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Path: senator-bedfellow.mit.edu!bloom-beacon.mit.edu!hecate.umd.edu!cs.umd.edu!zombie.ncsc.mil!newsgate.duke.edu!nntprelay.mathworks.com!newsfeed.internetmci.com!199.117.161.1!csn!nntp-xfer-1.csn.net!boulder!spot.colorado.edu!rparson From: rparson@spot.colorado.edu (Robert Parson) Newsgroups: sci.environment,sci.answers,news.answers Subject: Ozone Depletion FAQ Part IV: UV Radiation and its Effects Followup-To: poster Date: 24 Dec 1997 20:51:43 GMT Organization: University of Colorado, Boulder Lines: 926 Approved: news-answers-request@MIT.Edu Expires: Sun, 1 Jan 1998 00:00:00 GMT Message-ID: <67rskv$2uq@peabody.colorado.edu> Reply-To: rparson@spot.colorado.edu NNTP-Posting-Host: spot.colorado.edu NNTP-Posting-User: rparson 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. Keywords: ozone layer depletion UVB UVA skin cancer phytoplankton Originator: rparson@spot.colorado.edu Xref: senator-bedfellow.mit.edu sci.environment:158246 sci.answers:7538 news.answers:119486 Archive-name: ozone-depletion/uv Last-modified: 16 Dec 1997 Version: 5.9 ----------------------------- Subject: How to get this FAQ These files are posted to the newsgroups sci.environment, sci.answers, and news.answers. They are also archived at a variety of sites. These archives work by automatically downloading the faqs from the newsgroups and reformatting them in site-specific ways. They usually update to the latest version within a few days of its being posted, although in the past there have been some lapses; if the "Last-Modified" date in the FAQ seems old, you may want to see if there is a more recent version in a different archive. Many individuals have archived copies on their own servers, but these are often seriously out of date and in general are not recommended. 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.faqs.org/faqs/ozone-depletion/ 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. Anonymous 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. ---- C. Regular email Send the following messages to mail-server@rtfm.mit.edu: send usenet/news.answers/ozone-depletion/intro send usenet/news.answers/ozone-depletion/stratcl send usenet/news.answers/ozone-depletion/antarctic send usenet/news.answers/ozone-depletion/uv Leave the subject line blank. If you want to find out more about the mail server, send a message to it containing the word "help". ----------------------------- 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 * * 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.) For a good introduction to many aspects of UV and UV measurements, see the web page for Biospherical Instruments: http://www.biospherical.com/research/uvhome.htm ----------------------------- 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, http://uvb.nrel.colostate.edu/UVB/uvb_climate_network.html.) 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 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, 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 radiation levels. [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_, at http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html ----------------------------- 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] at http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html ----------------------------- 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 (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 documented. ----------------------------- 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 Cancer", _Scientific American_ July 1996, p. 52. [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. 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