OZONE DEPLETION FAQ Part IV: UV Radiation and its consequences. Copyright 1993 Robert Parson 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, just as in warfare one needs to consider not only what the enemy 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. _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. 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@rintintin.colorado.edu parson_r@cubldr.colorado.edu CONTENTS OF PART IV 1. What is "UV-B"? 2. How does UV-B vary from place to place? 3. If the ozone layer is depleted, won't the UV light just penetrate deeper into the atmosphere and make more ozone? 4. *Is* UV-B increasing? 5. What is the relationship between UV radiation and skin cancer? 6. Is ozone loss responsible for the melanoma upsurge? 7. Does UV Radiation cause cataracts? 8. Are sheep going blind in Chile? 9. What effects does increased UV have on agriculture? 10. What effects does increased UV have on marine life? References _________________________________________________________________ 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 believed to be especially dangerous. (See, however, question #6.) 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 takes a longer path through the atmosphere so that more UV-B is absorbed. Overall, UV-B fluence decreases by a factor of 15 upon travelling from the equator to the poles. For this reason, ozone depletion is likely to have a greater impact on _local_ ecosystems, such as the Antarctic marine phytoplankton, than on humans or 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) Seattle, Washington 477 Minneapolis, Minnesota 570 Chicago, Illinois 637 Washington, DC 683 San Francisco, California 715 Los Angeles, California 824 Atlanta, Georgia 875 Denver, Colorado 951 Miami, Florida 1028 Honolulu, Hawaii 1147 3. If the ozone is lost, won't the UV light just penetrate deeper into the atmosphere and make more ozone? This does happen to some extent - it's called "self-healing" - and has the effect of moving ozone from the upper to the lower stratosphere. It is not a very effective stabilizing mechanism, however. Recall that ozone is _created_ by UV with wavelengths less than 240 nm, but functions by _absorbing_ UV with wavelengths greater than 240 nm. The peak of the ozone absorption band is at ~250 nm, and the cross-section falls off at shorter wavelengths. The O2 and O3 absorption bands do overlap, however, and UV radiation between 200 and 240 nm has a good chance of being absorbed by _either_ O2 or O3. (Below 200 nm the O2 absorption cross-section increases dramatically, and O3 absorption is insignificant in comparison.) Since there is some overlap, a decrease in ozone does lead to a small increase in absorption by O2. This is a weak feedback, however, and it does not compensate for the ozone destroyed. Negative feedback need not imply stability, just as positive feedback need not imply instability. Numerical calculations of ozone depletion take the "self-healing" phenomenon into account, by letting the perturbed ozone layer come back into equilibrium with the exciting radiation. Even the simple one-dimensional models used in the early 1970's included this rather obvious effect. 4. Is UV-B at the earth's surface increasing? Yes, in some places; no, in others. Very large increases - up to a factor of 2 - have been seen even in the outer portions of the Antarctic hole. [Frederick and Alberts] 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] [Grant] There have been questions about the reliability of the instruments used by Scotto et al., but it seems clear that so far ozone depletion over US cities is small enough to be 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. Things are different 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 scanning radiometers being used in Antarctica and Patagonia, are the most informative. 5. What is the relationship between UV radiation 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. These are relatively mild and easily treated if detected in time. Melanomas are much more dangerous, but their connection with UV exposure is not well understood. There seems to a correlation between melanomas and brief, intense exposures to UV (long before the cancer appears.) Melanoma incidence is definitely correlated with latitude, with twice as many deaths (relative to state population) in Florida or Texas as in Wisconsin or Montana, but this correlation need not imply a causal relationship. Some claim that UV-A, which is not absorbed by ozone, is involved, perhaps as a promoting factor rather than an initiator. [Skolnick] 6. Is ozone loss to blame for the melanoma upsurge? A few physicians have said so, but most others think not. [Skolnick] First of all, UV-B has not, so far, increased very much, at least in the US and Europe. (I have not investigated the literature concerning Australia. There seems to be a lot of controversy there.) 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.) 7. 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 results in short-term "snow blindness", but the effects of prolonged, repeated exposure are not known. People living in naturally high UV environments such as Bolivia or Tibet do have a higher incidence of cataracts. 8. Are sheep going blind in Chile? If they are, it's probably not because of ozone depletion. For a few days out of 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, this appears to be a local infection, according to unnamed veterinarians. (Washington Post, 15 April 1993, p. A19.) Rumen Bojkov of the World Meteorological Organization in Geneva, a leading authority on ground-based ozone and UV measurements, expressed skepticism about the story in a recent interview given to _Die Zeit_: " 'There are blind sheep in Chile', says Rumen Bojkov, 'but they were there already twenty years ago.' While Chilean journalists and politicians have been constantly claiming such things, to date there has not been a single scientific study which confirms such effects." (Ulrich Schnabel, Wie gross ist das Ozonloch u"ber uns ? Die Zeit Nr. 14, 2. April 1993, p 37-38. I am grateful to Jan Schloerer for providing this quotation. I am trying to find a more definitive statement, for example a statement from a veterinarian who has actually inspected the animals - no luck so far.) This is _not_ 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. As discussed in Part I, 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. 9. What effects does increased UV have upon agriculture? 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, and do not tell us about the effects on species that are restricted to high latitudes. The measured effects vary markedly from one species to another; some adapt very readily to increased UV while others are seriously affected. 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] and the references therein. The same journal issue contains several other papers dealing with the biological effects of UV-B radiation. 10. 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, some work at night to repair the damage done during the day. These natural mechanisms however, are often triggered by _visible_ light intensities, in which case they do not protect against an increase in the _ratio_ of UV to visible light. 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 difficult to draw general conclusions at this stage. We can infer 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 large depletions at high latitudes since UV intensities at high latitudes are always low. (One must be a little careful with the second inference if the organism's natural defenses are stimulated by visible light.) 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. To my knowledge, only one field study has been published so far. [Smith et al.]. These workers 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 with shipboard UV measurements. They concluded that 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% averaged over 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 - yet. (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 reduction, although they are poorly understood at present. ___________________________ 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. Introductory Reading: [Graedel and Crutzen] T. E. Graedel and P. J. Crutzen, _Atmospheric Change: an Earth System Perspective_, Freeman, NY 1993. [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. ---------------------------- Books and Review Articles: [Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion", _Ann. Rev. Phys. Chem._ _42_, 731, 1991. [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 1991] World Meteorological Organization, _Scientific Assessment of Ozone Depletion: 1991_ Global Ozone Research and Monitoring Project - Report #25. ----------------------------------- More Specialized: [Blumthaler and Ambach] M. Blumthaler and W. Ambach, "Indication of increasing solar ultraviolet-B radiation flux in alpine regions", _Science_ _248_, 206, 1990. [Bruehl and Crutzen] C. Bruehl and P. Crutzen, "On the disproportionate role of tropospheric ozone as a filter against solar UVB radiation",_Geophys. Res. Lett._ _16_, 703, 1989. [Frederick and Alberts] J. Frederick and A. Alberts, "Prolonged enhancement in surface ultraviolet radiation during the Antarctic spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991. [Grant] W. Grant, "Global stratospheric ozone and UVB radiation", _Science_ _242_, 1111, 1988. (a comment on [Scotto et al.]) [Jerlov] N.G. Jerlov, "Ultraviolet Radiation in the Sea", _Nature_ _166_, 112, 1950. [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. [Madronich] 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. [Roach] M. Roach, "Sun Struck", _Health_, May/June 1992, p. 41. (See especially the sidebar by Steven Finch on p. 50). [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 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. [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. [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.)