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The Antarctic
Ozone Hole
The seasonal
thinning of the ozone layer of the earth's atmosphere above Antarctica,
so allowing abnormal amounts of ultra-violet light to reach the earth's
surface in those regions.
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1/ What is ozone? |
Ozone is a gas*
made of oxygen atoms. Usually oxygen atoms hang around in pairs
- this is the sort of oxygen that we breathe and that helps things to
burn. Oxygen sometimes however will form a molecule with three oxygen
atoms, this is what we call ozone:
O2
- two
oxygen atoms - ordinary common or garden oxygen
O3
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three oxygen atoms - Ozone
Ozone has a the particularly
useful characteristic that it can absorb large quantities of ultra-violet
(uv) light - more of that soon.
*Jumping Jack Flash is
also a gas (ask your Dad)
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2/ Where is the ozone? |
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Most of the ozone on earth
isn't on earth at all, but in the layer of the earth's atmosphere called
the stratosphere. This is the upper layer of the atmosphere and
starts between 12.9 to 19.3 km (8 to 12 miles) above our heads and goes
upwards to almost 50 km (30 miles). The stratosphere has virtually no
clouds or other form of weather, it's thinnest at the equator and thickest
at the poles.
Ozone is formed in
the stratosphere by the action of sunlight on oxygen molecules.
In particular it is the high energy ultra-violet light in sunlight that
is effective, it causes an oxygen molecule to split into two oxygen
atoms:
O2
---------> O + O
One of these then joins
with another oxygen molecule to form a molecule of ozone:
O +
O2 --------->
O3
Ozone may also be destroyed
by joining with a lone oxygen atom to get back to oxygen again. Ultra-violet
light is required for ozone to form in the stratosphere, but then the
ozone absorbs the ultra-violet light so stopping it reaching deeper
into the earth's atmosphere. The result is that levels of ozone are
greatest at around 20km up. This is good news for us as it stops
lots of ultra-violet light getting through to us and also keeps the
ozone high up in the atmosphere out of the way.
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3/ Isn't ozone bad news? I'm sure I saw something on the weather forecast.... |
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Ironically, at ground level ozone is very bad news.
It is a major component of photochemical smog. It is caused by the
effect of ultra-violet light on nitrogen oxide from vehicle exhausts
and so particularly affects built up areas in regions of high sunshine.
Ozone affects lung function, it can aggravate asthma
and other chronic respiratory tract and lung diseases and can reduce
lung function in the short term or even permanently on repeated exposure.
Ozone has an effect like sunburn on the lining of the respiratory tract
damaging the cells.
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4/ Why is a ozone hole a
problem? |
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Ozone in the stratosphere is nicely out of the
way and has the wonderful benefit to life on earth that it specifically
absorbs the harmful ultra-violet light from the sun while letting other
light wavelengths through.
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Though we talk about a "hole"
in the ozone layer, it's not really a hole at all, just
a thin bit. Ozone is spread thinly throughout the stratosphere
- and in low quantities too - if all the ozone above
your head was collected together in a continuous layer,
it would only be about 3-5mm (1/8 of an inch) thick.
It's better to think of the
ozone as being like orange squash in a glass of water where
the water is the rest of the atmosphere. When we talk about
an "ozone hole" we actually mean a region where the squash
is more diluted than we'd like it to be.
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If there is a hole in the ozone layer then this means
that more harmful ultra-violet rays get through than are good for us
or many other life forms, plant or animal. Too much ultra-violet
light can result in:
 | Skin cancer |
| Eye damage such as cataracts |
| Immune system damage |
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Reduction in phytoplankton
in the oceans that forms the basis of all marine food
chains including those in Antarctica. |
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Damage to the DNA
in various life-forms So far this has been as observed in
Antarctic ice-fish that lack pigments to shield them from the ultra-violet
light (they've never needed them before) |
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Probably other things
too that we don't know about at the moment.
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5/ Why does
a ozone hole form over Antarctica? |
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The ozone hole is caused by the effect of pollutants
in the atmosphere destroying stratospheric ozone. During the Antarctic
winter something special happens to the Antarctic weather.
| Firstly, strong winds blowing around the continent
form, this is known as the "polar vortex" - this isolates
the air over Antarctica from the rest of the world. |
| Secondly, special clouds form
called Polar Stratospheric Clouds. Clouds don't normally form
in the stratosphere and these turn out to have the effect of concentrating
the pollutants that break down the ozone, so speeding the process
up. |
 Polar
stratospheric clouds at about 80,000 feet altitude. These
are the highest flying of all clouds and only occur in polar
regions where the temperature in the upper atmosphere dips
below minus 100F. They are sometimes called "nacreous clouds"
as they are coloured like the nacre of mother-of-pearl with
coloured bands that move the position of cloud and observer.
Picture courtesy NSF
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By the time spring arrives and the
sun comes back after the long polar night, the ozone levels are severely
depleted around the Antarctic continent causing the "ozone hole". Unfortunately,
there then follows a particularly long period of high sunshine and long
days, just to make the effect of the ozone hole worse.
