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OCR for page 103
9
The Ozone Layer and
Ultraviolet Radiation
The first unmistakable sign of human-induced change in
the global environment arrived in 1985 when a team of British
scientists publishecl findings that stunned the world community
of atmospheric chemists. Joseph Farman, of the British Meteoro-
Togical Survey, and colleagues reported in the scientific journal
Nature that concentrations of stratospheric ozone above Antarc-
tica had plunged more than 40 percent from 1960s baseline
levels during October, the first month of spring in the Southern
Hemisphere, between 1977 and 1984.
Most scientists greeted the news with disbelief. Existing the-
ory simply had not predicted it. It meant that for several months
of the year a hole forms in the ozone layer, which protects an-
imals and plants from ultraviolet solar radiation. Suddenly it
seemed that the chemical processes known to deplete ozone
high in the earth's atmosphere were working faster and more
efficiently than predicted. The discovery brought home a critical
fact about the planet. No matter how much we learn about the
workings of the earth system, the unexpected can always occur.
Ground-based observations conducted by the National Oce-
anic and Atmospheric Administration (NOAA) since 1964 had
103
OCR for page 104
104
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
not revealed the drop. Measurements taken by the Total Ozone
Mapping Spectrometer aboard the Nimbus 7 satellite operated
by the National Aeronautics and Space Administration (NASA)
since 1978 reflected the change but had not yet been analyzed.
When researchers scrutinized the data, Farman's findings were
confirmed, but tough questions remained. What processes were
causing the hole? Would the thinning of the ozone layer spread
to other latitudes, or was it confined to the Antarctic?
To gather more information about antarctic ozone chemistry
and the ozone hole anct its causes, a team of scientists led by
atmospheric chemist Susan Solomon, of NOAA in Boulder, Col-
orado, headed off in 1986 on the first National Ozone Expedition
to the Antarctic. By 1987 they and other teams of researchers
had learned that the ozone over Antarctica had been reduced
by more than 50 percent of values recorded in 1979, the first Oc-
tober of satellite operation, and that at altitudes between 15 and
20 kilometers, depletion was as great as 95 percent. In 1988 tem-
peratures (which influence processes in the stratosphere) were
milder than in 1987, and in October the ozone declined by about
15 percent of 1979 values (aIreacly 20 percent below the baseline
values of the 1960s). in 1989 temperatures dropped again, and
ozone levels matched the severe depletion of 1987.
CHEMISTRY OF THE OZONE LAYER
Until the hole was discovered, scientists were fairly sure
that they understood the chemical processes at work in the
ozone layer. Oxygen molecules (O2), abundant throughout the
atmosphere, are split apart into individual atoms (O+O) when
energized by radiation from the sun. These atoms are free to
collide with other O2 molecules to form ozone (Oily. The partic-
ular configuration of the ozone molecules allows them to absorb
the sun's radiation in ultraviolet wavelengths that are harmful
to life if they penetrate to the earth's surface.
The ozone molecules formed by collision are partially re-
moved by other naturally occurring chemical reactions, and so
OCR for page 105
THE OZONE LAYER AND ULTRAVIOLET RADIATION
60
Ozone is ford In varying concentrations
from the earth's surface to a height of
some 60 kilometers. Its concentration ~n-
creases sharply In the stratosphere. Max-
~mum ozone concentrations occur at a
height of 25 to 35 kilometers but even
here never exceed about 10 parts per mil-
lion by volume. (Adapted from U.N. En-
v~ronment Programme. 1987. The Ozone
Layer, Fig. 2, p. 9. Copyright ~ 1987,
U.N. Environment Programme.)
105
Troposphere
o
0 2 4 6 8 10
OZONE CONCENTRATION (ppm)
the overall concentration of stratospheric ozone remains con-
stant. High above the stratosphere, the density of gases is so
low that oxygen atoms rarely find other molecules to collide
with, and ozone does not form in abundance. Below the ozone
layer, too little solar radiation penetrates to allow appreciable
amounts of ozone to form. Thus most of the worId's ozone is in
a stratospheric layer bulging with ozone at altitudes from 10 to
35 kilometers.
