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20 December 2009
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Over 190 countries are signatory to the Montreal Protocol
and are legally bound to phase out ozone-depleting substances (ODSs) in
accordance with it.
In the upper troposphere and lower
stratosphere, from about 12 to 50 km altitude, much of the oxygen in the air
is converted to ozone by the action of radiation from the sun. This is the
so-called ozone layer. This is a continuous process of both the production
and disintegration of ozone molecules, in a natural equilibrium. The total
quantity of ozone at any one time is very small, but it does serve to stop
intense ultraviolet radiation (this is the energy used to convert the oxygen
to ozone) from reaching the earth's surface. Without the ozone layer, life
on earth, as we know it, would be impossible.
This was realised in the 1920s by Dobson, who
devised instruments for measuring the equivalent thickness of the layer, the
first monitoring station being set up at Davos, Switzerland in 1924. In
1970, Paul Crutzen hypothesised that exhaust gases from stratospheric flying
may upset the natural balance of the ozone layer. In 1974, Mario Molina and
Sherwood Rowland added that certain chlorine-containing molecules, such as
in chlorofluorocarbons, could also have a similar effect. (All three of
these scientists shared the 1995 Nobel Prize for Chemistry for this work.)
Many scientists were alarmed at this possibility, although others were
opposed to the theories. In 1977 and 1978, a scientist, Joseph Farman, in
the British Antarctic Survey at Halley Bay observed that the ozone layer
became thin in the Antarctic spring. This was not confirmed by Nimbus 7
satellite observations. Not believing his Dobson instruments, he obtained a
new one and observed the same phenomenon in succeeding years. Consultation
with NASA revealed that the Nimbus 7 instruments were calibrated to ignore
abnormally low ozone levels as faulty readings. By sifting through the
archives, it was found that the Nimbus 7 records, over several years, had
shown ignored levels over the Antarctic from September to October. Thus the
so-called "ozone hole" was discovered. Farman published his results in 1985
and this was the first scientific evidence that the ozone layer was thinning
to dangerous levels, vindicating the theoretical work of the other
scientists.
This situation was very alarming, even though
there was no formal scientific proof, at that time, that the ozone depletion
theories were explaining the observations. The groundwork for international
studies were laid at an ad hoc conference in Vienna, 1985, and this
culminated in the Montreal Protocol being signed in September 1987, with a
programme to phase down the emissions of what were believed to be
ozone-depleting gases and to support intense scientific studies as to the
causes and effects of the depletion. Results were quick to arrive. In the
following year, the formal scientific proof that CFCs and similar gases were
responsible was obtained. It was also shown that the situation was not only
worse than had been imagined, predictions showed that the Protocol
phase-down programme would not stop the depletion or even significantly slow
it down. Later amendments introduced drastic measures to totally ban most of
the gases in a very short time scale.
Nowadays, the mechanisms of ozone-depletion are
very well known and the main culprits are halons and CFCs, plus a few other
substances, used mainly for:
-
aerosol bomb propulsion
-
some fire extinguishers
-
refrigeration and freezing
-
air conditioners (mobile and fixed)
-
solvents
-
pesticides
-
plastic foams
-
miscellaneous uses
These uses represent over 90 per cent of the
man-made ozone-depletion.
Very bad, and getting worse every year! The
problem is that most of the gases and vapours that cause ozone-depletion
take a long time (10 - 15 years) to homogenise throughout the atmosphere,
but they can last up to 100 years for CFCs and 300 years for halons (to
remove roughly half of them). Fortunately, one substance that was used very
massively as a solvent, 1,1,1-trichloroethane, has a half-life of only 12
years, so that, as it was banned in most major using countries in 1996, the
levels of this compound in the air are already measurably improving.
It should be noted that the "ozone hole" is not
really a hole but simply an important thinning of the ozone levels over the
Antarctic in each southern spring. The cause is quite complex but is simply
driven by a weather situation, in conjunction with the man-made chemicals in
the stratosphere. On average, it has become worse each year since it was
first discovered. 2007 and 2003 were the worst yet, with 2006 a very close
third. In 2002, a peculiar event happened, in that there were two "holes",
each of slightly less individual importance. The 2004 "hole" was similar to
the 2003 one, very slightly less in depletion but covering a larger area.
Even with the application of the Montreal Protocol, it will become
significantly worse before it gets better, probably peaking around 2040 -
2050 and then starting to improve. It is expected to heal itself by about
the end of this century, provided the Protocol is followed to the letter.
This is a map of the "hole" as it
was measured in September 2003, just after the equinox. To explain it, the
contours are in Dobson Units (DU), which is defined as the column thickness
of ozone in 1/100ths of a millimetre, with the ozone converted to NTP (760
mm Hg pressure, 293.15K or 20°C temperature). The global average thickness
is 380 - 400 DU or 3.8 - 4 mm, which shows how little ozone there is, at the
best of times, to protect us. It can be seen that, with the exception of a
patch centred to the south of Australia, the whole of the area of this map
has some considerable thinning, down to under 100 DU at the Pole itself.
The British Antarctic Survey
issued the following statement at about the time of this map:
"Temperatures in the ozone layer are low enough that Polar Stratospheric
Clouds (PSCs), the key precursors to significant ozone depletion, have
formed widely over the continent. Ozone levels dropped quickly during
August and September with the return of sunlight and depletion exceeds
55% in places. The ozone hole grew rapidly and peaked in size at around
28 million square kilometres (roughly twice the size of Antarctica) in
mid September. It was larger than previously for the time of year in
August and early September, but the maximum size only equalled the all
time record. Ozone sonde flights from Rothera show substantial depletion
between 12 and 23 kilometres, with effectively 100% depletion at times
at some altitudes."
Although the "ozone hole" has captured the
public's imagination, it is not this that is causing concern; it is
depletion over inhabited areas. In Argentina, Chile, New Zealand and
Australia, the situation is very preoccupying, with peak depletion levels
likely to cause damage to inhabitants, marine life, livestock and plant
life. However, there is 10 - 15 per cent depletion over most of Europe, Asia
(N. of the Himalayas) and North America, as well. This is already causing
problems, such as a significant increase of skin cancer (melanomas).
Of course, the depletion itself is harmless; it
is the increased levels of ultraviolet light that causes the problems. These
include, in humans, skin cancer, cataracts and reduced immune responses.
Livestock in affected areas suffer similar problems plus a reduced growth
rate. Some plants, notably soya beans, produce smaller crops. Possibly the
worst long-term effect is to reduce the fertility of krill, which is the
basic bottom of the food chain for marine life on which the viability of
life in W. South America is highly dependent. It is not impossible that
this, combined with over-fishing, will cause severe famine in coastal areas,
especially of Chile and Peru.
With care, the effects of ozone-depletion will be felt, to some extent,
worldwide until the end of the 21st century. It is estimated that the peak
effect will occur-mid-century, on condition that the Montreal Protocol and
its amendments are followed to the letter by every country. If laxity
occurs between now and 2050, then the consequences may be very severe.
Unfortunately, there are signs that "ozone fatigue" (laxity because of
people being sick of hearing about ozone depletion) is setting in and some
nations are turning a blind eye to users disobeying the rules or applying
for frivolous exemptions (and sometimes obtaining them!). Another lacunary
feature within the Protocol is the lack of a foolproof means of adding new
controlled substances of a different chemical family from the traditional
ones, even though they are known to be just as ozone-depleting and are used
industrially in large quantities.
UNEP Ozone Secretariat
Substances
which damage the ozone layer
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