One of the biggest events in Earth's four billion year history is the oxygenation of its atmosphere. Around two and a half billion years ago, it is believed that some newly evolved organisms, cyanobacteria, developed a kind of photosynthesis, which produced free oxygen and used up carbon dioxide. At the beginning of this process, the atmosphere was mostly nitrogen with the rest largely carbon dioxide. At the end of the process, lasting perhaps hundreds of millions of years, the atmosphere was still mostly nitrogen, but oxygen had replaced almost all the carbon dioxide.
This process involved a huge number of geochemical and biological changes, but the bottom line is that Earth developed a very unusual atmophere, with free oxygen. The oxygen provided much more energy for organisms to use, expecially on land, and this led to the evolution of man. Hurrah!
Once there were large organisms on the surface of Earth, they went through their life processes, and in a few places, were buried along with the carbon they were composed of. One of these processes involved the burial, maybe under blown dust or dirt, in large number of layers, of carbon residues, which were carried by tectonic processes deeper underground, where the pressure would transform them into coal, oil and natural gas. Another of these processes happened in the frozen taiga, where the surface melts in the summer and plants form, only to die in the winter except for their seeds. All the rest were buried under layers and more layers of frozen ground and ice. There may have also been underwater processes, resulting in buried carbon compounds in the sea floor. There may be even more processes which extract carbon from organisms. All the buried carbon comes from organisms which extracted it from the residual carbon dioxide in the atmosphere, leading to a continued dropping of the concentration of this molecule. Since carbon dioxide is the most essential foodstuff for organisms, this extraction means that it is growing harder and harder for life to exist on Earth.
Those organisms which required more carbon dioxide in the atmosphere than we have now have already become extinct. Over periods measuring in millions of years, the lowering of carbon dioxide concentrations in the atmosphere would result in more and more extinctions, until Earth would be left with only the best carbon dioxide scavengers, living on a planet with little atmospheric carbon dioxide. Someday, if this plan were to continue, they would, one by one, die out as well. Thus the Earth may have been on its way to becoming a bare, lifeless planet.
Enter man. For most of its existence, man had no effect on this process, and indeed no knowledge of it. Fortunately for the rest of life on Earth, a couple of hundred years ago, mankind discovered the bountiful energy that was buried in what we usually call fossil fuels, coal, oil and natural gas, and began burning it. A large amount has been found and burned, and the Earth's horrendous shortage of carbon dioxide in its atmosphere is being reversed. It looks like this will continue, and the change in temperature caused by this, utilizing the greenhouse effect, may melt some of the frozen carbon storage in the northern part of the globe, leading to an even greater rescue of Earth's life. It is not beyond imagination that mankind will someday release some of the carbon buried in the sea floor.
We should not take credit for too much. There are other places the carbon can be hidden on the Earth, and eventually, these will take over and get rid of whatever is left over from the oxidation of the atmosphere plus whatever mankind has found and brought back for life to use. Hopefully, that will be long from now, when the sun is heating up and the Earth is becoming uninhabitable because of its solar-generated temperature. If mankind is successful in the short term in raising the average temperature of the planet a few degrees, life may evolve to endure higher temperatures, but this will only extend the span of life's duration on Earth by some millions of years. It is inevitable that Earth will become lifeless one day, but thanks to mankind and fossil fuels, that day may be in the far, far future, rather than at some sooner time.
Please excuse this tangential note. If you would like to read what I am projecting for the next seven hundred and fifty years of human life, the period of the most exciting changes in all of mankind's history, you can read my book, Looking Back From Luna.
