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Subject: Ozone Depletion FAQ Part IV: UV Radiation and its Effects
This article was archived around: 24 Dec 1997 20:51:43 GMT
Last-modified: 16 Dec 1997
Subject: How to get this FAQ
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Subject: Copyright Notice
* Copyright 1997 Robert Parson *
* This file may be distributed, copied, and archived. All such *
* copies must include this notice and the paragraph below entitled *
* "Caveat". Reproduction and distribution for personal profit is *
* not permitted. If this document is transmitted to other networks or *
* stored on an electronic archive, I ask that you inform me. I also *
* ask you to keep your archive up to date; in the case of world-wide *
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* request that you inform me before including any of this information *
* in any publications of your own. Students should note that this *
* is _not_ a peer-reviewed publication and may not be acceptable as *
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* to the original published source, not to this document. *
Subject: General Remarks
This file deals with the physical properties of ultraviolet
radiation and its biological consequences, emphasizing the
possible effects of stratospheric ozone depletion. It frequently
refers back to Part I, where the basic properties of the ozone
layer are described; the reader should look over that file first.
The overall approach I take is conservative. I concentrate on what
is known and on most probable, rather than worst-case, scenarios.
For example, I have relatively little to say about the
effects of UV radiation on plants - this does not mean that the
effects are small, it means that they are as yet not well
quantified (and moreover, I am not well qualified to interpret the
literature.) Policy decisions must take into account not only the
most probable scenario, but also a range of less probable ones.
will probably do, but also the worst that he could possibly do.
There have been surprises, mostly unpleasant, in this field in the
past, and there are sure to be more in the future. In general,
_much_ less is known about biological effects of UV-B than about
the physics and chemistry of the ozone layer.
Subject: Caveats, Disclaimers, and Contact Information
| _Caveat_: I am not a specialist. In fact, I am not an atmospheric
| scientist at all - I am a physical chemist studying gas-phase
| reactions who talks to atmospheric scientists. In this part in
| particular I am well outside the range of my own expertise.
| I have discussed some aspects of this subject with specialists,
| but I am solely responsible for everything written here, including
| any errors. On the other hand, if you find this document in an
| online archive somewhere, I am not responsible for any *other*
| information that may happen to reside in that archive. This document
| should not be cited in publications off the net; rather, it should
| be used as a pointer to the published literature.
*** Corrections and comments are welcomed.
- Robert Parson
Department of Chemistry and Biochemistry,
University of Colorado (for which I do not speak)
Subject: TABLE OF CONTENTS
How to get this FAQ
Caveats, Disclaimers, and Contact Information
TABLE OF CONTENTS
1.) What is "UV-B"?
2.) How does UV-B vary from place to place?
3.) Is UV-B at the earth's surface increasing?
4.) What is the relationship between UV and skin cancer?
5.) Is ozone loss to blame for the melanoma upsurge?
6.) Does UV-B cause cataracts?
7.) Are sheep going blind in Chile?
8.) What effects does increased UV have upon plant life?
9.) What effects does increased UV have on marine life?
10.) Is UV-B responsible for the amphibian decline?
REFERENCES FOR PART IV
Books and General Review Articles
More Specialized References
Subject: 1.) What is "UV-B"?
"UV-B" refers to UV light having a wavelength between 280 and
320 nm. These wavelengths are on the lower edge of ozone's UV
absorption band, in the so-called "Huggins bands". They are
absorbed by ozone, but less efficiently than shorter wavelengths
("UV-C"). (The absorption cross-section of ozone increases by more
than 2 orders of magnitude between 320 nm and the peak value at
~250 nm.) Depletion of the ozone layer would first of all result
in increased UV-B. In principle UV-C would also increase, but it is
absorbed so efficiently that a very large depletion would have to
take place in order for significant amounts to reach the earth's
surface. UV-B and UV-C are absorbed by DNA and other biological
macromolecules, inducing photochemical reactions. UV radiation with
a wavelength longer than 320 nm is called "UV-A". It is not
absorbed by ozone, but it is not usually thought to be especially
dangerous. (See, however, question #6.)
For a good introduction to many aspects of UV and UV measurements, see
the web page for Biospherical Instruments:
Subject: 2.) How does UV-B vary from place to place?
A great deal. It is strongest at low latitudes and high altitudes.
