The unusual, oxygenated planet
(from the book Diversity, globalization and the ways of nature)
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 also approximately 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 1 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 CO2 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 greenhouse effect, probably weak due to the small quantities, although this is uncertain.
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, perhaps with a cooling effect.
Through successive adaptations of photosynthetic processes, biosystems were able to reduce the CO2 content in the air to 0.04%, the current level. If that was so, and if solar radiation increases, there is little room for additional cooling (that is, for continued lowering of CO2 levels, which fortunately is not happening). In that respect, biological systems might be “living on the edge.” It is uncertain if the sun’s radiation increased during geological times, but an augmentation of the heat received by the Earth might be dangerous for the survival of some ecosystems. There are still many uncertainties in the future Earth evolution.
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