At that time the Sun was young and weak. The thermonuclear reaction in its core had made it start to shine, but it was about 25 percent to 30 percent dim mer and cooler than it is today. Nevertheless, this was enough to warm the surface layers of water and to provide light for the first cells to begin manufacturing sugars by photosynthesis. Those cells were cyanobacteria and they lived in microbial mats; the fossil remains of those mats are called stromatolites. Oxygen is a by-product of photosynthesis and cyanobacteria released it.
By around 2.1 billion years ago the atmosphere contained oxygen— about 15 percent of the amount it contains now—and the amount was increasing. The ozone layer formed in the stratosphere when the air contained 1 percent of its present concentration of oxygen. Ozone is a form of oxygen in which each molecule comprises three oxygen atoms (O3) rather than the two (O2) of ordinary oxygen.
Photosynthesis was producing the oxygen, but no one knows for sure how it came to accumulate, because photosynthesis is always accompanied by respiration. Photosynthesis removes carbon dioxide from the air and releases oxygen, but respiration releases energy by oxidizing carbon back into carbon dioxide, which is released into the air. When organisms die their tissues are decomposed by organisms that use their carbon as a source of energy, releasing it by respiration. Consequently, respiration removes the oxygen from the air and returns the carbon dioxide, so the composition of the air remains unchanged.
The first photosynthesizers lived in water. Most scientists think that when these single-celled organisms died, a small proportion of their material—about 0.1 percent—sank to the bottom and was buried in mud. This prevented further decomposition and would have allowed oxygen to remain in the atmosphere.
Other scientists believe a second process may have contributed. Modern microbial mats, very similar to those that existed more than 2 billion years ago, release large amounts of hydrogen at night, when photosynthesis ceases and so no oxygen is being released. Some of the hydrogen would have been used by other organisms, but a proportion would have risen through the atmosphere and escaped into space. Hydrogen is released by breaking the bonds that hold water molecules together: H2O ^ H + OH. In the presence of oxygen, the water molecule reforms: 4H + O2 ^ 2H2O, but at night, when oxygen was not being released, this reaction would not have taken place, allowing the oxygen to accumulate.
Oxygen continued to accumulate and the amount of carbon dioxide continued decreasing until the atmosphere reached its present composition. This is the Earth's third atmosphere, the one it has had for about 600 million years, during which time its composition has not changed. The table on page 5 lists the gases that constitute our present atmosphere.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.