Visualizing Environmental Science

Ozone Depletion in the Stratosphere 231

Causes of Ozone Depletion

The primary chemicals responsible for ozone loss in the

stratosphere are a group of industrial and commercial

compounds called chlorofluorocarbons (CFCs). Scien-

tists first discovered that CFCs can deplete stratospheric

ozone in the mid-1970s. Chloro-

fluorocarbons such as Freon were

used as propellants for aerosol

cans and coolants in air condi-

tioners and refrigerators. Other

CFCs were used as solvents and

as foam-blowing agents for insula-

tion and packaging (Styrofoam,

for example).

Other compounds that destroy ozone include halons,

used as fire retardants; methyl bromide, a pesticide; methyl

chloroform and carbon tetrachloride, industrial solvents; and

nitrous oxide, released from the burning of fossil fuels

(particularly coal) and from the breakdown of nitrogen

fertilizers in the soil.

Effects of Ozone Depletion

With depletion of the ozone layer, more UV radia-

tion reaches the Earth’s surface. Increased levels of UV

radiation may disrupt ecosystems. For example, the pro-

ductivity of Antarctic phytoplankton, the microscopic drift-

ing algae that are the base of the Antarctic food web, has

declined due to increased exposure to UV radiation. (The

UV radiation inhibits photosynthesis.) Biologists have

documented direct UV damage to natural populations of

Antarctic fish. Widespread decline of amphibian popula-

tions may be linked to increased UV radiation. Because

organisms are interdependent, the negative effect on one

species has ramifications throughout the ecosystem.

Excessive exposure to UV radiation is linked to

several health problems in humans, including eye cat-

aracts, weakened immunity, and skin cancer. Exposure

to any amount of UV radiation increases the risk of skin

cancer. Radiation can cause mutations in cells that then

reproduce abnormally and rapidly. This creates a par-

ticular concern for younger people, since cancers often

only appear years after exposure. For this reason, many

Ozone Depletion in the Stratosphere

LEARNING OBJECTIVES

1. Describe the importance of the stratospheric
   ozone layer.


2. Explain how ozone thinning takes place and
   relate some of its harmful effects.


3. Relate how the international community is
   working to protect the ozone layer.

A

lthough ozone (O 3 ) is a human-made pollut-
ant in the troposphere, it is a naturally
produced, essential component in the
stratosphere, which encircles our planet
some 10 to 45 km (6 to 28 mi) above the surface. The
ozone layer shields Earth’s surface from much of the
high-energy ultraviolet (UV) radiation coming from
the sun (ˆ}ÕÀiʙ°£x> and b in
Environmental Insight: The Ozone
Layer). If ozone disappeared from
the stratosphere, Earth would be-
come uninhabitable for most
forms of life, including humans.
A slight ozone thinning oc-
curs naturally over Antarctica for
a few months each year. In 1985,
however, the thinning was first ob-
served to be greater than it should
have been if natural causes were
the only factor inducing it. This
increased thinning, which occurs
each September, is commonly
referred to as the “ozone hole”
(ˆ}ÕÀiʙ°£xV). There, ozone levels decrease as much as
70 percent each year.
During the subsequent two decades the ozone-
thinned area continued to grow, and by 2006 it had
reached the record size of 29.5 million km^2 (11.4 million
mi^2 ), which is larger than the North American continent.
A smaller thinning was also detected in the stratospheric
ozone layer over the Arctic. In addition, world levels of
stratospheric ozone have been decreasing for several de-
cades (ˆ}ÕÀiʙ°£x`). According to the National Center
for Atmospheric Research, ozone levels over Europe and
North America have dropped almost 10 percent since
the 1970s.

ultraviolet (UV)

radiation Radia-

tion from the part of

the electromagnetic

spectrum with wave-

lengths just shorter

than visible light; can

be lethal to organ-

isms at high levels of

exposure.

ozone thinning

The removal of ozone

from the stratosphere

by human-produced

chemicals or natural

processes.

chlorofluoro-

carbons (CFCs)

Human-made organic

compounds that

contain chlorine and

fluorine; now banned

because they attack

the stratospheric

ozone layer.