Ozone hole


The ozone hole is an annual thinning of the ozone layer over Antarctica,
caused by stratospheric chlorine. Other more moderate thinnings have also
been called "ozone holes", such as that over the North Pole during certain
weather conditions.

The discovery of the annual depletion of ozone above the Antarctic was first
announced in a paper by Joe Farman, Brian Gardiner and Jonathan Shanklin
which appeared in Nature in May 1985.

The most pronounced decrease in ozone has been in the lower stratosphere.
However, the ozone hole is most usually measured not in terms of ozone
concentrations at these levels (which are typically of a few parts per
million) but by reduction in the total column ozone, above a point on the
earth's surface, which is normally expressed in Dobson units. Marked
decreases in column ozone in the antarctic spring and early summer compared
to the early 1970s and before have been observed using instruments such as
the Total Ozone Mapping Spectrometer (TOMS).

Cause of the ozone hole

The cause of the ozone holes is generally agreed to be CFC
(Chlorofluorocarbon) compounds which break down (due to UV light) and become
free radicals containing chlorine high in the Earth's atmosphere. These
radicals then break down the ozone catalytically. Ozone destruction due to
chlorine radicals from CFCs can take place in the gas phase, but occurs
particularly rapidly on the surface of polar stratospheric clouds (PSC),
which form over the poles (particularly the south pole) during winter.

The photochemical processes involved are complex but well understood, with
UV radiation being involved in both the natural production and destruction
of ozone, as well as the breakdown of CFCs into free radicals and the
destruction of ozone by chlorine radicals. The role of sunlight in ozone
depletion is the reason why the antarctic ozone depletion is greatest during
spring; during winter, even though PSCs are at their most abundant, there is
no light over the pole to drive the chemical reactions.

CFCs are a byproduct of some chemical processes, and were also used in air
conditioning/cooling units. They were also used as aerosol propellants. What
makes CFCs so effective in breaking down ozone is that one CFC radical acts
as a catalyst and can break down many ozone molecules. Also these radicals
stay in the atmosphere for a very long time.

Scientists have increasingly been able to attribute the observed ozone
depletion to the increase of anthropogenic halogen compounds from CFCs, by
the use of complex chemical transport models and their validation against
observational data (e.g. SLIMCAT). These models work by combining satellite
measurements of chemical concentrations and meteorological fields with
chemical reaction rate constants obtained in lab experiments, and are able
to identify not only the key chemical reactions but also the transport
processes which bring CFC photolysis products into contact with ozone.

Increased UV due to the ozone hole

Although ozone, O3, is a minority constituent in the earth's atmosphere, it
is responsible for most of the main absorption of ultraviolet (UV) radiation
in the atmosphere. Correspondingly, a significant decrease in atmospheric
ozone could be expected to give rise to significantly increased levels of UV
near the surface.

Increases in surface UV due to the ozone hole can be partially inferred by
radiative transfer model calculations, but cannot be calculated from direct
measurements because of the lack of reliable historical (pre-ozone-hole)
surface UV data, although more recent surface UV observation measurement
programmes exist (e.g. at Lauder, New Zealand).

Because it is this same UV radiation that creates the ozone in the ozone
layer from O2 (regular oxygen) in the first place, a reduction in
stratospheric ozone would actually tend to increase photochemical production
of ozone at lower levels (in the troposphere), although the overall observed
trends in total column ozone are still a decrease, largely because ozone
produced lower down has a naturally shorter photochemical lifetime, so it is
destroyed before the concentrations could reach a level which would
compensate for the ozone reduction higher up.

Biological effects of increased UV

The main public concern regarding the ozone hole has been the effects of
surface UV on human health. As the ozone hole over Antarctica has in some
instances grown so large as to reach southern parts of Australia and New
Zealand, environmentalists have been concerned that the increase in surface
UV could be significant.

UVB (the higher energy UV radiation absorbed by ozone) is generally accepted
to be a contributory factor to malignant melanoma (skin cancer) -- for
example one study showed that a 10% increase in the UVB was associated with
a 19% increase in melanomas for men and 16% for women (Fears et al, Cancer
Res. 2002, 62(14):3992-6).