Closer to the ground, in the troposphere, ozone procluced
through a series of chemical reactions involving hydrocarbons
and nitrogen oxide emissions from vehicles and industrial activ-
ity is an effective greenhouse gas (in addition to having adverse
impacts on human health at high concentrations). Thus ozone
plays two very different roles in global environmental change:
one in the stratosphere as a shielct against harmful ultraviolet
radiation, and another nearer the ground in the troposphere as
a greenhouse gas and a health hazard.
It is now known that in addition to the naturally occurring
chemical reactions in the stratosphere, certain reactions involv-
ing chemical species of industrial origin, including chlorine and
bromine, also chemically destroy ozone molecules. Atmospheric
chemists F. Sherwood Rowland, of the University of California
at Irvine, and Mario T. Molina, now at Massachusetts institute of
OCR for page 106
106
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
Technology, first hypothesized the link between natural ozone
in the stratosphere and chlorine released into the atmosphere
from inclustrial sources. In 1973 they began to wonder: What
happens to the industrially produced chlorinated molecules that
are releasecT into the lower atmosphere ancT for which no nat-
ural mechanisms for removal are known? The only long-lived
natural source of chlorine in the earth's atmosphere is methyl
chioricle, which comes from the ocean and is present in the
atmosphere at low levels.
The researchers hypothesized in 1974 that increasing con-
centrations of chIorofluorocarbons (CFCs), synthetic compounds
that are chemically very stable in the lower atmosphere, rise
unchanged through the lowest atmospheric layer, the tropo-
sphere. Even though CFCs are produced mostly in the indus-
trialized countries of Europe and North America- where they
are used in a wide variety of applications such as for solvents
and refrigerants they mix throughout the Tower atmosphere,
so that there are as many CFC molecules over Antarctica as
over Colorado or Washington, D.C. The researchers surmised
that upon reaching the stratosphere, the CFCs encounter high-
energy ultraviolet light, which breaks them down, releasing
their chlorine atoms. The chlorine atoms can then engage with
ozone in a catalytic reaction in which each chlorine fragment
can destroy up to 100,000 ozone molecules before other chemi-
cal processes remove the chlorine from the atmosphere.
The hypothesis was borne out and improved by measure-
ments and observations. In 1970 chlorine was present in the
stratosphere at 1.2 parts per billion, and at about 3 parts per
billion in 1985. Were CFC use to continue at the 1985 rates
(an eventuality precluded in 1987 by an international agreement
known as the Montreal Protocol, described below), the strato-
sphere would contain about 3.2 parts per billion of total chlorine
in the year 2050; current models of the chemistry and physics of
the stratosphere suggest that at this concentration, total global
ozone would drop by 5 percent.
Rowland and Molina believed that most of the chlorine
molecules that reached the stratosphere would form relatively
OCR for page 107
THE OZONE LAYER AND ULTRAVIOLET RADIATION
107
inactive and harmless compounds. The ozone depletion would
occur gradually, they hypothesized, and might not be detected
for many years. As ozone was lost, more ultraviolet radiation
would reach the earth's surface. The researchers said two of
the CFCs-CFC-~l, which is widely used as a blowing agent in
plastic foam, and CFC-12, mostly used as a refrigerant were
particularly likely to destroy ozone because of their widespread
use.
These two CFCs alone are increasing in the atmosphere at
an annual rate of about 5 percent. They are part of a class of
chemicals known as halocarbons, many of which attack and
destroy stratospheric ozone and also contribute to global warm-
ing as greenhouse gases. Another chIorofluorocarbon, CFC-~13,
is used as a solvent for cleaning electronic circuitry. Its atmo-
spheric concentration is going up at an annual rate of about I]
percent. Scientists are beginning to eye concentrations of still
other synthetic halocarbons with suspicion. These include car-
bon tetrachIoride, which is used as a cleaning fluid and in CFC
production; methyl chloroform, used in solvents and adhesives;
and halon 1301 and halon 121, which are used in fire extinguish-
ers. Bromine, a chemical element that is related to chlorine and
which is released from compounds used in fumigants and some
fire extinguishers, is accumulating rapidly in the atmosphere.
