For
nearly a billion years, ozone molecules in the atmosphere have protected life on
Earth from the effects of ultraviolet rays.
The
ozone layer resides in the stratosphere and surrounds the entire Earth. UV-B
radiation (280- to 315- nanometer (nm) wavelength) from the Sun is partially
absorbed in this layer. As a result, the amount of UV-B reaching Earth’s
surface is greatly reduced. UV-A (315- to 400-nm wavelength) and other solar
radiation are not strongly absorbed by the ozone layer. Human exposure to UV-B
increases the risk of skin cancer, cataracts, and a suppressed immune system.
UV-B exposure can also damage terrestrial plant life, single cell organisms, and
aquatic ecosystems.
Credit:Center
for Global Environmental Research, National Institute for Environmental Studies
Japan
In the past 60 years or so human
activity has contributed to the deterioration of the ozone layer.
NASA
Graphic
Only
10 or less of every million molecules of air are ozone. The majority of these
ozone molecules resides in a layer between 10 and 40 kilometers (6 and 25 miles)
above the Earth's surface in the stratosphere.
Each
spring in the stratosphere over Antarctica (Spring in the southern hemisphere is
from September through November.), atmospheric ozone is rapidly destroyed by
chemical processes.
As
winter arrives, a vortex of winds develops around the pole and isolates the
polar stratosphere. When temperatures drop below -78°C (-109°F), thin clouds
form of ice, nitric acid, and sulphuric acid mixtures. Chemical reactions on the
surfaces of ice crystals in the clouds release active forms of CFCs. Ozone
depletion begins, and the ozone “hole” appears.
Over the course of two to three months, approximately 50% of
the total column amount of ozone in the atmosphere disappears. At some levels,
the losses approach 90%. This has come to be called the Antarctic ozone hole.
In
spring, temperatures begin to rise, the ice evaporates, and the ozone layer
starts to recover.
The Antarctic ozone
hole was discovered in 1985 by British scientists Joesph Farman, Brian Gardiner,
and Jonathan Shanklin of the British Antarctic Survey.
The
ozone "hole" is really a reduction in concentrations of ozone high
above the earth in the stratosphere. The ozone hole is defined geographically as
the area wherein the total ozone amount is less than 220 Dobson Units. The ozone
hole has steadily grown in size (up to 27 million sq. km.) and length of
existence (from August through early December) over the past two decades.
After
a series of rigorous meetings and negotiations, the Montreal Protocol on
Substances that Deplete the Ozone Layer was finally agreed upon on 16 september
1987 at the Headquarters of the International Civil Aviation Organization in
Montreal.
NASA/NOAA
satellite data showing the rise in stratospheric chlorine and corresponding
decline in ozone layer thickness from 1979 to 1997. As stratospheric chlorine
declined in response to enactment of the Montreal Protocol, the first stage of
ozone recovery began.
The
Montreal Protocol stipulates that the production and consumption of compounds
that deplete ozone in the stratosphere--chlorofluorocarbons (CFCs), halons,
carbon tetrachloride, and methyl chloroform--are to be phased out by 2000 (2005
for methyl chloroform). Scientific theory and evidence suggest that, once
emitted to the atmosphere, these compounds could significantly deplete the
stratospheric ozone layer that shields the planet from damaging UV-B radiation.
Man-made
chlorines, primarily chloroflourobcarbons (CFCs), contribute to the thinning of
the ozone layer and allow larger quantities of harmful ultraviolet rays to reach
the earth.
NASA:
Exploring Ozone Video
Global
Total Ozone Change
Satellite
observations show a decrease in global total ozone values over more than two
decades. The graph above compares global ozone values (annual averages) with the
average from the period 1964 to 1980. Seasonal and solar effects have been
removed from the data. On average, global ozone decreased each year between 1980
and the early 1990s. The decrease worsened during the few years when volcanic
aerosol from the Mt. Pinatubo eruption in 1991 remained in the stratosphere. Now
global ozone is about 4% below the 1964- to-1980 average.
The
graph above compares ozone changes between 1980 and 2004 for different
latitudes. The largest decreases have occurred at the highest latitudes in both
hemispheres because of the large winter/spring depletion in polar regions. The
losses in the Southern Hemisphere are greater than those in the Northern
Hemisphere because of the Antarctic ozone hole. Long-term changes in the tropics
are much smaller because reactive halogen gases are less abundant in the
tropical lower stratosphere.
