ver 60
years ago, chlorofluorocarbons (CFCs)
were invented in the United States, and they soon found many uses
throughout the world in refrigeration, air conditioning, and other
industrial processes. Due to scientific evidence that CFCs and other
chemicals destroy ozone in the upper
atmosphere, the United States, the country which has traditionally
been the largest emitter of CFCs worldwide, is rapidly scaling back
the use of these chemicals and phasing
out their production.
The ozone (O3) layer in the
stratosphere protects life on earth from exposure to dangerous levels
of ultraviolet light. It does so by filtering out harmful ultraviolet
radiation from the sun. When CFCs and other ozone-degrading
chemicals are emitted, they mix with the atmosphere and eventually
rise to the stratosphere. There, the chlorine and the bromine they
contain catalyze the destruction of ozone. This destruction is occurring
at a more rapid rate than ozone can be created through natural processes.
The degradation of the ozone layer leads to higher levels of ultraviolet
radiation reaching Earth's surface. This in turn can lead to
a greater incidence of skin cancer, cataracts, and impaired immune
systems, and is expected also to reduce crop yields, diminish the
productivity of the oceans, and possibly to contribute to the decline
of amphibious populations that is occurring around the world.
The chemicals most responsible for the destruction of the ozone
layer are chlorofluorocarbons,
carbon tetrachloride, methyl
bromide, methyl chloroform,
and halons. Chlorofluorocarbons
have long been widely used as coolants in refrigerators and air
conditioners and as foaming agents, solvents, and aerosol propellants.
Carbon tetrachloride and methyl chloroform are solvents used for
essential industrial applications. In the United States, carbon
tetrachloride is now used almost entirely as a feedstock for the
production of chlorofluorocarbons. Hydrogenated CFCs (HCFCs) have
many of the same uses as CFCs and are increasingly employed as interim
substitutes for CFCs. Halons
have been used in fire extinguishers.
Long predicted, the degradation of the ozone layer was dramatically
confirmed when a large hole in
the layer over Antarctica was reported
in 1985. Smaller but significant stratospheric decreases have
been seen over more populated regions
of the Earth. Subsequent research established that industrial chemicals
are responsible for the observed depletions of ozone over Antarctica
and play a major role in global ozone losses.
Human Activities
Chlorine and bromine are emitted to the atmosphere from both natural
and human sources. These very stable human-made chemicals are not
soluble in water and are not broken down chemically in the lower atmosphere.
Thus, they survive long enough to reach the stratosphere. The CFCs
and carbon tetrachloride are relatively unreactive in the lower atmosphere
(the troposphere) and move unscathed into the stratosphere where they
are decomposed by intense sunlight, releasing chlorine to catalyze
the destruction of ozone molecules.
Certain
ozone-depleting chemicals (HCFC-22 and methyl chloroform) are more
reactive in the troposphere and deliver less of their initial chlorine
load to the stratosphere. Halons also are generally reactive in the
troposphere and deliver only a fraction of their initial load of bromine
to the stratosphere, but bromine is 40 times more efficient at destroying
ozone than chlorine. Increasing attention is being focused on the
ozone-depleting role of methyl bromide,
which has three potentially major human sources (soil fumigation,
biomass burning, and the exhaust of automobiles using leaded gasoline),
in addition to a natural oceanic source.
U.S. production of ozone-depleting gases has declined significantly
since 1988, and has now reached levels (measured by their ozone
depletion potential) comparable to those of 30 years ago. Because
of the international agreements to decrease production and ultimately
to phase out production of CFCs and halons, total equivalent chlorine
(total chlorine and bromine, with adjustments to account for bromine’s
higher ozone depletion potential) in the troposphere peaked between
1992 and 1994 and has since decreased. Total chlorine abundance
in the stratosphere is at or near peak; stratospheric bromine is
likely still increasing. Increasing ozone losses are predicted for
the remainder of the decade, with gradual recovery by the mid-21st
century.
State of the Environment
Worldwide monitoring has shown that stratospheric ozone has been
decreasing for the past two decades or more. The average loss across
the globe totaled about 3 percent at northern middle latitudes and
6 percent at southern middle latitudes since the mid-1960s, with
cumulative losses of about 10 percent in the winter and spring and
a cumulative 8 percent loss in the summer and autumn over North
America, Europe, and Australia.
