Page 27 - Curriculum Visions Dynamic Book

Changes through time
Another important characteristic of the Earth’s atmosphere is that it has changed in composition through time.
The primary feature responsible for this has been the development of life on Earth.
The early Earth’s atmosphere had little oxygen. But as simple plant life forms developed, they were able to make use of sunlight and the carbon dioxide in the air to build their tissues. As part of this process, they released free oxygen. This process is called photosynthesis, and it began to change the whole of the Earth’s atmosphere.
As long as there is enough plant life, free oxygen will remain part of the atmosphere. However, the oxygen is very reactive and quite unstable in the large amounts we currently find. Without life it would soon be locked up in compoUnds again. It does not occur in such high proportions on any other planet.
The oceans have also changed through the history of the planet. At first, water was spewed out of volcanoes. But at this early time the Sun was not as bright as it is today, and so it would not have heated the Earth as much as it now does. Temperatures on the surface would not normally have been high enough to keep the water from freezing.
However, out of the volcanoes also came carbon dioxide, which trapped heat in the air, making the surface warmer than it otherwise would have been.
But plants removed much of this carbon dioxide at a later date, thus keeping the atmospheric warming from getting out of control in the way that it has on Venus.
 This is a bloom, or rapid growth, of plAnkton in the Atlantic Ocean. It was the development of such primitive life in the oceans that changed the balance of gases
in the air, increasing the amount of oxygen, while reducing the amount of carbon dioxide.
compoUnd A substance made from two or more elements that have chemically combined.
cyclone A large storm in which the atmosphere spirals inward and upward.
hUrricane A very violent cyclone that begins close to the equator, and that contains winds of over 117 km/hr.
latitUde Angular distance north or south of the equator, measured through 90°.
photosynthesis The process that plants use to combine the substances in the environment, such as carbon dioxide, minerals, and water, with oxygen and energy-rich organic compounds by using the energy of sunlight.
plankton Microscopic creatures that float in water.
reactive The ability of a chemical substance to combine readily with other substances. Oxygen is an example of a reactive substance.
27