Earth) periods of hypoxia or low-oxygen, whereas the bottoms of the ocean have undergone not only hypoxia but, on occasion, anoxia, or the complete absence of oxygen, and also periods of higher than present oxygen.
There can be no discussion about the atmosphere without including the global ocean. The two are coupled; even small changes in the temperature or chemistry of the global ocean can produce enormous changes in the atmosphere.
The composition of Earth’s present-day atmosphere is basically known. Essentially it is made up of two gases: 78 percent of its volume is nitrogen, and 21 percent is oxygen. The remaining 1 percent is made up of trace amounts of other gases. Yet even at this small volume, this 1 percent has a huge effect on the planet, for within this 1 percent are both the important greenhouse gas of carbon dioxide and water vapor (itself a gas). “Greenhouse gas” is the term now used to describe any gas at work to trap heat in the atmosphere, thus warming the planet. How long has our planet had this atmosphere?
The atmosphere of our planet is as old as Earth itself. The two originated at the same time somewhere around 4.6 billion years ago—a date that is almost one-third the age of the Universe itself. The planet was molten soon after formation, but rapid cooling set in and as temperatures dropped, the planet rapidly evolved. Once formed, the solid Earth and its gaseous atmosphere evolved in quite different ways, even though each influenced the other over time. Like all planets, Earth formed through accretion of particles in a solar or planetary nebula. The formation of Earth was but one part of the formation of the entire solar system. As our planet accreted, it began to differentiate, with the heavier elements sinking toward the center and the lighter elements staying near the surface. In this fashion the major structural elements of our planet, its dense inner core, middle mantle, and outer crust regions formed. This process led to rapid changes in the atmosphere of the forming earth as well. Enormous quantities of gas were trapped in the differentiating Earth and sequestered far beneath the surface of the