The stability of the biosphere: The impossibility of computing the odds

The fundamental question about whether or not the stability of the biosphere is at risk was deferred. This is a very deep question indeed.

First, a quick review of the case for believing there may be a real threat to survival. Most people who have never thought deeply about the matter tend to assume that life is a passive "free rider" on the earth. In other words, most people suppose (or were taught) that life exists on earth simply because earth happened to offer a suitable environment for life to evolve. They imagine that earth was much like it is now (except for more volcanic activity) before life came along, and that if life were to be snuffed out by some cosmic accident-- say a massive solar flare - the animals and plants would disappear but the inanimate rivers, lakes, oceans, and oxygen-nitrogen atmosphere would remain much as they are today.

The above quasi-biblical vision is not in accord with the scientific evidence. It is true that life probably originated on earth (though some scientists speculate that the basic chemical components of all living systems may actually have originated in a cold interstellar cloud - Hoyle and Wickramasinghe 1978). Life certainly evolved on earth. The earliest living organisms appear to have been capable of metabolizing organic compounds (such as sugars) by fermentation, to yield energy and waste products such as alcohol's. The organic (but non-living) "food" for these simple organisms was created by still unknown processes in a reducing environment. The composition of the atmosphere of the early earth cannot be reconstructed with great accuracy, but it undoubtedly contained ammonia, hydrogen sulphide, and carbon dioxide, plus water vapour. There was certainly no free oxygen. It is less certain, but possible, that no free nitrogen was present. Life would have disappeared as soon as the supply of "food" was exhausted, if it had not been for the evolutionary "invention" of photosynthesis.⁷

The first photosynthetic organisms converted carbon dioxide and water vapour into sugars, thus replenishing the food supply. But they also generated free oxygen as a waste product. For a billion years or so, the free oxygen produced by photosynthesis was immediately combined with soluble ferrous iron ions dissolved in the oceans, yielding insoluble ferric iron. Similarly hydrogen sulphide and soluble sulphites were oxidized to insoluble sulphates. These were deposited on the ocean floors. Thanks to tectonic activity, some of them eventually rose above sealevel and became land. (Virtually all commercial iron ores and gypsum now being mined by humans are of biological origin.) When the dissolved oxygen acceptors were used up, oxygen began to build up in the atmosphere. As a metabolic waste product, oxygen was toxic to the anaerobic organisms that produced it. Again, there was a threat of self-extinction.

Once again, an evolutionary "invention" came to the rescue. This was the advent of aerobic respiration, which utilized the former waste product (oxygen) and also increased the efficiency of energy production sevenfold over the earlier fermentation process. Aerobic photosynthesis followed, thus closing the carbon cycle (more or less) for the first time. This occurred less than 1 billion years ago, though life has existed on the earth for at least 3.5 billion years. But the carbon cycle and the earth's atmosphere did not stabilize for several hundred million more years. The free oxygen in the atmosphere exists only because large quantities of carbon, with which it was originally combined, have been sequestered in two forms: (1) as calcium carbonate, in the shells of tiny marine organisms (which later reappear as chalk, diatomaceous earth, or limestone), or (2) as coal or shale. The carbon sequestering process took place over several hundred million years a period culminating in the so-called carboniferous era during which the carbon dioxide content of the atmosphere declined to its present very low level. In addition, sulphur has been sequestered, primarily as sulphates. Similarly, though somewhat less certainly, the free nitrogen in the earth's atmosphere was probably originally combined with hydrogen, in the form of ammonia of volcanic origin. Whereas the carbon has mostly been buried, the missing hydrogen has probably recombined with oxygen as water vapour.