At higher latitudes, the sun is always low in the sky so that it takes
a longer path through the atmosphere and more of the UV-B is absorbed.
For this reason, ozone depletion is likely to have a greater impact on
_local_ ecosystems, such as terrestrial plants and the Antarctic
marine phytoplankton, than on humans or their livestock. UV also
varies with altitude and local cloud cover. These trends can be seen
in the following list of annually-averaged UV indices for several US
cities [Roach] (units are arbitrary - I don't know precisely how this
index is defined though I assume it is proportional to some integral
over the UV-b region of the spectrum)
Minneapolis, Minnesota 570
Chicago, Illinois 637
Washington, DC 683
San Francisco, California 715
Los Angeles, California 824
Denver, Colorado 951
Miami, Florida 1028
Honolulu, Hawaii 1147
The effect of clouds on local UV-B irradiance is not straightforward
to determine. While the body of a cloud attenuates the radiation,
scattering from the sides of a cumulus cloud can actually enhance it.
[Mims and Frederick 1994.]
In comparing UV-B estimates, one must pay careful attention to
exactly what is being reported. One wants to know not just whether
there is an increase, but how much increase there is at a particular
wavelength, since the shorter wavelengths are more dangerous.
Different measuring instruments have different spectral responses,
and are more or less sensitive to various spectral regions. [Wayne,
Rowland 1991]. Wavelength-resolving instruments, such as the
spectroradiometers being used in Antarctica, Argentina, and Toronto,
are particularly informative, as they allow one to distinguish the
effects of ozone trends from those due to clouds and aerosols.
[Madronich 1993] [Kerr and McElroy]. When wavelength-resolved
data are available, they are frequently convolved with an "action
spectrum" that is relevant for a particular biological influence.
Thus the "erythemal action spectrum", designed to estimate the
tendency of UV radiation to redden human skin, places less emphasis
on short wavelengths that the action spectrum designed to estimate
the tendency of UV to damage DNA. When the ozone column overhead
decreases by 1%, erythemal UV increases by about 1% while DNA-damaging
UV increases by about 2.5%. [Madronich 1993] The widely-used broadband
Robertson-Berger meter has a spectral response that is close to
the erythemal action spectrum.
Subject: 3.) Is UV-B at the earth's surface increasing?
Yes, in some places; no, in some others; unknown, in most.
There is very little data on long-term UV trends, primarily because
with very few exceptions UV monitoring operations of the requisite
sensitivity did not exist until very recently. (See the US
Department of Agriculture's UV Monitoring Program web page,
Measurements over a period of a few years cannot establish long-term
trends, although they can be used in conjunction with ozone measurements
to quantify the relationship between surface UV-B intensities and
Very large increases, by as much as a factor of 2-3, have been seen
within the Antarctic ozone hole. [Frederick and Alberts] [Stamnes et
al.] UV-B intensity at Palmer Station (65 degrees S. Lat.) in late
October 1993 exceeded *summertime* UV-B intensity at San Diego,
California. [WMO 1994] At Ushaia at the tip of South America, the
noontime UV-B irradiance in the austral summer is 45% above what would
be predicted were there no ozone depletion. [Frederick et al. 1993]
[Bojkov et al. 1995] The effect is to expose Ushaia to UV intensities
that are typical of Buenos Aires.
Small increases, of order 1% per year, have been measured in the
Swiss Alps. [Blumthaler and Ambach] These _net_ increases are small
compared to natural day-to-day fluctuations, but they are actually
a little larger than would be expected from the amount of ozone
depletion over the same period.
In urban areas of the US, measurements of erythemal UV-B showed no
significant increase (and in most cases a slight decrease between 1974 and
1985. [Scotto et al.]. This may be due due to increasing urban
pollution, including low-level ozone and aerosols. [Grant]
Tropospheric ozone is actually somewhat more effective at absorbing UV
than stratospheric ozone, because UV light is scattered much more in
the troposphere, and hence takes a longer path. [Bruehl and Crutzen]
Increasing amounts of tropospheric aerosols, from urban and industrial
pollution, may also offset UV-B increases at the ground. [Liu et al.]