Bromine is believed to cause 10 to 30 percent of the antarctic
ozone depletion.
STUDYING THE ANTARCTIC OZONE HOLE
In the year before the discovery of the ozone hole, scien-
tists were estimating that increasing use of chIorofluorocarbons
might cause reductions in the total ozone at high latitudes by
about one percent in the 1980s and by 5 to 10 percent 50 to
100 years from now. "While those numbers were disturbing,"
Solomon said, "they were nevertheless small enough that it was
hard to argue that they were even real. They were not being
observed."
That viewpoint changed along with scientists' faith in their
OCR for page 108
108
400
_ ·
-
~ 300
o
rot
o
0 200
100
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
400 .
^ /
/
~ /
_ 300
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c
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~ 200
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.
)
· · ~ -
South Pole
~ October Meon
.~ .
60 1970 1980 1990
Year
1' /
\\ ~-ID-~-
/~
TOM S
Oct.7, 1987
Em_
at_
Halley Boy
October Mean
100, , 1 , 1 , I ,
/1950 1960 1970 1980 1990
Year
\a, 300
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100\ _
1960
.
S yowa
October Mean
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1970 1980 1990
Year
Observational data that first indicated the existence of the antarctic ozone hole. (DU,
Dobson units; TOMS, total ozone mapping spectrometer.) (Reprinted, by permission,
from Global Change and Our Common Future. Copyright A) 1989, National Academy
Press, Washington, D.C.)
models in the mid-19SOs as observations poured in from the
coldest place on earth. Now, many scientists describe the antarc-
tic ozone hole as the first unequivocal evidence of ozone loss
due to man-made chlorine and one of the first clearly definable
effects of human-~nduced global change.
When the antarctic ozone hole was first discovered, little
was known about the antarctic stratosphere beyond the ozone
measurements themselves. Virtually no data were available on
the other chemical compounds present in the stratosphere, nor
was there detailed meteorological information. This information
was gathered rapidly by means of aircraft and state-of-the-art
instrumentation. In short order, scientists were able to measure
a broad range of atmospheric compounds, including chlorine
monoxide, chlorine dioxide, hydrochloric and nitric acid, nitric
oxide and nitrogen dioxide, and nitrous oxide. They found that
OCR for page 109
THE OZONE LAYER AND ULTRAVIOLET RADIATION
109
the ozone levels dip at about the same latitudes where levels of
chlorine monoxide ascend. As one researcher quipped, "These
measurements are better than a smoking gun. This is more like
seeing the guy pull the trigger." Scientists can now calculate
how much ozone would be lost with a given amount of chlorine
monoxide. The answer is strikingly similar to the levels of ozone
depletion observed. Scientists are convinced that the elevated
levels of chlorine and bromine account for much, if not all, of
the antarctic ozone depletion.
For most of the year, the atmosphere over Antarctica has
fairly high ozone concentrations. The ozone molecules are
formed over the tropics and are delivered along with chlorine
to the Antarctic, as well as to the Arctic, via atmospheric mo-
tions. In Antarctica, a circulation pattern known as the antarctic
polar vortex traps the ozone over the South Pole for several
months. It is within this vortex that scientists have measured
such shockingly low ozone concentrations during the first two
weeks of October, shortly after the beginning of the Southern
Hemisphere spring.
