Environmental Engineering Reference
In-Depth Information
Active Figure 3-7
Solar capital:
flow of energy to
and from the earth.
See an animation based on this
figure and take a short quiz on the concept.
Solar
radiation
Energy in = Energy out
Reflected by
atmosphere (34%)
Radiated by
atmosphere
as heat (66%)
UV radiation
Lower Stratosphere
(ozone layer)
Troposphere
Absorbed
by ozone
Greenhouse
effect
Visible
light
Heat
Absorbed
by the
earth
Heat radiated
by the earth
so-called
greenhouse gases
(such as water vapor, car-
bon dioxide, methane, nitrous oxide, and ozone) in
the troposphere. As it interacts with these gaseous
molecules, it increases their kinetic energy, helping
warm the troposphere and the earth's surface. With-
out this
natural greenhouse effect,
the earth would
be too cold for life as we know it to exist.
and to keep the light gaseous molecules (such as N
2
,
O
2
, CO
2
, and H
2
O) in its atmosphere from flying off
into space.
On a time scale of millions of years, the earth
is enormously resilient and adaptive. During the 3.7
billion years since life arose here, the planet's average
surface temperature has remained within the narrow
range of 10-20 °C (50-68 °F), even with a 30-40% in-
crease in the sun's energy output. In short, this re-
markable planet that we call home is uniquely suited
for life as we know it.
Learn more about the flow of energy—from sun to earth and
within the earth's systems—at Environmental ScienceNow.
Why Is the Earth So Favorable for Life?
The earth's temperature range, distance from the sun,
and size result in conditions that are favorable for life
as we know it.
Life on our planet depends on the liquid water that
dominates the earth's surface. Temperature is crucial
because most life on earth needs average temperatures
between the freezing and boiling points of water.
The earth's orbit is the right distance from the sun
to provide these conditions. If the earth were much
closer to the sun, it would be too hot—like Venus—for
water vapor to condense to form rain. If it were much
farther away, the earth's surface would be so cold—
like Mars—that its water would exist only as ice. The
earth also spins; if it did not, the side facing the sun
would be too hot and the other side too cold for water-
based life to exist.
The earth is also the right size: It has enough grav-
itational mass to keep its iron and nickel core molten
3-3
ECOSYSTEM COMPONENTS
Biomes and Aquatic Life Zones
Life exists on land systems called biomes and in
freshwater and ocean aquatic life zones.
Viewed from outer space, the earth resembles an enor-
mous jigsaw puzzle consisting of large masses of land
and vast expanses of ocean.
Biologists have classified the terrestrial (land) por-
tion of the biosphere into
biomes
(“BY-ohms”). Each of
these large regions—such as forests, deserts, and grass-
lands—is characterized by a distinct climate and spe-
cific species (especially vegetation) adapted to it (see
Figure 1 in Science Supplement 2 at the end of this
topic). Figure 3-8 shows different major biomes along
the 39th parallel spanning the United States.
Scientists divide the watery parts of the biosphere
into
aquatic life zones,
each containing numerous eco-