Since the late 1970s, an ozone hole has formed over Antarctica each
austral spring (September / October), in which up to 66 percent
of the total ozone is depleted. Record low global ozone levels were
recorded in 1992 and 1993. These lows were due, in part, to large
amounts of stratospheric sulfate particles
from the volcanic eruption of Mount Pinatubo in the Philippines
in 1991; the sulfate particles temporarily accelerated the ozone
depletion caused by human-made chlorine and bromine compounds.
As expected from the increasing use of CFC
substitutes, observations from several sites have revealed rising
concentrations of these compounds in the atmosphere. These substitutes
have short tropospheric lifetimes, which tends to reduce their impact
on stratospheric ozone as compared to CFCs and halons. However,
some are potent greenhouse gases.
Ozone Depletion Over Antarctica, Mean October
Values at Halley Station, 1956-2003 The link between a decrease in stratospheric ozone and an increase
in surface ultraviolet (UV) radiation at the Earth's surface has
been strengthened during the last several years by simultaneous
measurements of total ozone and UV radiation in Antarctica and the
southern part of South America during the period of the seasonal
ozone "hole." The measurements show
that when total ozone decreases, UV increases. Furthermore, elevated
surface UV levels in mid-to-high latitudes were observed in the
Northern Hemisphere in 1992 and 1993, corresponding to the low ozone
levels of those years. However, the lack of long-term monitoring
of surface UV levels and uncertainties introduced by clouds and
ground-level pollutants have precluded the unequivocal identification
of a long-term trend in surface UV radiation. The graph to the left shows mean October values of total ozone over Halley Station in Antarctica from 1956 through 2003, and the graphic below shows Halley's location in northwest Antartica. In 1956, the mean October total ozone was 311 dobson units (DU). Mean total ozone in October reached a high of 322 DU in 1962, and a low of 122 DU in 1993. By October 2003, mean total ozone was 159 DU.
Response
Reacting to the environmental threat of ozone depletion, the nations
of the world came together to create a global treaty, the Vienna
Convention for the Protection of the Ozone Layer. The agreement
entered into force in 1988 and the subsequent Montreal
Protocol on Substances that Deplete the Ozone Layer entered
into force in 1989. Currently over 180 countries are parties to
the Montreal Protocol. The parties to the Protocol decided on a
timetable for countries to reduce and to end their production and
consumption of eight major halocarbons. The Protocol also provides
a ten-year delay in this timetable for those developing countries
consuming less than 0.3 kilograms per capita.
The Montreal Protocol timetable was accelerated in 1990 and 1992.
Amendments were adopted in response to scientific evidence that
stratospheric ozone is depleting faster than predicted. As part
of an effort to speed the phase-out of production and consumption
of ozone-depleting chemicals, the parties to the Protocol decided
to provide technology transfer and funds from industrial to developing
countries. Under the accelerated schedule, the production of most
controlled gases is to cease by January
1, 1996. The developing countries, however, may receive residual
production from industrialized countries, not to exceed 15 percent
of 1986 levels. Some individual governments have committed to even
earlier phaseout deadlines.
The U.S. Environmental Protection Agency (EPA),
under authority of the U.S. Clean
Air Act Amendments of 1990, issued regulations for the phaseout
of production and importation of ozone-depleting chemicals controlled
under the Protocol through a marketable permit program. In addition,
EPA established controls on refrigerant recycling to prevent emissions
in both motor vehicle
and stationary systems
, a ban on nonessential
products, labeling
requirements, a program to review safe
alternative substances, and requirements to revise federal procurement
specifications. Under the regulations, surplus or recycled substances
can in general be stored to service existing machinery.
Because of the importance of the ozone layer and the complexity
of the chemical reactions affecting it, the condition of the ozone
layer must continue to be monitored.
Acknowledgements: This bulletin is first in a series of
environment indicator bulletins covering major topics of environmental
protection. It is a product of a collaboration between the World
Resources Institute and the Environmental Indicators Team of EPA's
Office of Policy, Division
of Environmental Statistics and Information. This report was prepared
in collaboration with the EPA Office of Air
and Radiation's Global Programs Division (GPD).
The World Wide Web version was created by GPD based on the original
hard copy.
For Further Information: For additional information, please
contact the contacts for each chart or Ms. Susan Auby, Mail Code
2152, Office of Environmental Information, USEPA, 1200 Pennsylvania
Avenue NW, Washington, DC 20460. Phone (202) 260-4901, e-mail: auby.susan@epa.gov.
A separate technical supplement provides
source data, references, and contact information for this bulletin.
Links to Other Information
Ozone Monitoring
UV Index and UV Monitoring
Health Effects of Ozone Depletion |