The explanation for the decrease lies in the combination of
ozone-destroying chemistry and weather conditions that favor
formation of the high, thin clouds known as polar stratospheric
clouds (PSCs). The stratosphere is extremely dry, and the ice
crystals that make up the clouds form only when temperatures
drop to -80°C (-~12°F) or lower. The clouds foster a basic
change in stratospheric chemistry by allowing reactions to occur
on surfaces rather than between gas molecules. The chemical
reactions that take place on these surfaces convert chlorine from
forms that do not react with ozone to other, less stable forms that
readily break up in the presence of sunlight and go on to destroy
ozone. Both cold temperatures and sunlight are critical to the
process leading to ozone depletion in the Antarctic. Antarctic
ozone is depleted not during the winter, when temperatures are
coldest and the South Pole is immersed in darkness, but in the
southern spring, after sunlight returns but temperatures are still
low.
Researchers describe a process something like this: Usu
OCR for page 110
110
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
ally chlorine in the stratosphere becomes trapped in so-called
reservoir compounds, such as hydrogen chloride and chlorine
nitrate, which themselves do not destroy ozone. Once the strato-
sphere becomes cold enough that cloud particles freeze, the ice
crystals provide surfaces on which reactions can occur: chlorine
nitrate (ClONO2) reacts with hydrochloric acid (MCI) present
on the ice surface, producing molecular chlorine (CI:) and nitric
acid (HNO31. The nitric acid remains bound to the ice, and the
molecular chlorine is quickly broken down Into atomic chlorine
(CI). The chlorine atoms react with ozone (03), destroying it
through the production of chlorine monoxide (ClO) and molec-
ular oxygen (O:~. In a vicious cycle, the chlorine monoxide
undergoes further reactions that re-form a chlorine atom, which
is then free to destroy another ozone molecule.
As researchers improve their understanding of the antarctic
ozone hole, it seems less ominous than it did at first for most
of the rest of the world. Over the mid-latitudes in the Southern
Hemisphere, however, the hole may be spreading. Recent re-
search suggests that in the late spring, when the antarctic vortex
breaks up, the winds transport the polar, ozone-depleted air into
lower latitudes. The record low ozone values found over the
Antarctic in October 1987 were followed by record low levels
over Australia and New Zealand that December as the South-
ern Hemisphere summer began. NASA's Ozone Trends Panel
reports that the effect may persist year round and that since
1979 ozone levels at all latitudes south of 60°S have decreased
by 5 percent or more.
For the most part, the hole has not spread outside of Antarc-
tica and the lower Southern Hemisphere because it is limited
by the seasons and the frigid temperatures required for the
formation of the ice-laden polar stratospheric clouds. Yet, the
insights gained during several years of intense data-gathering
have raised concern about ozone in the stratosphere over the
rest of the globe.
OCR for page 111
THE OZONE LAYER AND ULTRAVIOLET RADIATION
OZONE DEPLETION IN OTHER LATITUDES
111
With ozone levels over the South Pole dropping up to 50
percent or more for several months each year, scientists are eager
to know whether the same processes are operating to cleplete
ozone over the Arctic. Results gathered by scores of atmospheric
scientists using sensors aboard airplanes and balloons suggest
that the arctic stratosphere differs from the antarctic stratosphere
in a number of important ways that make a northern ozone hole
of the same magnitude unlikely.
Measurements from satellites and grouncI-based stations re-
veal ozone losses of about 5 to 10 percent at northern high
latitudes during the arctic winter. This is much smaller than in
the Antarctic for several reasons. For one, the arctic stratosphere
generally warms up much earlier in the spring than does the
antarctic, and the average temperatures are warmer. This means
that cold temperatures and the sunlight necessary for the forma-
tion of polar stratospheric clouds and the ozone depletion they
promote-overlap for a much shorter interval. Another factor
is that the arctic vortex is not as tight as the antarctic vortex. As
Rowland explains, air drifts across the pole, through the polar
darkness, undergoes some polar stratospheric cloud chemistry,
emerges into sunlight still in arctic winter-and loses a little
ozone. Then the vortex warms up, and the ozone loss in the
air mass stops. Meanwhile, another air mass is coming, and the
process of successive small losses is repeated throughout the
winter.
