Subject: Ozone Depletion FAQ Part IV: UV Radiation and its Effects
Supersedes: <45muhb$oss@CUBoulder.Colorado.EDU>
Date: 21 Nov 1995 22:25:01 GMT
Expires: Mon, 8 Jan 1996 00:00:00 GMT
Summary: This is the fourth of four files dealing with stratospheric
         ozone depletion. It describes the properties of solar UV
         radiation and some of its biological effects.

Archive-name: ozone-depletion/uv
Version: 5.3

-----------------------------

Subject: How to get this FAQ

These files are (usually) posted monthly, towards the end of the month.
The current versions are stored on several archives:

A. World-Wide Web
(Limited) hypertext versions, with embedded links to some of the on-line
resources cited in the faqs, can be found at:

http://www.cis.ohio-state.edu/hypertext/faq/usenet/ozone-depletion/top.html
http://www.lib.ox.ac.uk/internet/news/faq/sci.environment.html 
http://www.cs.ruu.nl/wais/html/na-dir/ozone-depletion/.html

Plaintext versions can be found at:

ftp://rtfm.mit.edu/pub/usenet/news.answers/ozone-depletion/
ftp://ftp.uu.net/usenet/news.answers/ozone-depletion/

----
B. Anonynmous ftp

To rtfm.mit.edu, in the directory  /pub/usenet/news.answers/ozone-depletion
To ftp.uu.net, in the directory /usenet/news.answers/ozone-depletion
Look for the four files named intro, stratcl, antarctic, and uv.

----
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   send usenet/news.answers/ozone-depletion/intro
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   send usenet/news.answers/ozone-depletion/uv

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I have found a number of copies of the faqs tucked away in various corners
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above usually get the latest version within a few days of its being posted.

-----------------------------   

Subject: Copyright Notice

***********************************************************************
* Copyright 1995 Robert Parson                                        *
*                                                                     *
* This file may be distributed, copied, and archived. All             *
* 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  *
* web pages, this is most easily done by linking to the master at the *
* ohio-state http URL  instead of storing local copies. Finally, I    *
* 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   *
* a reference for school projects; it should instead be used as a     *
* pointer to the published literature. In particular, all scientific  *
* data, numerical estimates, etc. should be accompanied by a citation *
* 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
  Associate Professor
  Department of Chemistry and Biochemistry,
  University of Colorado (for which I do not speak)

  rparson@spot.colorado.edu
  Robert.Parson@colorado.edu

-----------------------------

Subject: TABLE OF CONTENTS 

 How to get this FAQ
 Copyright Notice
 General Remarks
 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
 Introductory Reading
 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.)

-----------------------------

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
 
 It should be noted that skin cancer rates show a similar trend.
 (See below).

-----------------------------
 
Subject: 3.) Is UV-B at the earth's surface increasing?

Yes, in some places; no, in others. 

There is very little data on long-term UV trends for the simple reason
that with very few exceptions UV monitoring operations of the
requisite sensitivity did not exist until very recently. Measurements
over a period of a few years cannot establish long-term trends, but can
be used in conjunction with ozone measurements to quantify the
relationship between surface UV-B intensities and ozone amounts. 
  
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, UV-B levels showed no significant increase
(and in most cases actually decreased a little) between 1974 and
1985. [Scotto et al.].  This is probably 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] 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] [Mims] [Seckmayer et al.]  [Zerefos
et al.] have presented evidence of short-term UV-B increases at middle
latitudes associated with the record low ozone levels in 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 n
atural, but the second is an example of a natural process enhancing an
anthropogenic mechanism since most of the chlorine comes ultimately
from manmade halocarbons.) One cannot deduce long-term trends from
such short-term measurements, but one can use them to quantify the
relationship between stratospheric ozone and surface UV-B intensities
under messy, "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.  In fact, [Michaels
et al.] have claimed that the winter "trend" arises entirely from a
four-day 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. Indeed, UV-B increases of
similar magnitude have been 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].

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 any given
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 the most informative, as they allow one to distinguish the
effects of ozone trends from those due to clouds and aerosols.
[Madronich 1993] [Kerr and McElroy]. 

-----------------------------

Subject: 4.) What is the relationship between UV and skin cancer?

There are three kinds of skin cancer, 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 does. The mechanism by which UV-B
induces carcinomas has been identified - the pyrimidine bases
in the DNA molecule form dimers when stimulated by UV-B radiation.
[Taylor] [Tevini] [Young et al.]. Fortunately, these cancers are 
relatively easy to treat if detected in time, and are rarely fatal. 
Skin carcinoma rates vary sharply with latitude, just as UV-B does. 
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
radiation levels.