[Madronich 1992, 1993] [Grant] There have been questions about the
suitability of the instruments used by Scotto et al.; they were not
designed for measuring long-term trends, and they put too much weight
on regions of the UV spectrum which are not appreciably absorbed by
ozone in any case. [WMO 1989] A thorough reassessment
[Weatherhead et al. 1997] found a number of problems:
"The RB meter network was originally established to determine the
relative amounts of UV at different locations around the earth,
with most sites in the United States. The data have been useful for
their intended purpose, that is, to help explain differences in skin
cancer at different locations. There was no original plan to use
the network to determine trends, and therefore the network was not
maintained using the high level of standards necessary for accurate
trend determination. The network management, calibration techniques,
and in some cases instrument location, underwent changes over the
20 years of operation. Unfortunately, most of the records documenting
the maintenance and calibration of the network were misplaced during
transfer of the network among different managers."
Nevertheless it seems clear that so far
ozone depletion over US cities is small enough to be largely offset by
competing factors. Tropospheric ozone and aerosols have increased in
rural areas of the US and Europe as well, so these areas may also be
screened from the effects of ozone depletion.
Several studies [Kerr and McElroy] [Seckmayer et al.] [Zerefos et
al.] have presented evidence of short-term UV-B increases at northern
middle latitudes (Canada, Germany, and Greece), associated with the
record low ozone levels seen in these areas in the years 1992-93. As
discussed in Part I, these low ozone levels are probably due to
stratospheric sulfate aerosols from the 1991 eruption of Mt.Pinatubo;
such aerosols change the radiation balance in the stratosphere,
influencing ozone production and transport, and accelerate the
conversion of inactive chlorine reservoir compounds into
ozone-destroying ClOx radicals. The first mechanism is purely natural,
while the second is an example of a natural process enhancing an
anthropogenic mechanism since most of the chlorine comes ultimately
from manmade halocarbons. (High UV levels associated with low ozone
levels were also reported in Texas [Mims 1994, Mims et al. 1995],
however in this case the low ozone is attributed to unusual
climatology rather than chemical ozone destruction.) One cannot
deduce long-term trends from such short-term measurements, but one can
use them to help quantify the relationship between stratospheric ozone
and surface UV-B intensities under real world conditions. Measurements
in Toronto, Canada [Kerr and McElroy] over the period 1989-93 found
that UV intensity at 300 nm increased by 35% per year in winter and 7%
per year in summer. At this wavelength 99% of the total UV is
absorbed, so these represent large increases in a small number, and do
not represent a health hazard; nevertheless these wavelengths play a
disproportionately large role in skin carcinoma and plant damage.
_Total_ UV-B irradiance, weighted in such a way as to correlate with
incidence of sunburn ("erythemally active radiation"), increased by 5%
per year in winter and 2% per year in summer. These are not really
"trends", as they are dominated by the unusually large, but temporary,
ozone losses in these regions in the years 1992-1993 (see part I), and
they should not be extrapolated into the future. Indeed, [Michaels et
al.] have claimed that the winter "trend" arises entirely from a brief
period at the end of March 1993 (they do not discuss the summer
trend.) Kerr and McElroy respond that these days are also reponsible
for the strong decrease in average ozone over the same period, so that
their results do demonstrate the expected link between total ozone and
total UV-B radiation. UV-B increases of similar magnitude were seen
in Greece for the period 1990-1993 [Zerefos et al.] and in Germany
for the period 1992-93. [Seckmeyer et al.]
Indirect evidence for increases has been obtained in the Southern
Hemisphere, where stratospheric ozone depletion is larger and
tropospheric ozone (and aerosol pollution) is lower. Biologically
weighted UV-B irradiances at a station in New Zealand were 1.4-1.8
times higher than irradiances at a comparable latitude and season in
Germany, of which a factor of 1.3-1.6 can be attributed to differences
in the ozone column over the two locations [Seckmeyer and McKenzie].
Record low ozone columns measured at Mauna Loa during the winter
of 1994-95 were accompanied by corresponding increases in the ratio
of UV-B to UV-A [Hofmann et al. 1996.]
The satellite-borne Total Ozone Mapping Spectrometer (TOMS) actually
measures the UV radiation that is scattered back into space from the
earth's atmosphere. [Herman et al. 1996] have combined ozone and
reflectivity data from TOMS with radiative transfer calculations to
arrive at an estimate of the ultraviolet flux at the surface. The
estimates are validated by comparison with ground-based UV measurements.
