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The Ozone Balance
Over Earth's lifetime, natural processes have
regulated the balance of ozone in the stratosphere. Scientists
are finding that ozone levels change periodically as part
of regular natural cycles such as seasons, periods of solar
activity, and changes in wind direction. Concentrations are
also affected by isolated events that inject materials into
the stratosphere, such as volcanic eruptions.
Polar regions reflect the greatest changes
in ozone concentrations, especially the South Pole. The topography
of Antarctica is such that a stagnant whirlpool of extremely
cold stratospheric air forms over the region during the long
polar night. The air stays within this polar vortex all winter,
becoming cold enough to allow the formation of polar stratospheric
clouds.
Polar stratospheric clouds speed up the natural
process of ozone destruction by providing ice crystal surfaces
on which the destructive reactions take place. After the long
polar winter, ozone within this extremely cold vortex is very
vulnerable to the arrival of sunlight. As spring arrives,
major ozone losses occur. In the Southern Hemisphere, the
area of most severe ozone depletion is localized above Antarctica
and is generally referred to as the ozone hole. The hole appears
in the southern spring, following the continent's coldest
season and polar night.
Ozone depletion over the Arctic is not as well
defined as in Antarctica. The rugged topography results in
an air circulation pattern that is quite different from that
of the South Pole, but expeditions have shown that the atmospheric
chemistry of the two polar regions is very similar. In the
Northern Hemisphere, the polar vortex is not as strong. It
can break up and reform several times during the course of
winter. One air mass after another enters the polar darkness
and soon emerges back into low sunshine. Thus, a bit of ozone
is lost from each parcel of air, rather than a large amount
from one parcel as in the Southern Hemisphere.
The end result is that we are losing ozone
in both hemispheres. Ozone levels in the atmosphere have been
monitored from the ground since the 1950s and by satellite since
the 1970s. Regional total ozone levels measured from satellites
over Antarctica have decreased 30-50% since their monitoring
began.
Since ozone is created and destroyed by solar
UV radiation, there is some correlation of ozone concentration
with 11-year sunspot cycles. Sunspots emit high levels of
electromagnetic radiation. The increased UV radiation contributes
to ozone production. Sunspot variations only account for 2
to 4% of the total variation in ozone concentrations. Natural
cycles in ozone variation are also associated with the quasi-biennial
oscillation in which tropical winds switch from easterly to
westerly every 26 months. This cyclic change in wind direction
accounts for approximately 3% of the natural variation in
ozone concentration.
Global agreements, regulations and social
considerations
In 1973, two scientists from the University
of California at Irvine, Mario Molina and F. Sherwood Rowland,
discovered that man-made substances called chlorofluorocarbons
(CFCs) could play a major role in the destruction of stratospheric
ozone. Their findings were published in the journal Nature
in June 1974. Since that time there has been much controversy
surrounding the subject of ozone depletion. Researchers have
struggled to understand the nature and severity of the problem
through numerous scientific studies. Nations from all over
the world have come together and agreed to establish international
industrial regulations in hope of protecting the ozone layer.
Because of uncertainty about how global
environmental systems work, and because the people affected
will probably live in circumstances very much different from
those of today and may have different values, it is difficult
to predict how present-day actions will affect future generations.
To project or forecast the human consequences of global change
at some point in the relatively distant future, one would
need to know at least the following:
- The future state of the
natural environment
- The future of social and
economic organization
- The values held by the
members of future social groups
- The proximate effects
of global change on those values
- The responses that humans
will have made in anticipation of global change or in response
to ongoing global change
International
Agreements
Even the value systems and technological advancements
of present day nations are extremely different. Nonetheless,
efforts to predict and protect are underway. Despite their
differences, the international community has made significant
progress in addressing ozone depletion as a serious global
environmental problem. Through the 1985 Vienna Convention
for the Protection of the Ozone Layer, the 1987 Montreal Protocol
on substances that deplete the ozone layer, and the 1990 London
Amendments to the protocol, members from nations around the
world have committed to phasing out the production and consumption
of CFCs, and a number of related chemicals, by the year 2000.
Ozone depletion control started in the early
1970s, when the United States, along with a handful of other
Western countries, expressed concern over emissions from supersonic
transport (SST) aircraft and aerosol spray cans. Environmental
groups organized opposition to the development of the SST
and to the extensive use of aerosols. Public response led
to a sharp drop in the sales of aerosol products. The U.S.
Congress, prodded by government studies supporting the CFC
ozone depletion theory and its links to skin cancer, approved
the Toxic Substances Control Act of 1976, which gave the Environmental
Protection Agency (EPA) authority to regulate CFCs. In 1978,
the United States became the first country to ban the nonessential
use of CFCs in aerosols. However, the EPA ruled that other
uses of CFCs, such as refrigeration, were essential and lacked
available substitutes.
Ozone depletion emerged as a major international
issue in the 1980s. This occurred primarily as a result of
initiatives by the United Nations Environmental Programme
and actions of the international scientific and environmental
communities. A United Nations Environment Program to protect
the ozone layer was signed in Vienna in 1985, and a protocol
outlining proposed protective actions followed. The Vienna
convention of 1985 embodied an international environmental
consensus that ozone depletion was a serious environmental
problem. However, there was no consensus on the specific steps
that each nation should take. The Montreal Protocol, signed
in September 1987, stated that there would be a 50% cut back
in CFC production by 2000. The United States ratified the
Montreal Protocol in 1988. The 1990 London Amendments to the
protocol state that production of CFCs, CCl4, and halons will
be completely halted by the year 2000. The phaseout schedule
for other compounds was accelerated by 4 years by the 1992
Copenhagan agreement.
All human activity potentially contributes
directly or indirectly to global change. Earth's atmosphere
consists of a delicate balance of gases essential to life.
Throughout the history of the planet, the atmospheric gases
have been influenced by Earth processes and by the living
organisms from both the oceans and land, and natural changes
have occurred in the type of gases and their concentrations.
Anthropogenic activities are now believed to be causing rapid
changes in atmospheric composition on an accelerated time
scale. Due to extended human life expectancies and greater
population densities, the influence of humans will continue
to grow.
Scientists are now confident that stratospheric
ozone is being depleted worldwide. However, how much of the
loss is the result of human activity, and how much is the
result of fluctuations in natural cycles, still needs to be
determined. To understand global atmospheric changes, we need
to understand the composition and chemistry of Earth's atmosphere
and how they are affected by human activity. To create accurate
models, scientists must account for all of the factors affecting
ozone creation and destruction, and conduct simultaneous,
global studies over the course of many years.
Text, images and videos
courtesy of Distributed Active Archive Center at NASA's Goddard
Space Flight Center.
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