, BIOSPHERE II seemed to be collapsing. Since the insects as pollen carriers were not surviving, important crop plants were dying. 19 ofthe 25 species of vertebrates became extinct. The clear winners were the cockroaches and busily multiplying ants. The desert became steppe, the ocean dead sea andthe eight human inhabitants lost a great deal of weight. The conclusion of this experiment is that currently, no one knows how to construct stable, life-sustaining systems that are characteristic ofthe natural terrestrial ecosystem. Salt water aquarists are familiar with the problem. Even in that mini biosphere it can take months to form stable microbial food chains and harmonize the communities. For a long time there is imbalance, fish or plants die, putrid gases form. The system meanders until apparently all of a sudden and without cause it ‘locks in’ and is stable. There is no recipe so far of how to achieve this more quickly or possibly in a more controlled manner. Just like the aquariums there is only one thing to do: do nothing and wait. Possibly, this is a trivial insight, but one which may be transferable. In BIOSPHERE II they did exactly the opposite. When things became critical, the attempt was made to control events, for example, by lowering the temperature (in order to store carbon dioxide in sea water) or changing the pH values. All possible environmental levels were tinkered with to stabilize the system and possibly it was precisely those actions that prevented stabilization. Whatever the case, in the wake of that experiment, the ability of life in BIOSPHERE I to control its environmental conditions for its own well-being can be seen in a completely new light. The key to understanding ‘pre-stabilized harmony’ in the Earth’s system andthe failure of BIOSPHERE II may well lie in more recent research on the Gaia hypothesis. F 1.3 A closer look at BIOSPHERE I The essential difference between BIOSPHERES I and II is certainly not the composition ofthe two systems. The structure ofthe regulatory systems seems much more significant. In the case of BIOSPHERE II the adjustments ofthe environmental levels took place via simple regulatory systems. The principle of variance compensation of individual variables as a rule only takes into consideration to a small degree linkages between the correcting variable. In light ofthe large number of regulatory levels andthe associated degrees of tolerance, however, such a system is extremely difficult to stabilize. Particularly, if no detailed climate or more general environmental model is available that would allow for a short-term prognosis, intervention on the principle of trial and error can have disruption rather than balance as the outcome and lead to fatal consequences. F 1.3.1 Homeostasis as a fundamental regulatory principle In our natural environment, however, homeostatic principles appear to dominate. Homeostasis means in this instance that a system controls itself to achieve balance and remains within a limited stable area (Rampino, 1993). Minimal disruptions do not change the character ofthe system because they can be neutralized. This observation demonstrates parallels to physiological systems: temperature regulation in the human organism as a physiological system demonstrates the phenomenon of homeostasis. There is no objective target temperature, rather the participating processes are linked to one another via negative feedbacks with the result that they more or less automatically establish a stable balance. A simple model by Watson and Lovelock (1983) aims to illustrate this effect. Picture a planet that is covered exclusively in black and white daisies. Their growth rate reduces as temperatures rise. At a given level of solar radiation the black daisies effect a higher temperature than the white ones as a result oftheir colour. If the rays lessen, however, the probability ofthe black daisies reproducing falls as the temperature rises. The more reflective white daisies are therefore more competitive and increase. The overall temperature ofthe planet remains more or less constant. The relationship between black and white daisies may be said to adapt in a self-organized manner to the increased radiation levels and regulates the disruption into non-existence. But if the planet were just an uninhabited lump of soil, it would warm up as the sun warms up. The competition between black and white daisies in the form of a Darwinian selection process therefore results for the individual in the possibility of controlling its environment together with other individuals. From this fundamental idea of ‘Daisy World’ far more complex and realistic models may be formed which show the homeostatic capacities and improve them (von Bloh et al, 1997). Self-regulation is therefore an extremely robust principle.