The advantage of this technique is that it gives truly global
coverage; the disadvantage is that it is indirect. Herman et al.
estimate that during the period 1979-92 UV irradiance, weighted for
DNA damage, increased by ~5% per decade at 45 degrees N latitude,
~7% per decade at 55 N, and ~10% per decade at 55 S. The increases
occurred primarily in spring and early summer.
Subject: 4.) What is the relationship between UV and skin cancer?
Most skin cancers fall into three classes, basal cell carcinomas.
squamous cell carcinomas, and melanomas. In the US there were
500,000 cases of the first, 100,000 of the second, and 27,600 of
the third in 1990. [Wayne] More than 90% of the skin carcinomas in
the US are attributed to UV-B exposure: their frequency varies
sharply with latitude, just as UV-B does. The mechanism by which UV-B
induces carcinomas has been identified - the pyrimidine bases
in the DNA molecule form dimers when they absorb UV-B radiation.
This causes transcription errors when the DNA replicates, giving
rise to genetic mutations.[Taylor] [Tevini] [Young et al.] [Leffell
and Brash]. Fortunately, nonmelanoma skin cancers are
relatively easy to treat if detected in time, and are rarely fatal.
Fair-skinned people of North European ancestry are particularly
susceptible; the highest rates in the world are found in Queensland,
a northerly province of Australia, where a population of largely
English and Irish extraction is exposed to very high natural UV
[Madronich and de Gruijl] have estimated the expected increases in
nonmelanoma skin cancer due to ozone depletion over the period 1979-1992:
Lat. % ozone loss % increase in rate, % increase in rate,
1979-1992 basal cell carcinoma squamous cell carcinoma
55N 7.4 +-1.3 13.5 +-5.3 25.4 +-10.3
35N 4.8 +-1.4 8.6 +-4.0 16.0 +-7.6
15N 1.5 +-1.1 2.7 +-2.4 4.8 +-4.4
15S 1.9 +-1.3 3.6 +-2.6 6.5 +-4.8
35S 4.0 +-1.6 8.1 +-3.6 14.9 +-6.8
55S 9.0 +-1.5 20.4 +-7.4 39.3 +-15.1
Of course, the rates themselves are much smaller at high latitudes,
where the relative increases in rates are large. A more extensive
evaluation of the effect of ozone layer depletion upon skin cancer
rates can be found in [Slaper et al. 1996]. They estimate that if
no restrictions had been placed upon halocarbon emissions, the resulting
excess skin cancer cases in the U.S. due to ozone depletion would
total 1.5 million for the next century. With current restrictions
under the Montreal Protocol and subsequent Amendments, this number
falls to 8000. These estimates do not take expected changes in
lifestyle (i.e. people taking better care to reduce their exposure
to solar UV) into consideration.
Malignant melanoma is much more dangerous, but its connection with UV
exposure is not well understood. [van der Leun and de Gruijl] [Ley].
There seems to a correlation between melanomas and brief, intense
exposures to UV (long before the cancer appears.) Melanoma incidence
is correlated with latitude, with twice as many deaths (relative to
state population) in Florida or Texas as in Wisconsin or Montana, [Wayne]
but this correlation does not necessarily imply a causal
relationship. There is some evidence that UV-A, which is not absorbed
by ozone, may be involved. [Skolnick] [Setlow et al.] [Ley] There is
a good summary [De Gruijl 1995] in the electronic journal _Consequences_,
Subject: 5.) Is ozone loss to blame for the melanoma upsurge?
A few physicians have said so, but most others think not.
[Skolnick] [van der Leun and de Gruijl]
First of all, UV-B has not, so far, increased very much, at least
in the US and Europe.
Second, melanoma takes 10-20 years to develop. There hasn't been
enough time for ozone depletion to play a significant role.
Third, the melanoma epidemic has been going on since the 1940's.
Recent increases in rates may just reflect better reporting, or
the popularity of suntans in the '60's and '70's. (This becomes
more likely if UV-A is in fact involved.)
Subject: 6.) Does UV-B cause cataracts?
While the evidence for this is indirect, it is very plausible.
