(introduction...)

People know that their actions can have considerable effect on local environments. When a campfire is lit, hundreds of hectares of forest may disappear in flames. Building a dam can flood an extensive low-lying area. Cities affect the local climate, increasing average temperatures and changing other characteristics of the overlying atmosphere. Large lakes can be rendered lifeless by contaminants discharged into them.

On a regional scale, it is not as easy to recognize environmental changes caused by human activities. However, it is now becoming clear that whole regions downwind of large industrial areas are being strongly affected by acid rain, that some species have disappeared from fishing regions, and that overgrazing or deforestation is affecting the regional climates of the Sahel and Amazon.

It is even more difficult to imagine the effects of anthropogenic action on the global environment. The Earth is so large and the atmosphere so extensive that past experience suggests that human activities will never reach the dimension necessary to produce changes on a planetary scale. Today, however, changes are occurring at an almost exponential rate, and many past theories may no longer apply.

Our sister planet

As the Earth moves around the sun, it is accompanied by the Moon. Before the 1960s, humans had no influence on the lunar environment. For 4 billion years or more, our sister planet evolved according to the general laws of celestial physics, its surface modified only by lava flows (in very ancient times), meteorite impacts, terrestrial tides, and solar radiation and particles.

For many years, even during the lunar landings of the late 1960s and early 1970s, it was believed that the Moon had no atmosphere. Now we know that it possesses a very thin one, consisting mainly of helium, argon, sodium, potassium, radon, and polonium (data from the 1972 Lunar Atmospheric Composition Experiment, see Stem 1993). The total mass of the lunar atmosphere is small - only about 30 tonnes for the whole planet.

The effect of the Apollo missions on the lunar environment was considerable. Each flight increased the mass of the lunar atmosphere by one-third. The gas escaped after a few weeks, but it was “renewed” curing each mission. The impact of establishing a settlement on the Moon would be enormous. The Moon missions showed that humans can change planets, even without meaning to.

The Earth is much bigger than the Moon: its diameter is four times greater and it is some 90 times more massive. Every day, however, the equivalent of several hundred thousand “Apollo missions” take place as aircraft take off and land. In addition, 500 million cars and 10 million factories use atmospheric gases and release others in ways quite contrary to natural cycles.

The production of carbon dioxide (CO2), for example, has been increasing exponentially since the beginning of the industrial revolution. During the first stages of the industrial era, coal was burned in large quantities. Later, factories fumed to petroleum, which is still used, and the volume of CO2 and other associated gases being emitted into the air is steadily increasing.

How much can the atmosphere of a planet like Earth absorb before changes start to occur in the gaseous layers and the crust? We don’t know the answer. Changes may already have started, and the situation may already be critical. We are “playing with fire” in both the literal and symbolic sense. We have good reason to worry, mainly because we still know so little. In the following section, some factors that might allow us to decipher the indicators of global change are discussed.

The unusual, oxygenated planet

Among the planets of the solar system, the Earth is an oddity. Although several bodies are similar in volume and mass (Venus, Mars, Mercury, Ganymede, and Titan), several features of the Earth make it unique. The Earth is the only known planet with a large oceanic area; its atmosphere contains very little CO2 (about 0.3%) and a large amount of free oxygen (21%).

The level of oxygen seems particularly high when we consider that it is a very active gas and combines with many other elements. It is found on many other planets, but usually combined with carbon or hydrogen as CO2 and water (in gaseous or solid forms) or with silicon, aluminium, and other elements to form the crystal lattices of minerals. Free oxygen does not exist in significant quantities on any other planet.

On Earth, oxygen occurs in water, ice, and rocks. In fact, oxygen represents 45% of the total mass of the Earth’s crust and 90% of the total volume. However, the huge amount of free oxygen in the atmosphere is unique in the solar system, and this oxygen has existed for many hundred million years. There is every indication that its proportion has increased during geological time, as a result of a long period of photosynthetic activity by algae and green plants.

Originally, Earth was probably more like Venus and Mars. Venus’ atmosphere is composed mainly of CO2 (95%) and nitrogen (4%); the Martian atmosphere is 94% CO2 and 5% nitrogen. Three billion years ago, the amount of CO2 in the Earth’s atmosphere was also high (perhaps over 90%); however, photosynthetic activity released the oxygen from CO2 to form organic matter. It is believed that noticeable volumes of free oxygen first appeared about 2 billion years ago. One billion years later, it probably made up I to 3% of the atmosphere and ozone started filtering out ultraviolet radiation. The 5% level was probably reached about 750 million years ago, and the current oxygen concentration was not reached until about 100 million years ago (Cloud and Gibor 1970). A large proportion of the carbon was buried in sediments as limestone, coal, petroleum, and gas. A small amount remained in the atmosphere or dissolved in ocean waters.