212 F The biosphere in the Earth System F 1.3.2 The metabolism ofthe Earth System Seen in that light, the particularities ofthe Earth’s atmosphere mentioned at the beginning may now be explained. Except for the inert gases, all other components are regulated by a complicated interaction of biological and geological processes. Photosynthesis plays a very important role in that context. The ‘waste product’ of this process, oxygen, is the second most important element in the atmosphere by about 20 per cent.This proportion is maintained at astoundingly constant levels. At an oxygen concentration below 15 per cent even dry wood will not ignite. But at levels over 25 per cent a rainforest will catch fire (Nisbet, 1994). Interestingly enough, nitrogen is involved here, too. It is fire retardant and protects plants. At the same time its 78 per cent share in the atmosphere also appears astonishingly stable: nitrogen fixing and nitrogen releasing processes hold one another in balance. Above all bacteria, but also processes such as lightning, fix nitrogen and other bacteria then release it. But the release is very important: all food chains on landand in the water are dependant on the availability of nitrate. Without this Gaia metabolism the nitrogen concentration in the atmosphere would reduce distinctly over the course of several million years. This is, however, just one small slice ofthe physiological-metabolic interplay that goes on in the Earth System.The concentration of oxygen is closely linked to that of carbon dioxide and phosphorus. More carbon dioxide would increase the oxygen levels via photosynthesis and thus increase the possibility of forest fires. This would result in a negative feedback that would stimulate algae growth in the seas through the release of phosphorus from the land’s biomass and at the same time defunct biomass would be deposited more quickly on the sea floor. From this withdrawal of phosphorus andthe lowering of carbohydrate content in atmosphere the original ratios would be re-established. Phosphorus itself is a limiting factor for the biosphere, but one which when sufficient carbon dioxide is available can possibly be ‘harvested’. An increase in the amount of carbon dioxide in the air leads via the greenhouse effect to increased precipitation that can above all speed up the erosion of minerals both chemically and biologically (via the increased activity of soil organisms and plants). The erosive process then in turn releases nutrients (eg phosphorus, sulphur) that further increase the activity ofthe biosphere until the atmospheric carbon dioxide levels are re-established. From what we know today, this closed-loop cycle appears primarily to have an effect over long time scales in the form of a biologically moderated silicon-carbon cycle (Berner et al, 1983), and not just to determine the ‘life expectancy’ of Gaia, but also appears to have controlled its early development (Franck et al, 1998). It is known that the sun in its early phase radiated about 30 per cent less energy than it currently does. Given the same atmospheric conditions, the Earth would have been an ice planet then that would never have thawed because ofthe high reflectivity of ice. At low solar radiation levels the silicon-carbon cycle however stabilizes a higher CO 2 concentration in the atmosphere thus ensuring through the greenhouse effect that the oceans flow andthe hydrological cycle continues (Walker et al, 1981). And above all, the latter is extremely important for the metabolism of Gaia: almost one quarter ofthe energy absorbed by the Earth System is used to drive the water cycle which – note the clear analogy to the living organism – produces nutrients for the biosphere, distributes them globally and makes them able to be absorbed, and regulates the energy regime by distributing thermal energy globally and directing energy to certain regions while keeping others cool with the help of clouds (Volk, 1998). The analogy to the living organism is self-evident.A host of organisms participate in each part of this terrestrial air-conditioning system: the sun may be the driving force, but it is controlled and structured by biological control mechanisms. It appears that evolution has not just kitted out individual species with harmonious, self-stabilizing physiological regulatory processes, but the entire biosphere also.This should not in any way be taken as an all clear for the greenhouse experiment ‘staged’ by humankind. Minimal fluctuations and disruptions can be regulated very easily it seems by the Earth System. But simple geo-physiological models show that there is a limit to the disruption beyond which homeostasis is overwhelmed and apparently collapses without prior warning. We do not know how far we are still away from that point and only know a handful ofthe less important regulatory elements. The International Geosphere Biosphere Programme (IGBP) has for this reason made analysis ofthese aspects a central research focus. The biological aspects ofthe (global) hydrological cycle or the analysis of biogeochemical metabolic and energy cycles are in activities within this international joint programme that, in the light ofthe Gaia theory, should provide additional key information towards understanding the Earth System.