LEARNING OBJECTIVES
Ozone depletion in the stratosphere is recognized as a major environmental
problem with serious effects. After reading this chapter, you should
understand:
•
What ozone is and how ozone is naturally formed and destroyed in the
stratosphere.
• What the so-called ozone shield is, and why it is important.
•
How chemical and physical processes and reactions link emissions of
chlorofluorocarbons (CFCs) to stratospheric ozone depletion.
• What role polar stratospheric clouds play in ozone depletion.
•
•
Why ozone depletion is a long-term problem.
What the environmental effects of ozone depletion are, and what options are
available to minimize ozone depletion.
• Why international cooperation, including significant economic aid from
wealthy to less wealthy nations, is necessary to encourage future reduction or
elimination of emissions of ozone-depleting chemicals into the atmosphere.
Summary
• Concentration of atmospheric ozone has been measured for more than 70
years. In the last decade, measurements have been taken from
instruments mounted on satellites. Evaluation of the available data
shows a clear trend: Ozone concentrations in the stratosphere have
been decreasing since the mid-1970s.
• In 1974, Mario Molina and F. Sherwood Rowland advanced the hypothesis
that stratospheric ozone might be depleted as a result of emissions
of chlorofluorocarbons (CFCs) into the lower atmosphere. Major
features of the hypothesis are: CFCs are extremely stable and have a
long residence time in the atmosphere; eventually, the CFCs reach the
stratosphere, where they may be destroyed by highly energetic solar
ultraviolet radiation, releasing chlorine; the chlorine may then
enter into a catalytic chain reaction that depletes ozone in the
stratosphere. An environmentally significant result of the depletion
is that more ultraviolet radiation reaches the lower atmosphere,
where it can damage living cells.
• The Antarctic ozone hole was first reported in 1985 and since has
captured the imagination of people around the world. Of particular
importance to understanding the ozone hole are the complex reactions
that occur in the polar vortex and the development of polar
stratospheric clouds. Reactions in the clouds tend to denitrify the
air mass in the vortex; and during the polar spring, chlorine is
released to react in the catalytic ozone depletion cycle. The
reactions can be very rapid, producing the observed 70% reduction in
stratospheric ozone in only a few weeks.
• Tropical and midlatitude ozone depletion is also hypothesized. Polar
stratospheric clouds may be present above the tropics; in addition,
belts of sulfur dioxide–rich aerosol clouds may result from volcanic
eruptions. These clouds may be related to processes that denitrify
the atmosphere and facilitate ozone depletion.
• Millions
of tons of chemicals with the potential to deplete
stratospheric ozone are now in the lower atmosphere and working their
way to the stratosphere. As a result, if all production, use, and
emissions of these chemicals were stopped today, the problem would
continue. The good news is that the concentrations of CFCs in the
atmosphere have apparently peaked and are now static or in slow
decline.
• Potential environmental effects related to ozone depletion include
damage to Earth’s food chain, both on land and in the ocean, and
human health effects, including increases in skin cancers, cataracts,
and suppression of the immune system.
• Many nations around the world have agreed to the Montreal Protocol,
which will reduce global emissions of CFCs to 50% of the 1986 levels.
The agreement called for elimination of production of the chemicals
by 1996 for industrialized nations and by 2006 for developing
nations. A serious hurdle to compliance for some nations relates to
the economic fact that most chemical replacements for CFCs are more
expensive than CFCs. Therefore, it appears that financial aid will be
required if the less wealthy nations are to eliminate CFC use.
• Potential management strategies for the ozone depletion problem,
along with eliminating production of CFCs include: (1) collecting and
reusing CFCs, and (2) using substitutes for CFCs.
• Given the lifetimes of the ozone-depleting chemicals, people will
have to continue to live with higher levels of exposure to
ultraviolet radiation over the next few decades. Banning chemicals
that can deplete stratospheric ozone is a step in the right direction
and will ultimately result in the reduction of atmospheric ozone
depletion.