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8 Use of Climatic Effects of Volcanic Eruptions and Extraterrestrial Impacts on the Earth as Analogs Very large explosive volcanic eruptions and asteroid or meteor impacts can inject large amounts of dust high into the atmosphere. It is important therefore to assess the extent to which data provided by such events can be useful in the attempt to understand the atmospheric modification that would follow a nuclear exchange. The committee did not, however, find any unambiguous evidence provided by volcanic and impact events to support or refute a conclusion that nuclear war may seriously affect the world's climate. No recent natural events have been energetic enough to provide more than a small atmospheric perturbation; furthermore, the only investigations of earlier, larger events, whose goals included dust lofting estimates have been those associated with the hypothesis that a very large meteor caused the extinction at the Cretaceous-Tertiary boundary some 65 million years ago. That event would have been more energetic than the baseline exchange by a factor of more than 104. Nevertheless, an account is included here of those aspects of volcanic and natural impact events that, if there were more data available, would be pertinent. It is important to consider the similarities and differences between volcanic explosions and nuclear explosions, and between extraterrestrial impacts and nuclear explosions. One must keep in mind that substantial unknowns exist in our understanding of the effects of these events on the terrestrial ecosystem, which parallel in many ways the uncertainties in our understanding of the potential effects of nuclear war. For example, it is only for the past 100 years that a reliable record exists of the optical depths of clouds of volcanic ~ origin. VOLCANIC ERUPTIONS Since large volcanic eruptions could introduce quantities of material into the atmosphere comparable to those from a nuclear war, it is pertinent to ask if we can use evidence from volcanic explosions to empirically determine the climatic effects that would be caused by a nuclear war. It turns out that volcanoes do not prove very useful in this regard. 174

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175 The largest explosive volcanic eruptions in the last 200 years have occurred at Tambora, Indonesia, in 1815; at Krakatau, Indonesia, in 1883; and at Katmai, Alaska, in 1912 (Simkin, 19811. Each of these eruptions produced 1016 to 1017g of 700 to 900C volcanic fragments within a few hours to a few~days. All three eruptions produced large ash clouds that reached into the stratosphere. The total thermal energies of these eruptions amounted to 1019 to 102 joules (J), of which only about 1 percent was converted to mechanical energy (as steam blasts, the initial velocities of ejected material, and the buoyant lift of the ash cloud). This volcanic mechanical energy (1017 to 1018 J) is equivalent to 25 to 250 Mt of nuclear energy. However, the release of volcanic energy takes place over minutes to days, and hence the power (energy release per second) of historic volcanic eruptions has been much less than would be the power of a single-megaton nuclear explosion. Nevertheless, some volcanic blasts devastate areas similar in size to areas that would be affected by nuclear explosions, and they loft large amounts of dust and gases into the stratosphere. Prehistoric volcanic eruptions have produced thick blankets of explosive volcanic debris called ash flows covering areas of up to 10,000 km2 with masses of up to 2 x 1018 g. Most of the enormous thermal energy in these huge eruptions (2 x 1021 J. equivalent to 500,000 Mt) is retained in the hot fragmental debris blanket and is slowly dissipated over tens to hundreds of years. In addition to energy release patterns, another important distinction between volcanic explosion clouds and nuclear explosion clouds involves composition. Volcanic clouds contain large quantities of sulfur gases. However, both volcanic explosion clouds and near-surface nuclear explosion clouds contain silicate glass and silicate mineral particles. Very large volcanic eruptions occur infrequently, on the average of about once every 10,000 to 100,000 years. They occur as locally isolated events, whereas the 25,000 potential nuclear explosions assumed in this report occur nearly simultaneously over large areas. Historic volcanic explosions have not generated large forest or brush fires, and the thick ash flow blankets of very large prehistoric eruptions would have tended to smother fires. Hence the ~soot" problem that results from multiple nuclear explosions probably has no counterpart in volcanic eruptions. The effects of gases and dust from volcanic eruptions on climate have been a subject of speculation ever since Benjamin Franklin alleged that the Laki eruption in Iceland in 1783 had caused a "dry fog" in Europe with attendant cold weather and poor crops (Humphreys, 1940~. Some investigators have concluded that increased volcanism was a major cause of the Pleistocene ice age (Kennett and Thunnel, 1977~; others have argued that apparent worldwide average temperature drops of up to 1C in the year following the large explosive eruptions of Tambora Volcano (Indonesia, 1815) and Krakatau Volcano (Indonesia, 1883) either could be errors of measurement (or analysis), or if real, could be coincidental to normal fluctuations in average world temperature (Landeberg and Albert, 19741.