So far, the timing of the warming in the Arctic has of-
ferecl some protection against wholesale ozone depletion. But
researchers worry that this may not always be the case. in the
winter of 1988-1989, the arctic winter was unusually cold- the
coldest for at least 25 years. In January 1989 the polar vortex
was relatively stable, giving rise to conditions similar to those
in the antarctic winter stratosphere. In late January near the
Arctic Circle at Kiruna, Sweden, researchers measured an ozone
deficit very similar to the initial stages of ozone depletion in
early September in the Antarctic. With so much extra chlorine
OCR for page 112
112
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
in the stratosphere, repeated occurrences of such winters could
cause sudden ozone reductions over the Arctic and perhaps over
much of the Northern Hemisphere.
Although the unusual chemistry of polar stratospheric
clouds has made the Antarctic ozone layer more vulnerable
than the rest of the atmosphere, there is the particularly trou-
bling possibility that similar chemical reactions couIcl occur in
warmer latitudes. Temperatures outside the polar regions are
20° to 30°C too warm for ice clouds to form, but droplets of sul-
furic acid and water can support reactions involving the same
chlorine reservoir compounds that deplete stratospheric ozone
over the Antarctic and may help to explain part of the 3 percent
ozone decrease observed over the Northern Hemisphere in the
past two decades.
One prospect is that sulfurous particles emitted by a large
volcanic eruption could team up with chlorine compounds to
accelerate ozone destruction. Solomon and David I. Hofrnann,
of the University of Wyoming, describe a sharp drop in strato-
spheric ozone at mid-latitudes in 1982 after E! Chichon erupted
in Mexico, vaulting tons of volcanic debris into the upper at-
mosphere. At the time, the ozone drop was unexplained; atmo-
spheric chemists still thought in terms of gases, not surfaces of
particles. in light of the recent ozone studies, it seems likely that
the sudden increase in the availability of surfaces provided by
the volcanic debris allowect the industrially produced chlorine
compounds to break down into chlorine atoms that could then
destroy ozone, though more slowly than in the Antarctic.
Many factors other than industrial chemicals affect the con-
centration of stratospheric ozone. Ozone ebbs and flows along
with the cycle of sunspots. This solar cycle affects ozone be-
cause during the height of sunspot activity ultraviolet radiation
increases at wavelengths that can split apart an oxygen molecule
to form a molecule of ozone, causing a change of a few (! to
2) percent in ozone concentrations. The solar cycle was wind-
ing down between 1979 and 1986, but it is currently increasing.
The upswing in sunspot activity will lead to ozone production
that could partially cancel the chlorine-caused decline, but this
OCR for page 113
THE OZONE LAYER AND ULTRAVIOLET RADIATION
113
will be temporary. The researchers warn against a sense of false
security: After 1991 ozone could decrease again. They also sus-
pect that ozone responds to a 26- or 27-month cycle of varying
wind direction in which shifts in winds from the equatorial
stratosphere change the flow of ozone to the poles.
Still other factors fuel concern for the global ozone layer.
Rowland and colleagues report that the amount of water in the
normally arid stratosphere could increase by 25 percent by the
middle of the next century (because of water vapor produced
with oxidation of increasing amounts of methane in the atmo-
sphere) and contribute to increased cloud formation.
EFFECTS ON LIFE
The ozone layer is essential to life because it shields it from
damaging ultraviolet radiation. ironically, much less is known
about the biological effects of increased ultraviolet radiation
than about the chemical processes of ozone depletion in the
atmosphere. Researchers are trying to learn how humans, veg-
etation, anct aquatic ecosystems each may be affected by ozone
depletion.