The lens of the eye is a good UV-filter, protecting the delicate
structures in the retina. Too much UV burns the lens, resulting in
short-term "snowblindness", but the cumulative effects of prolonged,
repeated exposure are not fully understood. People living in naturally
high UV environments such as Bolivia or Tibet do have a high incidence
of cataracts, and in general cataracts are more frequently seen at lower
latitudes. [Tevini] [Zigman] For more on this, see [De Gruijl 1995]
Subject: 7.) Are sheep going blind in Chile?
If they are, it's not because of ozone depletion.
For a short period each year, the edge of the ozone hole passes
over Tierra del Fuego, at the southern end of the South American
continent. This has led to a flurry of reports of medical damage
to humans and livestock. Dermatologists claim that they are seeing
more patients with sun-related conditions, nursery owners report
damage to plants, a sailor says that his yacht's dacron sails have
become brittle, and a rancher declares that 50 of his sheep,
grazing at high altitudes, suffer "temporary cataracts" in the
spring. (_Newsweek_, 9 December 1991, p. 43; NY Times, 27 July
1991, p. C4; 27 March 1992, p. A7).
These claims are hard to believe. At such a high latitude,
springtime UV-B is naturally very low and the temporary increase
due to ozone depletion still results in a UV fluence that is well
below that found at lower latitudes. Moreover, the climate of
Patagonia is notoriously cold and wet. (There is actually more of
a problem in the summer, after the hole breaks up and ozone-poor
air drifts north. The ozone depletion is smaller, but the
background UV intensity is much higher.) There may well be effects
on _local_ species, adapted to low UV levels, but even these are
not expected to appear so soon. It was only in 1987 that the hole
grew large enough to give rise to significant UV increases
in southern Chile, and cataracts and malignant melanomas take many
years to develop. To be sure, people do get sunburns and
skin cancer even in Alaska and northern Europe, and all
else being equal one expects on purely statistical grounds such
cases to increase, from a small number to a slightly larger number.
All else is definitely not equal, however - the residents are now
intensely aware of the hazards of UV radiation and are likely to
protect themselves better. I suspect that the increase in
sun-related skin problems noted by the dermatologists comes about
because more people are taking such cases to their doctors.
As for the blind sheep, a group at Johns Hopkins has investigated
this and ascribes it to a local infection ("pink eye"). [Pearce]
This is _not_ meant to dismiss UV-B increases in Patagonia as
insignificant. Damage to local plants, for example, may well emerge
in the long term, as the ozone hole is expected to last for 50
years or more. The biological consequences of UV radiation are real,
but often very subtle; I personally find it hard to believe that
such effects are showing up so soon, and in such a dramatic fashion.
Ozone depletion is a real problem, but this particular story is a red
Subject: 8.) What effects does increased UV have upon plant life?
Generally (though not exclusively) harmful, but hard to quantify.
Many experiments have studied the response of plants to UV-B radiation,
either by irradiating the plants directly or by filtering out some
of the UV in a low-latitude environment where it is naturally high.
The artificial UV sources do not have the same spectrum as solar
radiation, however, while the filtering experiments do not
necessarily isolate all of the variables, even when climate
and humidity are controlled by growing the plants in a greenhouse.
Out of some 200 agricultural plants tested, more than half show
sensitivity to UV-B increases. The measured effects vary markedly
from one species to another; some adapt very readily while others are
seriously damaged. Even within species there are marked differences;
for example, one soybean variety showed a 25% growth reduction under a
simulated ozone depletion of 16%, whereas another variety showed no
significant yield reduction. The general sense seems to be that
ozone depletion amounting to 10% or more could seriously affect
agriculture. Smaller depletions could have a severe impact on local
ecosystems, but very little is known about this at present.
I have not investigated the literature on this in detail, not
being a biologist. Interested readers should consult [Tevini and
Teramura], [Bornman and Teramura], or the book by [Tevini] and
the references therein. If any botanist out there would like to write
a summary for this FAQ, please let me know.
Subject: 9.) What effects does increased UV have on marine life?
Again, generally harmful but hard to quantify. Seawater is
surprisingly transparent to UV-B. In clear waters radiation at 315
nm is attenuated by only 14% per meter depth. [Jerlov]. Many marine
creatures live in surface waters, and they have evolved a variety
of methods to cope with UV: some simply swim to lower depths, some
develop protective coatings, while some work at night to repair the
damage done during the day. Often these natural mechanisms are
triggered by _visible_ light intensities, in which case they
might not protect against an increase in the _ratio_ of UV to visible
light. Also, if a photosynthesizing organism protects itself by
staying at lower depths, it will get less visible light and produce
less oxygen. An increase in UV-B can thus affect an ecosystem
without necessarily killing off individual organisms.
