Human effects on biogeochemical cycles

In discussing the chemistry of near-surface environments on Earth it is important to distinguish between different types of alteration to Earth systems caused by humans. Two main categories can be distinguished:

1 Addition to the environment of exotic chemicals as a result of new substances synthesized and manufactured by industry.

2 Change to natural cycles by the addition or subtraction of existing chemicals by normal cyclical and/or human-induced effects.

The first category of chemical change is probably easiest to understand. Some examples of substances which are found in the environment only as a result of human activities are given in Table 1.2 and include pesticides, such as 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane (DDT), which is broken down by bacteria in the soil to produce a number of other exotic compounds; polychlorinated biphenyls (PCBs), which have many industrial uses and are slow to degrade in the environment; tributyl tin (TBT), which is used in marine paints to inhibit organisms from settling on the hulls of ships; many drugs; some radionuclides; and a range of chlorofluorocarbon compounds (CFCs), which were developed for use as aerosol propellants, as refrigerants and in the manufacture of solid foams.

The list in Table 1.2 is by no means complete. It has been calculated that the chemical industry has synthesized several million different chemicals (mainly organic) never previously seen on Earth. Although only a small fraction of these chemicals are manufactured in commercial quantities, it is estimated that approximately a third of the total production escapes to the environment.

The impact of these exotic substances on the environment is difficult to predict, since there are often no similar natural compounds whose behaviour is

Table 1.2 Examples of substances found in the environment only as a result of human activities.

Name

Formula

Environmental impact

chlorophenyl)-

1,1,1-trichloroethane

Cl Cl

PCBs (polychlorinated x x biphenyls) xf^M^x x x X X

(x are possible chlorine positions)

(CH3(CH2)3)3Sn

TBT (tributyl tin)

Pesticide

Dielectric in transformers; hydraulic fluids and many other uses

Antifouling agent in marine paints

Unselective poison, concentrates up food chain

Resistant to breakdown carcinogens

Affects sexual reproduction of shellfish

(chlorofluorocarbons)

Aerosol propellant, Destruction of foam blower stratospheric ozone understood. A new substance may be benign, but our lack of knowledge can lead to unforeseen and sometimes harmful consequences. For example, because of the chemical inertness of the CFCs, when they were first introduced it was assumed that they would be completely harmless in the environment. This was true in all environmental reservoirs except the upper layers of the atmosphere (stratosphere), where they were broken down by solar radiation. The breakdown products of CFCs led to destruction of ozone (O3), which forms a natural barrier, protecting animal and plant life from harmful ultraviolet (UV) radiation coming from the sun (see Section 3.10).

The second category of chemical changes is concerned with natural or human-induced alterations to existing cycles. These types of changes are illustrated in Chapter 7 with the elements carbon and sulphur. The cycling of these elements has occurred throughout the 4.5 billion years of Earth history. Furthermore, the appearance of life on the planet had a profound influence on both cycles. As well as being affected by biology, the cycles of carbon and sulphur are also influenced by alterations in physical properties, such as temperature, which have varied substantially during Earth history—for example, between glacial and interglacial periods. It is also clear that changes in the cycles of carbon and sulphur can influence climate, by affecting variables such as cloud cover and temperature. In the last few hundred years, the activities of humans have perturbed both these and other natural cycles. Such anthropogenic changes to natural cycles essentially mimic and in some cases enhance or speed up what nature does anyway.

In contrast to the situation for exotic chemicals described earlier, changes to natural cycles should be easier to predict, since the process is one of enhancement of what already occurs, rather than addition of something completely new. Thus, knowledge of how a natural system works now and has done in the past should be helpful in predicting the effects of human-induced changes. However, we are often less able at such predictions than we would like to be, because of our ignorance of the past and present mode of operation of natural chemical cycles.

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