The early atmosphere and hydrosphere of the earth were quite alkaline compared with the present, because of the ammonia. The hydrogen-rich reducing atmosphere of the early earth has been replaced by an oxygenating atmosphere; the hydrosphere is correspondingly more acid than it once was before life appeared. The biosphere has stabilized the atmosphere (and the climate), at least for the last several hundred million years. If all life disappeared suddenly today, the oxygen in the atmosphere would gradually but inexorably recombine with atmospheric nitrogen and buried hydrocarbons and sulphides (converting them eventually to carbon dioxide, nitric acid, nitrates, sulphuric acid, and sulphates). Water would be mostly bound into solid minerals, such as gypsum (hydrated calcium sulphate). This oxygenation process would also further increase the acidity of the environment.

Suppose all possible chemical reactions among carbon, nitrogen, and sulphur compounds - including those currently sequestered in sediments and sedimentary rocks - proceed to thermodynamic equilibrium. The atmosphere would consist mainly of carbon dioxide. The final state of thermodynamic equilibrium would be totally inhospitable to life. (For one thing, the temperature would rise to around 300°C.) Once dead, the planet could never be revived (Lovelock 1979). Table 1.2 displays some of these "ideal" effects.

The point of the capsule history of the earth shown in table 1.2 is that our planet is, in reality, an extraordinarily complex interactive system in which the biosphere is not just a passive passenger but an active element. It is important to establish that the earth (atmosphere, hydrosphere, geosphere, biosphere) is a self-organizing system (in the sense popularized by Prigogine and his colleagues, e.g. Prigogine and Stengers 1984) in a stable state far from thermodynamic equilibrium. This system maintains its orderly character by capturing and utilizing a stream of high-quality radiant energy from the sun. Living organisms perform this function, along with other essential functions such as the closure of the carbon cycle and the nitrogen cycle (Schlesinger 1991).

Table 1.2 The stabilizing influence of the biosphere (Gaia)

Source: Lovelock (1972).
Note: Life is impossible if the average temperature is too high for liquid water, or if salinity exceeds 6 per cent.

Complex systems stabilized by feedback loops are essentially nonlinear. An important characteristic of the dynamic behaviour of some non-linear systems is the phenomenon known as chaos. Such systems are characterized by trajectories that move unpredictably around regions of phase-space known as strange attractors. "Stability" for such a system means that the trajectory tends to remain within a relatively well-defined envelope. However, a further characteristic of non-linear multi-stable dynamic systems is that they can "jump" also unpredictably - from one attractor to another. (Such jumps have been called "catastrophes" by the French mathematician Renhom, who has classified the various theoretical possibilities for continuous systems.) The resilience of a non-linear dynamic system - its tendency to remain within the domain of its original attractor - is not determinable by any known scientific theory or measurement. In fact, since the motion of a non-linear system along its trajectory is inherently unpredictable (though deterministic), the resilience of the earth system is probably unknowable with any degree of confidence. It is like a rubber band whose strength and elasticity we have no way of measuring.

The climate of the earth, with its feedback linkages to the biosphere, is a non-linear complex system. It has been stable for a long time. However, there is no scientific way to predict just how far the system can be driven away from its stable quasi-equilibrium by anthropogenic perturbing forces before it will jump suddenly to another stable quasi-equilibrium. Nor is there any way to predict how far the equilibrium will move if it does jump. The earth's climate, and the environment as a whole, may indeed be very resilient and capable of absorbing a lot of punishment. Then again, they may not.

What can be gained by more research? Probably we can learn a lot about the nature of the earth-climate-biosphere interaction. We will learn a lot about the specific mechanisms. We will learn how to model the behaviour of the system, at least in simplified form. We will learn something about the stability of the models. We may, or may not, learn something definitive about the stability of the real system. The real system is too complex, and too nonlinear, for exact calculations. There is no prospect at all of "knowing the odds" and making a rational calculation of risk. The problem we face is that the odds cannot be calculated, even in principle. In the circumstances, prudence would seem to dictate buying some insurance. The question on which reasonable people can still differ is: how much insurance is it worthwhile to buy? The answer depends, in part, on the technological alternatives.