Evolution of Earth's atmosphere

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The history of the Earth's atmosphere prior to one billion years ago is poorly understood; it is an active area of scientific research. The following discussion presents a plausible scenario.

The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from previous compositions. The original atmosphere was primarily helium and hydrogen. Heat from the still-molten crust, the sun, and a probably enhanced solar wind, dissipated this atmosphere.

About 4.4 billion years ago, the surface had cooled enough to form a crust. It was heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the early "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. This second atmosphere had approximately 100 times as much gas as the current atmosphere, but as it cooled much of the carbon dioxide was dissolved in the seas and precipitated out as carbonates. The later "second atmosphere" contained largely nitrogen and carbon dioxide. However, simulations run at the University of Waterloo and University of Colorado in 2005 suggest that it may have had up to 40% hydrogen.^[7] It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide and methane, kept the Earth from freezing.

One of the earliest types of bacteria was the cyanobacteria, which formed into colonies called stromatolites. Fossil evidence indicates that bacteria shaped like these existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the Earth's atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen) during the period 2.7 to 2.2 billion years ago. Being the first to carry out oxygenic photosynthesis, they were able to produce oxygen while sequestering carbon dioxide in organic molecules, playing a major role in oxygenating the atmosphere. This is often referred to as the Oxygen Catastrophe. The increase in the concentration of oxygen in the atmosphere required time because iron and other elements in the Earth's crust reacted with oxygen, removing it from the atmosphere.

Photosynthesising plants later evolved and continued releasing oxygen and sequestering carbon dioxide. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to release nitrogen. Bacteria also converted ammonia into nitrogen, but most of the nitrogen currently in the atmosphere resulted from sunlight-powered photolysis of ammonia released steadily over the aeons from volcanoes.

As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various elements, but eventually oxygen accumulated in the atmosphere, contributing to Cambrian explosion and further evolution. With the appearance of an ozone layer (ozone is an allotrope of oxygen) life-forms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere". Between 200 and 250 million years ago, up to 35% of the atmosphere was oxygen (as found in bubbles of ancient atmosphere preserved in amber).

This modern atmosphere has a composition which is enforced by oceanic blue-green algae as well as geological processes. O₂ does not remain naturally free in an atmosphere but tends to be consumed by inorganic chemical reactions, and by animals, bacteria, and even land plants at night. CO₂ tends to be produced by respiration and decomposition and oxidation of organic matter. Due to this, O₂ would vanish within a few million years by chemical reactions, and CO₂ dissolves in water and would be gone in millennia if not replaced. Both are maintained by biological productivity and geological forces seemingly working hand-in-hand to maintain reasonably steady levels over millions of years.

Currently, anthropogenic greenhouse gases are increasing in the atmosphere and this is a causative factor in global warming.^[8]