Significance of the Atmosphere
Without the atmosphere, planet Earth would be much more like the Moon than like the
planet we live on today. The Earth’s atmosphere, along with the abundant liquid water
on the Earth’s surface, are keys to our planet’s unique place in the solar system. Much of
what makes Earth exceptional depends on the atmosphere. Let’s consider some of the many
reasons we are lucky to have an atmosphere.
Atmospheric Gases Are Indispensable for Life on Earth
Without the atmosphere, Earth would be lifeless. Carbon dioxide (CO 2 ) and oxygen (O 2 )
are the most important gases for living organisms. CO 2 is vital for use by plants inphoto-
synthesis, in which plants use CO 2 and water to convert the Sun’s energy into food energy.
This food energy is in the form of the sugar glucose (C 6 H 12 O 6 ). Plants also produce O 2.
Photosynthesis is responsible for nearly all of the oxygen currently found in the atmosphere.
The chemical reaction for photosynthesis is:
6CO 2 + 6H 2 O + solar energy → C 6 H 12 O 6 + 6O 2
By creating oxygen and food, plants have made an environment that is favorable for animals.
Inrespiration, animals use oxygen to convert sugar into food energy they can use. Plants
also go through respiration and consume some of the sugars they produce.
The chemical reaction for respiration is:
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + useable energy
Notice that respiration looks like photosynthesis in reverse. In photosynthesis, CO 2 is con-
verted to O 2 and in respiration, O 2 is converted to CO 2.
The Atmosphere is a Crucial Part of the Water Cycle
Water moves from the atmosphere onto the land, into soil, through organisms, to the oceans
and back into the atmosphere in any order. This movement of water is called the water cycle
orhydrologic cycle(Figure15.1).
Water changes from a liquid to a gas byevaporation.Water vaporis the name for water
when it is a gas. When the Sun’s energy evaporates water from the ocean surface or from
lakes, streams, or puddles on land, it becomes water vapor. The water vapor remains in the
atmosphere until itcondensesto become tiny droplets of liquid. The tiny droplets may
come together to createprecipitation, like rain and snow. Snow may become part of the ice
in a glacier, where it may remain for hundreds or thousands of years. Eventually, the snow
or ice will melt to form liquid water. A water droplet that falls as rain, could become part
of a stream or a lake, or it could sink into the ground and become part ofgroundwater.