[Madronich and deGruijl] have estimated the expected increases in
skin carcinoma rates 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. These estimates do
not take changes in lifestyle 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 need not 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]

-----------------------------

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]

-----------------------------

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 
herring.

-----------------------------

Subject: 8.) What effects does increased UV have upon plant life?

Generally 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
documented.

-----------------------------

Subject: Introductory Reading

[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen, 
_Atmospheric Change: an Earth System Perspective_, Freeman, NY
1993.

[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

[Dobson] G.M.B. Dobson, _Exploring the Atmosphere_, 2nd Edition,
 Oxford, 1968. 

[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,
L. O. Bjorn, J. Mohan, and W. Nultsch, Plenum, N.Y. 1993.
 
-----------------------------

Subject: More Specialized References

[Blaustein et al.] A. R. Blaustein, P. D. Hoffman, D. G. Hokit,
J. M. Kiesecker, S. C. Walls, and J. B. Hays, "UV repair and
resistance to solar UV-B in amphibian eggs: A link to population
declines?", _Proc. Nat. Acad. Sci._ _91_, 1791, 1994.

[Blumthaler and Ambach] M. Blumthaler and W. Ambach, "Indication of
increasing solar ultraviolet-B radiation flux in alpine regions",
_Science_ _248_, 206, 1990.

[Bornman and Teramura]  J. F. Bornman and A. H. Teramura, "Effects of
Ultraviolet-B Radiation on Terrestrial Plants", in [Young et al.]

[Bojkov et al. 1995] R. D. Bojkov, V. E. Fioletov, and S. B. Diaz,
"The relationship between solar UV irradiance and total ozone from
observations over southern Argentina", _Geophys. Res. Lett._ _22_,
 1249, 1995.

[Bruehl and Crutzen]  C. Bruehl and P. Crutzen, "On the
disproportionate role of tropospheric ozone as a filter against
solar UV-B radiation",_Geophys. Res. Lett._ _16_, 703, 1989.

[Cullen et al.] J. J. Cullen, P. J. Neale, and M. P. Lesser, "Biological
weighting function for the inhibition of phytoplankton photosynthesis by
ultraviolet radiation", _Science_ _258_, 646, 1992.

[Cullen and Neale] J. J. Cullen and P. J. Neale, "Ultraviolet Radiation,
ozone depletion, and marine photosynthesis", _Photosynthesis Research_
_39_, 303, 1994.

[Frederick and Alberts] J.E. Frederick and A. Alberts, "Prolonged
 enhancement in surface ultraviolet radiation during the Antarctic
 spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991.

[Frederick et al. 1993] J.E. Frederick, P.F. Soulen, S.B. Diaz,
I. Smolskaia, C.R. Booth, T. Lucas, and D. Neuschuler,
"Solar Ultraviolet Irradiance Observed from Southern Argentina:
 September 1990 to March 1991", J. Geophys. Res. _98_, 8891, 1993.

[Grant] W. Grant, "Global stratospheric ozone and UV-B radiation",
  _Science_ _242_, 1111, 1988. (a comment on [Scotto et al.])

[Holm-Hansen et al.] O. Holm-Hansen, D. Lubin, and E. W. Helbling,
"Ultraviolet Radiation and its Effects on Organisms in Aquatic
Environments", in [Young et al.]

[Jerlov] N.G. Jerlov, "Ultraviolet Radiation in the Sea",
_Nature_ _166_, 112, 1950.

[Kerr and McElroy] J. B. Kerr and C. T. McElroy, "Evidence for Large
 Upward Trends of Ultraviolet-B Radiation Linked to Ozone Depletion",
 _Science_ _262_, 1032, 1993.

[Ley] R. D. Ley, "Animal Models for Melanoma Skin Cancer", in [Mukhtar].

[Livermore] B. Livermore, "Amphibian alarm: Just where have all the 
frogs gone?", _Smithsonian_, October 1992.

[Liu et al.] S.C. Liu, S.A. McKeen, and S. Madronich, "Effect of
 anthropogenic aerosols on biologically active ultraviolet
radiation", _Geophys. Res. Lett._  _18_,  2265, 1991.

