The Ozone Hole

The
Ozone Hole

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. 

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.

Consumption
of ozone depleting substances expressed as million tonnes of ozone depletion
potential (ODP), 1989-2009.

ODP is
a number that refers to the amount of ozone depletion caused by a chemical
substance. Consumption of ozone depleting substances has largely been reduced in
the past 20 years. Data
source: GEO Data Portal, compiled from the UNEP Secretariat for the Vienna
Convention and the Montreal Protocol (UNEP 2010)

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