The Earth as a Solar-Driven System The Atmosphere
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Life has, in a sense, created its own environment on earth.

Over eons of time, life has profoundly transformed the surface of the earth and its atmosphere. The atmosphere was, in fact, originally composed of chemicals hostile to life. We make think of the earth's atmosphere as a given, as an inherent characteristic of the planet; it is, instead, a collective creation and sustainer of Earth's life forms. The gases which form the atmosphere were much different in composition when the earth was young.

Original atmosphere without life

CO2 98%

Nitrogen 1.9%

Oxygen 0%

Argon traces

Hydrogen

Helium Atmosphere as it is

CO2 .03%

Nitrogen 79%

Oxygen 21%

Argon traces As the Earth aged, water vapor and other gases, once chemically bound inside the planet erupted from the surface in huge quantities, creating Earth's primitive oceans and atmosphere. Earth's primitive atmosphere held no free oxygen and consisted chiefly of water vapor, nitrogen, and carbon dioxide. The heat and ultraviolet radiation made life on the land impossible. Eventually, however, the increasing amount of water vapor in the atmosphere caused rain. The rain caused erosion which washed sediment and chemicals into the oceans, enriching it. The oceans, sheltered from the heat and ultraviolet radiation provided a safe haven for life to begin.

Scientists have found evidence of a simple cells in 3.5 billion year old rocks. The first simple cells nourished themselves by absorbing nutrients from the surrounding sea. Then some cells evolved a new way to sustain themselves: photosynthesis, using the energy from the sun to make their food. These first plants, the blue green algae, formed colonies. More importantly, as a by product of photosynthesis, they produced oxygen. Slowly, the biological activity of plants changed the composition of the earth's atmosphere.

As the free oxygen in the atmosphere increased, oxygen molecules linked to form ozone in the stratosphere. The ozone filtered out much of the sun's ultraviolet light, for the first time allowing plant populations to thrive on dry land. As green plants spread, iron remaining in the Earth's crust rusted to iron oxide, a record in rock of oxygen's first appearance on Earth.

The earth's atmosphere thus dynamically interacts with and sustains existing life forms. Plants process carbon dioxide through photosynthesis, storing the carbon in their tissues and releasing free oxygen into the atmosphere, where it can be used in the metabolism of animal life forms.

These interactions between the plant and animal kingdoms collectively maintain the atmospheric composition and range of temperature which we take for granted--and which is congenial to existing life forms. Life is thus a part, and an integral part, of the planetary system, and this system as a whole acts very much like the system of the body, for example, in regulating temperatures of the planet within tolerable ranges.

The biological and physical systems of the planet have evolved into a passive control system with many feed-back loops. Without life, for example, the average temperature of the Earth would lie in the range of 240-340 degrees C (464-644 F)--that is, at a temperature fatal to all life. As it is, with a life-sustained atmosphere, the earth's temperature averages 13 degrees C (55.4 F) overall and about 16 C (60.8 F) on land.

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