Energy and Changes in the Environment - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 41

The trace gases in the earth's atmosphere are only a few percent of
its composition but they make the planet livable. They absorb radiant
energy at infrared wavelengths much more efficiently than they absorb
radiant energy at solar wavelengths, thus trapping most of the radiant
heat emitted from the earth's surface before it escapes.
THE GREENHOUSE CYCLE
Greenhouse gases include water vapor, carbon dioxide (CO 2 ) and
other particles such as water droplets in clouds. They absorb infrared
energy and also give it off. The infrared radiation is emitted upward
cooling the planet and maintaining a balance with incoming sunlight.
Some of it goes back to the earth's surface creating the greenhouse effect.
This downward reradiation warms the earth's surface and makes it 33°C
(60°F) warmer.
In the greenhouse analogy, gases and clouds in the earth's atmosphere
allow a larger amount of the sun's shorter wavelength radiation in while
allowing the longer wavelength infrared radiation to escape to space. This
theory has been tested with millions of measurements in the atmosphere,
space and laboratories.
Over 4 billion years ago the heat from the sun was about 30% less
powerful than it is today. A number of elements, including carbon and
oxygen, condensed to form the earth and as the earth's crust cooled and
hardened, hot gases from the interior were ejected including carbon
dioxide.
The amount of carbon dioxide in the atmosphere then has been
estimated to be many times greater than today. This explains how the
earth's climate was warm enough for liquid water and the life that
evolved from it about 4 billion years ago. As life on earth evolved, the
solar output increased and photosynthetic organisms used much of this
carbon dioxide.
CO 2 is a major factor in the cycles that built up our mineral resources.
Fossil fuels developed over several hundred million years during the
Phanerozoic era. There was abundant life for about 600 million years.
The richest fossil fuel deposits are thought to occur at times when
the earth was much warmer and contained much more CO 2 than today.
During the last 2 million years, the permanent polar ice descended
and most of the evidence indicates that CO 2 levels decreased compared