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5.20 Earth's Atmosphere and Oceans

The primordial atmosphere four billion years ago was very different from today's atmosphere. It was dominated by carbon dioxide (CO2) and water vapor (H2O) in gaseous form. This primitive atmosphere would have seemed quite alien to us. You’re probably aware that our metabolism requires oxygen to breathe, and too much carbon dioxide is toxic to us. Studies of ancient sediments show that the primitive atmosphere lacked oxygen (O2). The early Earth was far from suitable for human life! But the earliest life forms, whose fossils are 2.5 to 3.5 billion years old, are types of algae found today only in oxygen-poor environments, such as salt marshes along sea coasts. An oxygen-rich atmosphere like today’s would have been as poisonous to these early life forms as the early atmosphere would be to present-day oxygen-dependent life forms like us.

The?Mars Global Surveyor?acquired this image of the Martian north polar ice cap in early northern summer. The polar cap is a mixture of 85% carbon dioxide and 15% water ice. Click here for original source URL.

Cloud structure in?the Venusian atmosphere?in 1979, revealed by ultraviolet observations by?Pioneer Venus Orbiter. Click here for original source URL.

How did the Earth’s atmosphere reach its present state? Three processes were important. First, water vapor in the atmosphere condensed and fell as rain, forming oceans. If the Earth had been closer to the Sun (and therefore hotter), the water would have stayed in gaseous form. If the Earth had been farther from the Sun (and colder), the water would have frozen. So the most common "landscape" on our planet — vast oceans of liquid water — is unique in the Solar System. By looking at other planets, we can see examples of what might have happened to the Earth if it had been slightly warmer, like Venus, or slightly cooler, like Mars. Water on Venus stayed its in gaseous form. In the upper atmosphere, solar radiation broke it up into H2 and O. The hydrogen was light enough to escape Venus’s gravity, and the oxygen combined with other gases or surface rocks. So there is very little water vapor left in Venus’s atmosphere. On Mars, water is currently present only in its solid state, frozen beneath the surface or trapped in polar ice caps. At one point in its history, liquid water did flow on the Martian surface, but it would have required a much different climate than the one we see on Mars today.

The presence of oceans then initiated a second process. Large amounts of carbon dioxide from the atmosphere dissolved in the ocean water (like the carbon dioxide dissolved in soft drinks), to make weak carbonic acid, H2CO3. That acid solution reacted with sea floor sediments to create carbonate-rich sedimentary rocks, effectively trapping the CO2 on the ocean floor. With the carbon dioxide and water vapor removed, the dominant remaining gas in the atmosphere was nitrogen (N2).

The oxygen that we depend on for life has an interesting history. Before biology "invented" photosynthesis, there was no oxygen in the atmosphere. Prokaryotic organisms (primitive cells without nuclei) started producing oxygen as a waste product about 3.5 billion years ago. This oxygen was locked up in rocks as soon as it was produced, leaving us with "rusted" iron formations that are found all around the world. The first significant amount of oxygen in the atmosphere dates from about 2.3 billion years ago, caused by something called the Great Oxygenation Event. Cyanobacteria started to produce oxygen by photosynthesis and ironically, the oxygen was toxic to many other microbes around at the time. For more than a billion years, the level of oxygen was low and variable, probably 10% of today's level.

Last, and most importantly, plant life evolved. Plants consumed much of the remaining carbon dioxide and emitted oxygen. Oxygen content rose to the levels we enjoy today. In fact, the levels may hav peaked around 300 million years ago at the end of the Carboniferous period, contributing to the large size of the insect and reptiles at that time. Today's atmosphere is 76% nitrogen and 23% oxygen by weight, with only modest traces of carbon dioxide and water vapor. Like our oceans, the Earth’s atmosphere is also unique in the solar system. The postscript on this story is written by humans. All of the changes described so far played out over time scales of millions of years. Since the Industrial Revolution, the concentration of carbon dioxide in the atmosphere has increased from 200 parts per million to almost 400 parts per million. While this is still a small component of the atmosphere, carbon dioxide is a potent greenhouse gas and we are now witnessing the effects of our progress as a species. Humans have substantially altered the global atmosphere and climate in just a few generations.


Carbon dioxide bubbles, dissolving in a soft drink. Click here for original source URL.