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 Earths 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 Earths 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