Life on earth has built up enormously complex structures and systems in which the myriad forms of life interact with the physical systems and cycles of the planet. The continuous flow-through of solar energy has made evolution of life forms and these complex systems possible.

Some of the sun's energy is captured by simple absorption by the earth's atmosphere and surface. Terrestrial life, however, has a direct energy-capture system in the process of photosynthesis, which is carried on in plants, about 80% of which are microscopic plankton in the oceans.

Plants use the sun's energy to bond atoms of elements taken from the atmosphere, water, and minerals from the soil or, in the case of plankton, from the ocean. The plant kingdom functions like a huge biochemical factory, a machine for making, cycling, and recycling the chemicals of life. Terrestrial life is based on carbon, one of the most fundamental building block of life. (The element carbon makes up at least 50% of every living thing; together, carbon, oxygen, hydrogen, and nitrogen, the main atmospheric gases, constitute 99% of all living things.) Billions of tons of carbon are contained in the living tissues of plants and animals on the earth's surface, and billions more tons of carbon are trapped in fossil fuels under ground.