So far, ozone depletion in most locations has been typically a few percent.
Were the high levels of depletion seen in the ozone hole ever to be common
across the globe, the effects could be substantially more dramatic. For
example, recent research has analyzed a widespread extinction of
plankton 2 million years ago that coincided with a nearby supernova.
Researchers speculate that the extinction was caused by a significant
weakening of the ozone layer at that time when the radiation from the
supernova produced nitrogen oxides that catalyzed the destruction of ozone
(plankton are particularly susceptible to effects of UV light, and are
vitally important to marine food-webs).

Aside from the direct effect of ultraviolet radiation on organisms,
increased surface UV leads to increased tropospheric ozone, as noted above.
Paradoxically, at ground-level increased ozone is generally recognised to be
a health risk, as ozone is toxic due to its strong oxidant properties.

Public policy in response to the ozone hole

Environmentalists assert that the CFCs have caused so much damage to the
ozone layer that the use of CFCs should be banned. The full extent of this
damage CFCs have caused is not known and will not be known for decades;
however marked decreases in column ozone have already been observed (see
above).

In 1987, the Montreal Protocol was signed, controlling the emissions of
CFCs. To some extent, their role has been replaced by the less damaging
hydro-chloro-fluoro-carbons (HCFCs), although concerns remain regarding
HCFCs also.

Controversy regarding ozone science and policy

Any counter-measures which have a negative economic impact will remain a
controversial issue due to the strong economic interests involved, with key
questions regarding whether the scientific understanding is strong enough to
warrant the proposed countermeasures. In this context it is worth noting
that it is commonly believed that one reason for the relative ease of
introduction of the Montreal protocol was the availability of CFC
replacements at little extra cost.

The consensus amongst most atmospheric physicists and chemists is that the
scientific understanding has now reached a level where countermeasures to
control CFC emissions are justified, although the decision is ultimately one
for policy-makers and society.

Despite this general consensus, the science behind ozone depletion remains
complex, and some who oppose the enforcement of countermeasures point to
some of the difficulties experienced in these studies. For example:

   * Initial studies of the ozone hole were hampered with difficulties. Most
     notably, satellite measurements showing massive depletion of ozone
     around the south pole were initially rejected as unreasonable by data
     quality control algorithms; the ozone hole was only detected in
     satellite data when the raw data was reprocessed with modified
     processing algorithms following evidence of an ozone hole in in situ
     observations. This, however, was simply a problem with the
     data-processing algorithms for the satellite data and has long been
     corrected, and so has no bearing on the current situation.

   * Predictions of ozone remains a difficult science. The World
     Meteorological Organization Global Ozone Research and Monitoring
     Project - Report No. 44, which on balance comes out strongly in favour
     of the Montreal protocol, nonetheless notes that projections of ozone
     loss for the 1994-1997 period made in the UNEP 1994 Assessment had been
     an overestimate.

   * Although increased UVB has been shown to constitute a melanoma risk
     (see above), it has been difficult for statistical studies to establish
     a direct link between ozone depletion and increased rates of melanoma.
     Although melanomas did increase significantly during the period
     1970-1990, it is difficult to separate reliably the effect of ozone
     depletion from the effect of changes in lifestyle factors (e.g. time
     spent outdoors).

One prominent opponent of CFC reduction strategy has been the atmospheric
scientist Fred Singer, who has noted the scientific uncertainties such as
the lack of direct observations of surface UV increases (as mentioned
above). However, Singer goes far beyond this to claim, for example, that
"CFCs with lifetimes of decades and longer become well-mixed in the
atmosphere, percolate into the stratosphere, and there release chlorine" is
controversial [4], when there is clear evidence for it (though Singer is
wrong to use the word "percolate"). Singer, who is also a leading skeptic of
strategies on global warming, has consistently insisted that the remaining
level of scientific uncertainty about these issues is too high to justify
taking the control measures recommended by most other atmospheric
scientists, given their possible economic impact.

As noted above, Singer's objections go beyond reasonable skepticism.
Moreover, he is a retired scientist who has produced no new research since
the mid-1970s. His only recent publication in the peer-reviewed scientific
literature is a single technical comment published in 1994 in Science
magazine.[5] In 1995 testimony before the US Congress, Singer himself stated
that his last original, peer-reviewed research was in 1971. His
contributions to the recent debates over ozone deption and global warming
have consisted entirely of commentaries and letters, mostly self-published
or published in newspapers and other popular media rather than in scientific
journals. Environmentalists critical of Singer's role also allege a conflict
of interest, pointing out that he has financial ties to oil companies
(Exxon, Shell, ARCO, Unocal, and Sun Oil).