The total global environment consists
of four major realms: a gaseous atmosphere, liquid hydrosphere,
solid lithosphere, and living biosphere.
From space, Earth's atmosphere looks like a
thin blue veil. This fragile, nearly transparent envelope
of gases supplies the air that we breathe each day. It also
regulates the global temperature and filters out dangerous
levels of solar radiation. In recent years, scientific research
has shown that the chemical composition of the atmosphere
is changing because of both natural and human induced causes.
There is growing concern over the impact of human activities.
Human impact may be increasing levels of heat absorbing gases,
thereby contributing to global warming and destroying ozone,
the fragile atmospheric ingredient that shields the planet
from ultraviolet (UV) radiation.
Earth is believed to have formed about 5 billion
years ago. In the first 500 million years a dense atmosphere
emerged from the vapor and gases that were expelled during
degassing of the planet's interior. These gases may have consisted
of hydrogen (H2), water vapor, methane (CH4),
and carbon oxides. Prior to 3.5 billion years ago the atmosphere
probably consisted of carbon dioxide (CO2), carbon
monoxide (CO), water (H2O), nitrogen (N2),
The hydrosphere was formed 4 billion years
ago from the condensation of water vapor, resulting in oceans
of water in which sedimentation occurred.
The most important feature of the ancient
environment was the absence of free oxygen. Evidence of such
an anaerobic reducing atmosphere is hidden in early rock formations
that contain many elements, such as iron and uranium, in their
reduced states. Elements in this state are not found in the
rocks of mid-Precambrian and younger ages, less than 3 billion
the Ozone Layer
One billion years ago, early aquatic organisms
called blue-green algae began using energy from the Sun to
split molecules of H2O and CO2 and recombine
them into organic compounds and molecular oxygen (O2).
This solar energy conversion process is known as photosynthesis.
Some of the photosynthetically created oxygen combined with
organic carbon to recreate CO2 molecules. The remaining
oxygen accumulated in the atmosphere, touching off a massive
ecological disaster with respect to early existing anaerobic
organisms. As oxygen in the atmosphere increased, CO2
High in the atmosphere, some oxygen (O2)
molecules absorbed energy from the Sun's ultraviolet (UV)
rays and split to form single oxygen atoms. These atoms combined
(27k jpeg) with remaining oxygen (O2) to form ozone
(O3) molecules, which are very effective at absorbing
UV rays. The thin layer of ozone that surrounds Earth acts
as a shield, protecting the planet from irradiation by UV
The amount of ozone required to shield Earth
from biologically lethal UV radiation, wavelengths from 200
to 300 nanometers (nm), is believed to have been in existence
600 million years ago. At this time, the oxygen level was
approximately 10% of its present atmospheric concentration.
Prior to this period, life was restricted to the ocean. The
presence of ozone enabled organisms to develop and live on
the land. Ozone played a significant role in the evolution
of life on Earth, and allows life as we presently know it
and videos courtesy of Distributed Active Archive Center at
NASA's Goddard Space Flight Center.