Nature and Sources of the Problem:
The stratosphere, located about 6 to 30 miles above the Earth, contains
a layer of ozone gas that protects living organisms from harmful ultraviolet
radiation (UV-b) from the Sun. Over the past 2 decades, however, this protective
shield has been damaged. Each year, an "ozone hole" forms over
the Antarctic, and ozone levels fall to 70 percent below normal. Even over
the U.S., ozone levels are about 5 percent below normal in the summer and
10 percent below normal in the winter. The figure below shows ozone levels
over North America in dobson units (DU) in March 1979 and March 1994. One
hundred DU of ozone would form a layer 1 millimeter thick at the Earth's
surface. Each color band represents an area with a similar amount of ozone
overhead. Comparing the colors of the bands over a particular city, such
as Seattle, shows lower ozone levels in 1994 than in 1979. This figure
is a snapshot in time that shows one example of reduced ozone levels. Long-term
trends are based on numerous data sets taken over several years, as opposed
to single observations.
This figure compares of satellite measurements of ozone levels over
North America. Each color band represents an area with a similiar amount
of ozone overhead; lower levels of dobson units indicate less protective
ozone overhead. Ozone trends are based on detailed statistical analysis
of large data sets, and not on simple graphs like these.
As the ozone layer thins, more UV-b radiation reaches the Earth. In
1996, scientists demonstrated for the first time that UV-b levels over
most populated areas have increased. Scientists have linked several substances
associated with human activities to ozone depletion, including the use
of chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl
chloroform. These chemicals are emitted from home air conditioners, foam
cushions, and many other products. Strong winds carry them through the
lower part of the atmosphere, called the troposphere, and into the stratosphere.
There, strong solar radiation releases chlorine and bromine atoms that
attack protective ozone molecules. Scientists estimate that one chlorine
atom can destroy 100,000 ozone molecules.
Health and Environmental Effects:
Some UV-b reaches the Earth's surface even with normal ozone levels. However,
since the ozone layer normally absorbs most UV-b radiation from the Sun,
ozone depletion is expected to lead to increases in harmful effects associated
with UV-b radiation. In humans, UV-b is linked to skin cancer, including
melanoma, the form of skin cancer with the highest fatality rate. It also
causes cataracts and suppression of the immune system.
The effects of UV-b radiation on plant and aquatic ecosystems are not
well understood. However, the growth of certain food plants can be slowed
by excessive UV-b radiation. In addition, some scientists suggest that
marine phytoplankton, which are the base of the ocean food chain, are already
under stress from UV-b radiation. This stress could have adverse consequences
for human food supplies from the oceans. Because they absorb CO2
from the atmosphere, significant harm to phytoplankton populations could
increase global warming (see following section on Global Warming and
Programs to Restore the Stratospheric Ozone Layer:
In 1987, 27 countries signed the Montreal Protocol, a landmark treaty that
recognized the international nature of ozone depletion and committed the
world to limiting the production of ozone-depleting substances. Today,
over 150 nations have signed the protocol, which has been strengthened
twice and now calls for the elimination of these chemicals.
The 1990 Clean Air Act Amendments established a U.S. regulatory program
to protect the stratospheric ozone layer. In January 1996, U.S. production
of many ozone-depleting substances virtually ended, including CFCs, carbon
tetrachloride, and methyl chloroform. Production of halons ended in January
1994. EPA regulations control the handling and emissions of CFCs and the
use of substitutes. Many new products that are either harmless or less
damaging to the ozone layer are now gaining popularity. For example, computer-makers
are using ozone-safe solvents to clean circuit boards, and automobile manufacturers
are using HFC-134a, an ozone-safe refrigerant, in new motor vehicle air
conditioners. In some sectors, the transition away from ozone-depleting
substances has already been completed.
Trends in Stratospheric Ozone Depletion:
Scientific evidence shows that the approach taken under the Montreal Protocol
has been effective. In 1995, measurements showed that the tropospheric
concentrations of methyl chloroform had started to fall, indicating that
emissions had been greatly reduced. Tropospheric concentrations of other
ozone-depleting substances, like CFCs, are also beginning to decrease.
It takes several years for these substances to reach the stratosphere and
release chlorine and bromine. For this reason, stratospheric chlorine levels
are expected to continue to rise, peak between 1997 and 1999, and then
slowly decline. Because of the stability of most ozone-depleting substances,
chlorine will be released into the stratosphere for many years, and the
ozone layer will not fully recover until well into the next century.
In 1996, scientists developed a new technique allowing them to draw
conclusions about UV-b radiation at ground level. According to satellite-based
trend analyses, major populated areas have experienced increasing UV-b
levels over the past 15 years. As shown by the figure below, at latitudes
that cover the U.S., UV-b levels are 4 to 5 percent higher than they were
10 years ago.
|A 1996 study using satellite-based analyses of UV-b
trends demonstrated that UV-b levels had increased at ground level.
This figure shows the percent increases in average annual UV-b reaching
the surface over the past 10 years. UV-b incidence is strongly dependent
on latitude. At latitudes that cover the U.S., UV-b levels are 4 to
5 percent higher than they were 10 years ago.