[Lubin and Jensen] D. Lubin and E. H. Jensen, "Effects of clouds
and stratospheric ozone depletion on ultraviolet radiation trends",
_Nature_ _377_, 710, 1995.

[Madronich 1992] S. Madronich, "Implications of recent total 
atmospheric ozone measurements for biologically active ultraviolet 
radiation reaching the earth's surface", 
_Geophys. Res. Lett. _19_, 37, 1992.

[Madronich 1993] S. Madronich, in [Tevini].

[Madronich 1995] S. Madronich, "The radiation equation" _Nature_ _377_,
682, 1995. (News and Views column.)

[Madronich and de Gruijl] S. Madronich and F. R. de Gruijl,
"Skin Cancer and UV radiation", _Nature_ _366_, 23, 1993.

[McMinn et al.] A. McMinn, H. Heijnis, and D. Hodgson, "Minimal effects
of UVB radiation on Antarctic diatoms over the past 20 years", _Nature_
_370_, 547, 1994.

[Michaels et al.] P. J. Michaels, S. F. Singer, and P. C. 
Knappenberger, "Analyzing Ultraviolet-B Radiation: Is There
a Trend?", _Science_ _264_, 1341, 1994. (Technical Comment)

[Mims et al.] F. M. Mims III, J. W. Ladd and R. A. Blaha, "Increased
solar ultraviolet-B associated with record low ozone over Texas", 
_Geophys. Res. Lett._ _22_, 227, 1995.

[Pearce] F. Pearce, "Ozone hole 'innocent' of Chile's ills",
 _New Scientist_ #1887, 7, 21 Aug. 1993.

[Scotto et al.] J. Scotto, G. Cotton, F. Urbach, D. Berger, and T.
Fears, "Biologically effective ultraviolet radiation: surface
measurements in the U.S.",  _Science_ _239_, 762, 1988.

[Seckmeyer et al.] G. Seckmeyer, B. Mayer, R. Erb, and G. Bernhard,
"UV-B in Germany higher in 1993 than in 1992", _Geophys. Res. Lett._
 _21_, 577-580, 1994.

[Seckmeyer and McKenzie] G. Seckmeyer and R. L. McKenzie,
"Increased  ultraviolet radiation in New Zealand (45 degrees S)
relative to Germany (48 degrees N.)", _Nature_ _359_, 135, 1992.

[Setlow et al.] R. B. Setlow, E. Grist, K. Thompson and
A. D. Woodhead, "Wavelengths effective in induction of Malignant
Melanoma", PNAS _90_, 6666, 1993.

[Skolnick] A. Skolnick,  "Is ozone loss to blame for melanoma
upsurge?" JAMA, _265_, 3218, June 26 1991. 

[Smith et al.] R. Smith, B. Prezelin, K. Baker, R. Bidigare, N.
Boucher, T. Coley, D. Karentz, S. MacIntyre, H. Matlick, D.
Menzies, M. Ondrusek, Z. Wan, and K. Waters, "Ozone depletion:
Ultraviolet radiation and phytoplankton biology in antarctic
waters", _Science_ _255_, 952, 1992.

[Stamnes et al.] K. Stamnes, Z. Jin, and J. Slusser, "Several-fold
enhancement of biologically effective Ultraviolet radiation levels at
McMurdo Station Antarctica during the 1990 ozone 'hole'", _Geophys. Res.
Lett._ _19_, 1013, 1992.

[Taylor] J.-S. Taylor, "Unraveling the Molecular Pathway from Sunlight
to Skin Cancer", _Acc. Chem. Res._ _27_, 76-82, 1994.
 
[Tevini and Teramura] M. Tevini and A. H. Teramura, "UV-B effects
on terrestrial plants", _Photochemistry and Photobiology_, _50_,
479, 1989. (This issue contains a number of other papers dealing
with biological effects of UV-B radiation.)

[van der Leun and de Gruijl] J. C. van der Leun and F. R. de Gruijl,
"Influences of Ozone Depletion on Human and Animal Health", in [Tevini].

[Wake] D. B. Wake, "Declining Amphibian Populations", _Science_
 _253_, 860, 1991.

[Zerefos et al.] C. S. Zerefos, A. F. Bias, C. Meleti, and I. C. Ziomas,
"A note on the recent increase of solar UV-B radiation over northern
middle latitudes", _Geophys. Res. Lett._ _22_, 1245, 1995.

[Zigman] S. Zigman, "Ocular Damage by Environmental Radiant Energy 
and Its Prevention", in [Young et al.]