While the level of CO: decreased and carbon was trapped in geological layers, oxygen molecules were being released into the atmosphere, increasing slowly to a concentration of about 20%. The upper limit for oxygen concentration is related to the probability of natural fires occurring; the more free oxygen there is, the more likely spontaneous fires will break out. Fires oxidize the carbon in the organic matter, such as wood, to produce CO2, thus reducing the amount of oxygen in the air relative to CO2.

The decrease in CO2 concentration during geological times brought about important climatic changes, the main one likely being a decrease in average temperature. Carbon dioxide in the atmosphere produces a strong greenhouse effect, and its elimination promotes a general cooling of the atmosphere. The decrease in CO2 was not continuous. It occurred in leaps, and qualitative changes were determined by the development of new, more sophisticated biological systems to use it.

According to Lovelock (1988, p. 164), the decrease in CO2 was also a way for the planet to cool in spite of increasing solar heat. In other words, life seems to possess a “thermostat” that has ensured a relatively constant temperature throughout geological times, a temperature that allows survival of life. Every time the solar heat increased to a certain level, new biological systems developed to use smaller proportions of CO2, causing the concentration of this gas to decrease further, cooling the biosphere.

Through successive adaptations of photosynthetic processes, biosystems were able to reduce the CO2 content in the air to 0.3%, the current level. If solar radiation continues to increase, there is little room for additional cooling (that is, for continued lowering of CO2 levels). In that respect, biological systems are “living on the edge.” If additional CO2 is released into the air, and if the volume and activity of CO2 users (algae and plants) are reduced because of deforestation and water pollution by pesticides and oil, there is a risk that the thermostat may break down (Cloud and Gibor 1970). When that happens, it may be too late to change course.

We must seriously consider a rapid, drastic reduction in systems that burn fossil fuels and produce large quantities of CO2 and other greenhouse gases. Postponing action will put at risk not only the survival of humankind, but that of “Gala” itself.

The paradox of ozone

Ozone can be a problem gas: in the lower atmosphere, there may be too much of it and it is an indicator of pollution; in the upper atmosphere, there is not enough to block undesirable solar radiation. In both cases, the problem results from anthropogenic contamination of the air.

Oxygen is a basic building block of our planet. The crust, the oceans, and the atmosphere all contain important proportions of oxygen. Free oxygen, which is only present in the atmosphere, occurs as the diatomic molecule O2. In some cases, as a result of various natural or artificial causes, oxygen may occur as a triatomic molecule or ozone (O3).

When normal diatomic oxygen molecules reach the stratosphere, they are exposed to high-energy ultraviolet radiation, resulting in the formation of ozone. Ozone in the stratosphere filters out an important part of the ultraviolet solar spectrum. Without this protective layer, the amount of ultraviolet radiation reaching the Earth’s surface would increase to the detriment of all living organisms, including humans. The main effects would be at the molecular level, resulting in genetic malformations, cancers, and other diseases.

The concentration of ozone in the stratosphere has been gradually decreasing (despite seasonal variations), particularly over both polar regions. In Antarctica, where the process has received more attention, an “ozone hole” was observed in the early 1980s. More recently, an “Arctic hole” has also been found. In other parts of the ozone layer, there is also widespread thinning, which is becoming significant enough to affect biological activities.

The culprits identified as being responsible for this change are the chlorofluorocarbons (CFC-11 and CFC-12) contained in aerosol sprays, refrigerants, solvents, and foams. About 1 million tonnes of CFCs are emitted into the air every year. They remain in the atmosphere for 60 to 100 years - the current atmospheric concentration of chlorine is about 3 parts per billion (ppb) (Graedel and Gutzen 1989).

In the early 1970s, it was already possible to detect CFCs in Antarctica (Lovelock 1988). At that time, the concentration in the southern hemisphere was about 40 parts per trillion (ppt) and 50 to 70 ppt in the northern hemisphere. The threat to the ozone layer was not yet recognized. In 1974, Rowland and Molina (see Lovelock 1988) developed the hypothesis that CFCs were a source of chlorine and, therefore, a threat to the ozone layer. Since then, considerable scientific research has been done and, although not unanimously, it is generally believed that CFCs are indeed having deleterious effects on the ozone layer.