Scientists do know that direct exposure to ultraviolet radi-
ation can damage the human immune system, cause cataracts,
and increase the incidence of skin cancer. The EPA estimated in
1986 that the incidence of skin cancers would rise 2 percent for
each ~ percent depletion of stratospheric ozone. (Today, mostly
because of lifestyles that encourage skin exposure to strong sun-
light, there are about 300,000 to 400,000 new cases of skin cancer
each year in the United States.)
As part of the effort to understand the effects on vegetation
and crops, researchers have tested more than 200 plant species,
two thirds of which show sensitivity to increased ultraviolet
exposure. Soybeans, one of civilization's staple food crops, is
particularly susceptible to ozone damage, as are members of
the bean and pea, squash and melon, and cabbage families.
Plant responses to ultraviolet radiation include reduced leaf size,
OCR for page 114
114
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
stunted growth, poor seed quality, and increased susceptibility
to weeds, disease, and pests.
Scientists are also in the early stages of understanding how
ultraviolet radiation might affect marine ecosystems and ani-
mals. Concern about these systems begins with phytoplankton,
microscopic marine algae that form the base of the marine food
web. Studies in the tropics have shown that significant amounts
of ultraviolet radiation can kill them, while lesser amounts can
slow photosynthesis and thus productivity. In Antarctica, this
could affect Frill, tiny crustaceans a notch up the food chain,
and then fish, birds, and marine mammals including seals and
whales. While water provides some protection from radiation,
crude estimates indicate that ultraviolet radiation can penetrate
to depths of 10 to 20 meters. Some phytoplankton are known to
be tolerant of ultraviolet radiation, whereas others cannot toler-
ate any. A likely response will be for tolerant species to replace
sensitive ones, though no one knows how this would affect the
fish that eat them.
NATIONS JOINING TO PROTECT
THE OZONE LAYER
The strong scientific consensus that CFCs deplete the ozone
layer prompted nations to come together in unprecedented co-
operation. The Montreal Protocol on Substances That Deplete
the Ozone Layer, negotiated in September 1987, calls for a 50
percent reduction in CFC production from 1936 levels by 1999.
Forty-nine nations including Canada, the United States, Japan,
and many nations in Europe, which together consume 80 per-
cent of the chemicals controlled have ratified the protocol.
An important factor in the discussion leading to the protocol
was the recognition that, because the chlorine compounds are so
stable, CFC molecules emitted today will exist to deplete ozone
for a century or more. The average lifetime of CFC-~l, for
instance, is believed to be about 75 years and for CFC-12, 110 to
140 years. With a 100-year average lifetime, Rowland explains,
37 percent of the CFCs will still be in the stratosphere after 100
OCR for page 115
THE OZONE LAYER AND ULTRAVIOLET RADIATION
115
years, about 13 percent after 200 years, and about 4 percent
after 300 years. Researchers agree that CFC concentrations will
continue to increase for 10 to 20 years after we stop releasing
them to the atmosphere because they will escape from existing
reservoirs such as automobile air conditioners, and because of
the lag between emission, arrival in the high stratosphere, and
decomposition. Thus, if the nations that ratified the protocol
comply with the terms established, average global ozone Tosses
will still continue, but at a slower rate. These facts, and the
growing bocly of scientific data on the threat to the ozone layer,
are prompting nations to consider a 100 percent reduction in
CFC production by the year 2000.
The protocol is a clelicate balance between the most up-
to-date scientific information, reliable inclustrial expertise, and
committed political leadership, all supported by strong and in-
formed public interest. The Montreal Protocol may prove to
be a mode! for actions that span national boundaries and in-
terests as the world addresses common environmental issues
such as greenhouse warming and other forms of global change.
It is perhaps the best illustration of the emerging role of sci-
entific information and scientists in discussions about policies
to manage global change. As Norway's former Prime Minister
and chairperson of the World Commission on Environment and
Development Gro Harlem BrundtIand explains, "The scientist's
chair is now firmly drawn up to the negotiating table, right next
to that of the politician, the corporate manager, the lawyer, the
economist, and the civic leader."
Representative terms from entire chapter:
ultraviolet radiation