The concentration of ozone in
the atmosphere is measured in "Dobson Units", the average
concentration of ozone in the atmosphere is about 300 Dobson Units.
The ozone hole is considered to be wherever the concentration drops
below 220 Dobson Units.
The following pictures are
provided courtesy of NASA. They show the extent of ozone thinning.
Dark blue and purple colors correspond to the thinnest ozone, while light
blue, green, and yellow pixels indicate progressively thicker ozone.
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October 1999 (average)
Historically, the Antarctic ozone hole was largest during
October. In recent years however, September has been the
peak month. |
September 7th 2000
The ozone hole grew quicker than usual and exceptionally
large. By the first week in September the hole was the largest
ever at that time. For the first time
it reached towards South America and to regions of high population. |
September 2006 average
From September 21 to 30, 2006, the average area of the
ozone hole was the largest ever observed, at 10.6 million
square miles. A little over a week later, instruments
recorded the lowest concentrations of ozone ever observed
over Antarctica, making the ozone hole the deepest it had
ever been. |
The ozone hole builds up over the winter months,
peaking at around September and breaking up again by December, this
data set from 2005.
July |
August |
September |
October |
November |
December |
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6/ What causes the ozone depletion? |
 Ozone
is mainly broken down by chemicals called ChloroFluoroCarbons CFC's
and also by nitrogen oxides. CFC's ironically were first used in large
quantities because they were thought to be safe and inert (unreactive)
chemicals. They are a group of chemically similar gases used in refrigeration
systems, air conditioners, aerosols, solvents and in the production
of some types of packaging. Nitrogen oxides are a by-product of fuel
burning, e.g. aircraft exhausts.
CFC's don't occur naturally, they are man-made chemicals.
They are very useful when they are where they are supposed to be, and
doing what they are supposed to be doing. But once released into the
atmosphere they are a serious pollutant. The problem is it took us
many years to realise this during which we thought they were
perfectly harmless, but in fact were building up to levels that will
take decades for them to disappear again even if we stop producing
them altogether.
The actual reactions that destroy ozone are very complicated.
They take place on the surface of the ice particles of the Polar Stratospheric
Clouds and it takes only a small amount of CFC to destroy an awful
lot of ozone.
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7/ Is the
ozone hole going to stay over Antarctica? |
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Since the annual thinning of the ozone layer over
Antarctica was first discovered, measurements have been carried out
in all regions. Ozone
depletion has been measured everywhere in the world except in the tropics.
Depletion is usually worse the further from the equator and recently
an Ozone hole (as defined by a distinct area of very low ozone levels)
has been detected above the North Pole in the arctic.
There
is a lot to learn about the breakdown of ozone in the atmosphere. Warmer
region, non polar depletion of ozone in particular is not properly understood.
So for
the time being the "ozone hole" seems to be an Antarctic phenomena,
but a less severe thinning of the ozone layer is pretty much a world-wide
thing. How acute and important it will be in the future is not known.
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8/
Can the ozone hole recover? |
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The way to stop the formation, growth and spread
of ozone thinning is to reduce the production of those chemicals that
cause the destruction of ozone, namely CFC's and nitrogen oxides.
In 1987, the Montreal Protocol was signed by
many nations whereby those nations that signed agreed to reduce their
emissions of CFC's to a half (of the 1987 levels) by 2000.
Potential problems come from nations that do not see
the reduction of CFC's to be a priority, and also from the huge quantity
of refrigeration and air conditioning systems in the world that still
contain CFC's. If they are not disposed of correctly, then the CFC's
will escape into the atmosphere and continue to destroy ozone.
The problem is far from settled and is under investigation
by research teams all over the world. The latest estimates are that
as long as production and release of CFC's is regulated properly, global
ozone levels should recover by 2050.
Latest Ozone news
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| 17th September
2005 "The 2005 hole is larger and deeper
than the holes that formed when the discovery was made but the situation
would be much worse if the Montreal Protocol had not come into force.
This agreement shows us that global action by governments to stop the
release of ozone depleting chemicals really can help society to successfully
mitigate a global environmental problem. We are still experiencing large
losses of Antarctic ozone each spring because CFCs and other chemicals
live for a long time in our atmosphere. However, the ban ensures that
we will see an improvement in the future. We now need to take similar
actions to control greenhouse gasses, otherwise we will bequeath future
generations a significantly different climate from that of today."
- Jonathan Shanklin - one of the original discoverers of the ozone
hole
Covering an area of around 22 million square kilometres this year's
hole is a little smaller than the record-breaking event in August 2003.
Measurements made during August and September at BAS Halley and Rothera
Research Stations reveal a 50% reduction on normal ozone levels over
the base of the Antarctic Peninsula and the Weddell Sea, and a 20% reduction
over the tip of South America and the Falkland Islands. The increased
ultra violet light reaching the surface poses a medical hazard to people
living under the ‘hole' and without suitable protection people face
the prospect of rapid sunburn and potentially more serious skin damage.
British Antarctic Survey press release
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