Many experiments have been carried out to determine the
response of various marine creatures to UV radiation; as with land
plants the effects vary a great deal from one species to another,
and it is not possible to draw general conclusions at this stage.
[Holm-Hansen et al.] We can assume that organisms that live in tropical
waters are safe, since there is little or no ozone depletion there, and
that organisms that are capable of living in the tropics are probably
safe from ozone depletion at high latitudes since background UV
intensitiesat high latitudes are always low. (One must be careful
with the second inference if the organism's natural defenses are
stimulated by visible light.) The problems arise with organisms
that have adapted to the naturally low UV levels of polar regions.
In this case, we have a natural laboratory for studying UV
effects: the Antarctic Ozone hole. (Part III of the FAQ discusses
the hole in detail.) The outer parts of the hole extend far out
into the ocean, beyond the pack ice, and these waters get
springtime UV-B doses equal to or greater than what is
seen in a normal antarctic summer. [Frederick and Alberts] [Smith
et al.]. The UV in shallow surface waters is effectively even
higher, because the sea ice is more transparent in spring than in
summer. There has been speculation that this UV could cause a
population collapse in the marine phytoplankton, the microscopic
plants that comprise the base of the food chain. Even if the plankton
are not killed, their photosynthetic production could be reduced.
Laboratory experiments show that UV-A and UV-B do indeed inhibit
phytoplankton photosynthesis. [Cullen and Neale] [Cullen et al.]
In one field study, [Smith et al.]. measured the photosynthetic
productivity of the phytoplankton in the "marginal ice zone" (MIZ),
the layer of relatively fresh meltwater that lies over saltier
deep water. Since the outer boundary of the ozone hole is
relatively sharp and fluctuates from day to day, they were able to
compare photosynthesis inside and outside the hole, and to
correlate photosynthetic yield with shipboard UV measurements.
They concluded that the UV-B increase brought about an overall
decrease of 6-12% in phytoplankton productivity. Since the "hole"
lasts for about 10-12 weeks, this corresponds to an overall decrease
of 2-4% for the year. The natural variability in phytoplankton
productivity from year to year is estimated to be about + or - 25%,
so the _immediate_ effects of the ozone hole, while real, are far
from catastrophic. To quote from [Smith et al.]: "Our estimated
loss of 7 x 10^12 g of carbon per year is about three orders
of magnitude smaller than estimates of _global_ phytoplankton
production and thus is not likely to be significant in this
context. On the other hand, we find that the O3-induced loss to a
natural community of phytoplankton in the MIZ is measurable and the
subsequent ecological consequences of the magnitude and timing of
this early spring loss remain to be determined." It appears, then,
that overall loss in productivity is not large.
The cumulative effects on the marine community are not known. The
ozone hole first became large enough to expose marine life to large
UV increases in 1987, and [Smith et al.] carried out their survey in
1990. Ecological consequences - the displacement of UV-sensitive
species by UV-tolerant ones - are likely to be more important than
a decline in overall productivity, although they are poorly
understood at present. [McMinn et al.] have examined the relative
abundance of four common phytoplankton species in sediment cores from
the fjords of the Vestfold hills on the Antarctic coast. They conclude
that compositional changes over the past 20 years (which should include
effects due to the ozone hole) cannot be distinguished from long-term
natural fluctuations. Apparently thick coastal ice protects the
phytoplankton in these regions from the effects of increased UVB;
moreover, these phytoplankton bloom after the seasonal hole has closed.
McMinn et al. emphasize that these conditions do not apply to ice-edge
and sea-ice communities.
For a general review, see [Holm-Hansen et al.]
Subject: 10.) Is UV-B responsible for the amphibian decline?
UV-B may be part of the story, although it is unlikely to be the
principal cause of this mysterious event.
During the past decade, there has been a widespread decline in
amphibian populations [Livermore] [Wake]. The decline appears to be
global in scope, although some regions and many species appear to be
unaffected. While habitat destruction is undoubtedly an important
factor, many of the affected species are native to regions where
habitat is relatively undisturbed. This has led to speculation that
global perturbations, such as pesticide pollution, acid deposition,
and climate change, could be involved.