At ground level, ozone is a secondary photochemical oxidant, which is formed as a result of various human activities, including automobile engine combustion. Although it is not part of the emissions themselves, ozone is formed as an immediate result and is an important component of smog. Contamination in urban and industrial areas can be measured in terms of “ozone concentrations.” The gas is a clear indicator of air quality: the more ozone occurring in the lower atmospheric layers, the more contaminated the air. An improvement in air quality in large metropolitan areas will be accompanied by a reduction in the concentration of ozone. Ozone, itself, at low concentrations, is not a toxic gas, but its presence reveals that pollution emissions are taking place.

Oceans can be degraded too

Oceans and large bodies of water are also being degraded by human activities. Although oceans are very large, occupying nearly three quarters of the Earth’s surface, the continuous outflow of untreated effluents into the sea has had persistent and increasing effects, particularly along the shores. Sediment accumulations have increased at the outlets of several large rivers, “plumes” of industrial and urban pollutants are flowing into many coastal areas, and overfishing has had a profound negative effect on marine ecosystems. Thin films of petroleum, foam from detergents, and various floating wastes can be found even far from populated areas. The degradation of oceanic basins has become a planetary phenomenon.

The rivers are becoming muddy

Some time ago, Erhart (1968), traveling by ship along the Congo and Amazon rivers, was puzzled by the lack of turbidity in the water - no sediments, no clays, nothing of the brown colour that one expects of mighty rivers draining such large basins. Eventually, he realized that the clear water was natural. These large streams flowed from rain-forest basins, where there was no erosion. Chemical processes of organic origin predominated. Although the water in these rivers was carrying salts, resulting from the leaching of ions from the soils they drained, no sediments were being transported. Ions of calcium, sodium, potassium, magnesium, and silicon and carbonates, phosphates, and chlorides were carried in the water in small proportions, producing a gradual increase in the salinity of the sea and supplying raw materials for the shells of sea organisms.

Erhart also realized that the old processes of soil formation (weathering) in rain-forest environments were the origin of limestone. Today’s calcareous mud at the bottom of the ocean is the current equivalent of the ancient limestones formed (by biostasy) 100 or 200 million years ago during the Mesozoic era, when dinosaurs roamed the Earth. However, the calcareous muds of the past were buried by younger sediments, composed of claystones, siltstones, and associated sandstones (what geologists call “flysch”), during a drier period that followed the humid period that produced the limestones. Erhart concluded that the forest had disappeared and that subsequently the soils had been eroded; he called this situation, in which mechanical processes predominated, rhexistasy.

Today, large forests are disappearing even faster as a result of human action. Deforestation is widespread. Forests are logged or burned, leading to soil erosion; rivers are becoming filled with muddy sediments. Flying over the Amazon brings new surprises every year: its tributaries are becoming yellow or brown in colour; the Amazon itself is no longer dark green; and in geological terms, the forest is starting to die.

In ancient times, forests would die, but others were born. There were always sufficient trees to maintain a low level of CO2. Now, as all forests are being cut back at the same time, we suspect there is considerable risk to the planet’s dynamics.

Overshooting

It is difficult to predict the outcome of current changes. Exponential growth of some components (such as world population) or some factors (temperature of the oceans, level of CO2) indicates the direction of change, but cannot provide sufficient information to allow us to guess the future of the Gaia system. The Earth is an extremely complex environment, and growth curves are crude instruments for understanding it. In reality, we do not know where or when “overshooting” of limits will take place. At best, these tools give us a slight indication of the risk.

We must remember that natural processes never follow a linear or exponential path indefinitely. Once they reach a ceiling, a change takes place, and new relationships are established. Sometimes factors that are overlooked may be increasing or decreasing exponentially, and their effects may be felt suddenly. The greenhouse effect produces an increase in temperature, which in turn increases evaporation; this leads to increased cloudiness and an increase in the albedo of the planet, reducing radiation and decreasing temperature. Even a relatively simple model like this can be difficult to quantify, however, mainly because the data and relationships are poorly understood. For example, if we introduce the role of algae and photosynthesis in the upper layer of the oceans or the effect of ice melting at the poles, the situation becomes more complex. A model of the planet requires understanding and measuring thousands of variables, some of which are biological or anthropogenic in nature.

Although much can be done toward solving the riddle of our environmental future, we must remain cautious about forecasts. Because so little is known and the risk is so great, survival strategies must rely on the best interpretation of existing data. We may, in the end, “go beyond the limits inadvertently” because of inattention, inadequate information, a slow response, or simply the momentum (Meadows et al. 1992). On this “spaceship Earth,” however, we cannot afford to risk overshooting the limits, whatever they may be; we may not have a second chance.