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What is the ozone layer and why is it important?
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The ozone layer is a concentration
of ozone molecules in the stratosphere. About 90% of the planet's
ozone is in the ozone layer. The layer of the Earth's atmosphere
that surrounds us is called the troposphere.
The stratosphere, the next
higher layer, extends about 10-50 kilometers above the Earth's
surface. Stratospheric ozone is a naturally-occurring gas that
filters the sun's ultraviolet (UV) radiation. A diminished ozone
layer allows more radiation to reach the Earth's surface. For
people, overexposure to UV rays can lead to skin cancer, cataracts,
and weakened immune systems. Increased UV can also lead to reduced
crop yield and disruptions in the marine food chain. UV also
has other harmful effects.
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How does ozone depletion occur?
It is caused by the release of chlorofluorocarbons (CFCs)
and other ozone-depleting substances (ODS),
which were used widely as refrigerants, insulating foams, and
solvents. The discussion below focuses on CFCs, but is relevant
to all ODS. Although CFCs are heavier
than air, they are eventually carried into the stratosphere in
a process that can take as long as 2 to 5 years. Measurements
of CFCs in the stratosphere are made from balloons, aircraft,
and satellites.
When CFCs reach the stratosphere, the ultraviolet radiation
from the sun causes them to break apart and release chlorine
atoms which react with ozone, starting chemical cycles of ozone
destruction that deplete the ozone layer. One chlorine atom
can break apart more than 100,000 ozone molecules.
Other chemicals that damage the ozone layer include methyl
bromide (used as a pesticide), halons
(used in fire extinguishers), and methyl
chloroform (used as a solvent in industrial processes for
essential applications). As methyl bromide and halons are broken
apart, they release bromine atoms, which are 40 times more destructive
to ozone molecules than chlorine atoms.
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How do we know that natural sources are not responsible for ozone
depletion?
While it is true that volcanoes and
oceans release large amounts of chlorine, the chlorine from these
sources is easily dissolved in water and washes out of the atmosphere
in rain. In contrast, CFCs are not broken down in the lower atmosphere
and do not dissolve in water. The chlorine in these human-made
molecules does reach the stratosphere. Measurements show that
the increase in stratospheric chlorine since 1985 matches the
amount released from CFCs and other ozone-depleting substances
produced and released by human activities.
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What is being done about ozone depletion?
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In 1978, the use of CFC propellants in spray cans was banned
in the U.S. In the 1980s, the Antarctic "ozone
hole" appeared and an international science assessment
more strongly linked the release of CFCs and ozone depletion.
It became evident that a stronger worldwide
response was needed. In 1987, the Montreal Protocol was
signed and the signatory nations committed themselves to a reduction
in the use of CFCs and other ozone-depleting substances.
Since that time, the treaty has been amended to ban
CFC production after 1995 in the developed countries, and
later in developing countries. Today, over 180 countries have
ratified the treaty. Beginning January 1, 1996, only recycled
and stockpiled CFCs will be available for use in developed countries
like the US. This production phaseout is possible because of
efforts to ensure that there will be substitute
chemicals and technologies for all CFC uses.
EPA coordinates numerous regulatory
programs designed to help the ozone layer and continues
to be active in developing international ozone protection policies.
Individuals can also help,
primarily by ensuring that technicians working on air conditioning
and refrigeration equipment are certified by EPA, that refrigerants
are recaptured and not released, and by educating themselves
about the issue of ozone depletion. A
longer article explains EPA's ozone protection efforts in
more detail.
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Is there general agreement among scientists on the science of
ozone depletion?
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Yes. Under the sponsorship of the UN Environment Programme
(UNEP) and the World Meteorological Organization (WMO), the
scientific community issues periodic reports. Almost 300 scientists
worldwide drafted and reviewed the WMO/UNEP Scientific Assessment
of Ozone Depletion: 2002. An international consensus about the
causes and effects of ozone depletion has emerged. To obtain
a copy of the executive summary of the assessment, please visit
NOAA's
web site.
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Will the ozone layer recover? Can we make more ozone to fill in
the hole?
The answers, in order, are: yes and no. We can't make enough
ozone to replace what's been destroyed, but provided that we stop
producing ozone-depleting substances,
natural ozone production reactions should return the ozone layer
to normal levels by about 2050. It is very important that the
world comply with the Montreal Protocol; delays in ending production
could result in additional damage and prolong the ozone layer's
recovery. More detail on these questions is provided elsewhere
on this web site.
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