Recently, [Blaustein et al.] have investigated the effects of UV-B
radiation on the reproduction of amphibians living in the Cascade
Mountains of Oregon. In their first experiment, the eggs of several
amphibian species were analyzed for an enzyme that is known to
*repair* UV-induced DNA damage. The eggs of the Cascades frog,
R. cascadae, and of the Western toad, Bufo Boreas, showed low levels
of this enzyme; both species are known to be in serious decline
(R. Cascadae populations have fallen by ~80% since the 1970's [Wake].)
In contrast, much higher levels of the enzyme are found in the eggs of
the Pacific Tree Frog, _Hyla Regilla_, whose populations do not appear
to be in decline.
Blaustein et al. then studied the effects of UV-B upon the
reproductive success of these species in the field, by screening the
eggs with a filter that blocks the ambient UV. Two control groups were
used for comparison; in one no filter was present and in the other a
filter that *transmitted* UV-B was put in place. They found that for
the two species that are known to be in decline, and that showed low
levels of the repair enzyme, filtering the UV dramatically increased
the proportion of eggs surviving until hatch, whereas for the species
that is not in decline and that produces high levels of the enzyme,
filtering the UV made little difference. Thus, both the laboratory and
the field experiments suggest a correlation between amphibian declines
and UV sensitivity, albeit a correlation that at present is based on a
very small number of species and a limited time period.
Contrary to the impression given by some media reports, Blaustein and
coworkers did *not* claim that ozone depletion is "the cause" of the
amphibian decline. The decline appears to be world-wide, whereas ozone
depletion is restricted to middle and high latitudes. Also, many
amphibian species lay their eggs under dense canopies or underground
where there is little solar radiation. So, UV should be regarded
as one of many stresses that may be acting on amphibian populations.
Subject: REFERENCES FOR PART IV
A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival journals such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions". Readers who want to
see "who did what" should consult the review articles listed below.
or, if they can get them, the WMO reports which are extensively
Subject: Introductory Reading
[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
_Atmospheric Change: an Earth System Perspective_, Freeman, NY 1993.
[Leffell and Brash] D. J. Leffell and D. E. Brash, "Sunlight and Skin
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[Roach] M. Roach, "Sun Struck", _Health_, May/June 1992, p. 41.
[Rowland 1989] F. S. Rowland, "Chlorofluorocarbons and the
depletion of stratospheric ozone", _American Scientist_ _77_, 36, 1989.
[Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises
Challenge Atmospheric Scientists", _Chemical and Engineering News_,
24 May 1993, pp. 9-18.
Subject: Books and General Review Articles
[Chamberlain and Hunten] J. W. Chamberlain and D. M. Hunten,
_Theory of Planetary Atmospheres_, 2nd Edition, Academic Press, 1987
[De Gruijl 1995] F. R. de Gruijl, "Impacts of a Projected Depletion
of the Ozone Layer", _Consequences_ _1_, #2, 1995, on the web at
[Dobson] G.M.B. Dobson, _Exploring the Atmosphere_, 2nd Edition,
[Mukhtar] H. Mukhtar, editor: _Skin Cancer: Mechanisms and Human
Relevance_, CRC series in dermatology, CRC, 1995.
[Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion",
_Ann. Rev. Phys. Chem._ _42_, 731, 1991.
[Tevini] M. Tevini, editor: _UV-B Radiation and Ozone Depletion:
Effects on humans, animals, plants, microorganisms, and materials_
Lewis Publishers, Boca Raton, 1993.
[Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd Ed., Oxford, 1991.
[WMO 1988] World Meteorological Organization,
_Report of the International Ozone Trends Panel_,
Global Ozone Research and Monitoring Project - Report #18.
[WMO 1989] World Meteorological Organization,
_Scientific Assessment of Stratospheric Ozone: 1989_
Global Ozone Research and Monitoring Project - Report #20.
[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_
Global Ozone Research and Monitoring Project - Report #25.
[WMO 1994] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1994_
Global Ozone Research and Monitoring Project - Report #37.
[Young et al.] _Environmental UV Photobiology_, Ed. by A. R. Young,
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