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178 Most present investigators of the influence of volcanic eruptions on climate (Pollack et al., 1976; Robock, 1978; Toon and Pollack, 1982) agree that there is a measurable effect and that it is largely caused by the sulfuric acid aerosol particles that form in the stratosphere from sulfur dioxide gases in volcanic ash clouds that reach stratospheric altitudes. Actual sampling of the amount of dust and gases from volcanic eruptions reaching the stratosphere began following the eruption of Agung Volcano (Indonesia, 19631. Data on silicate dust (mainly volcanic glass and small silicate mineral fragments) and sulfur gases reaching the stratosphere from recent eruptions of Agung, Fuego (Guatemala), Mount St. Helens, and E1 Chichon (Mexico) volcanoes are listed in Table 8.1. Rough estimates of the amounts of dust and sulfur gases reaching the stratosphere from the eruptions of Tambora, Krakatau, and Agung are given in Table 8.2. Since the Tambora and Krakatau eruptions produced 10 to 100 times more volcanic debris than the Agung, Fuego, Mount St. Helens, and E1 Chichon eruptions, it is apparent from the tables that larger volcanic eruptions do not generate linearly proportional amounts of dust and gases that reach the stratosphere. One probable reason for this lack of proportionality is that larger eruptions produce denser ash clouds, which are less buoyant. Both Settle (1978) and Wilson et al. (1978) have demonstrated by theory and observation of historic volcanic eruptions that the height of explosive eruption clouds increases as the rate of emission of fragmental volcanic material increases. However, Wilson et al. calculate that eruption rates of about 106 m3/s will generate maximum ash cloud heights of 55 km. Larger eruption rates will produce dense clouds that will fall back under their own weight before reaching this maximum altitude. This lack of linear scaling is important in considering the possible climatic effects of extremely large volcanic explosions in prehistoric times. In the last 2 million years, there have been six explosive volcanic eruptions in the western United States (three at Yellowstone; two at Valles Caldera, New Mexico; and one at Long valley, California) that have produced 100 to 2000 km3 of fragmental volcanic material (ash flows) during apparent time intervals of a few hours to weeks (Francis, 1983~. An estimate for the total of both fine dust and sulfate aerosols injected into the stratosphere by a single very large volcanic eruption is 1015 g; however, there is a high degree of uncertainty in this estimate. Linearly proportional scaling of the 1013 g of stratospheric dust and aerosols produced by the E1 Chichon eruption up to a Yellowstone-type eruption yields an estimate of 1016 g as an upper limit. The approximately 1014 g fallout of sulfate from the Tambora eruption (Table 8.2) provides an apparent lower limit. However, large sulfate loadings may be self-limiting due to the nonlinear dependence of growth rate and sedimentation of injected sulfur. Therefore it is not clear that volcanoes have ever exceeded the apparent lower limit. Loading the global stratosphere with 1015 g of fine dust and sulfate aerosols from a great volcanic explosion would produce a worldwide average temperature drop of about 10C for

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179 TABLE 8.2 Comparison of Estimates of Fine Dust, Aerosols, and Sulfate Fallout Eruption, Total Year, and <2-pm Volumea DUstb (g) Total Global SO4 Aerosols (g ~Falloutd (g Tambora, 1815 1.S x 1013 to ? 1.5 x 1014 About 150 km3 1.2 x 1018 (?) (7.5) (150) Krakatau, 1883 2 x 1018 to 3 x 1013 5.5 x 1013 About 20 km3 1.6 x 1014 (~3) (~3) (20) Agung, 1963 1 x 1011 to 0~9 x 1013 2 x 1013 About 1 km3 8 x 1013 (1) (1) (1) aVolumes of eruptions are expressed as bulk volumes of near-source and distal ejecta. busing the method of Murrow et al. (1980), assumes an average bulk density of fine ash of 1 g/cm3. This volume represents the total mass of <2-pm-diameter dust ejected into the atmosphere. Only a fraction of this dust entered the stratosphere. Evidence from Mount St. Helens eruptions (Rose and Hoffman, 1982) demonstrates that a large, but as yet undetermined, portion of the fine dust will be quickly removed by particle aggregate formation. CDeirmendjian (1973~. Refers to stratospheric loading. dHammer et al. (1980~. Refers to stratospheric loading. NOTE: Figures in parentheses represent relative quantities of dust, aerosol, and sulfate fallout, with Agung (1963) as base figure. SOURCE: Rampino and Self (1982~. several months. This hypothetical climate anomaly is calculated by scaling upward the optical effect of the E1 Chichon eruption. In Table 8.1 it can be seen that the silicate loading following the eruption of Mount St. Helens was dominated by large ash particles and the submicron mass was probably smaller than the 1013 g of submicron dust estimated for the baseline case. Likewise, the E1 Chichon silicate mass loadings consisted of large particles and the estimates are unreliable since no in situ measurements were made in the E1 Chichon cloud for several months, at which time the silicate dust loadings were very small (about 1012 g). Unfortunately, the possibly large mass loadings of stratospheric submicron debris for prehistoric giant volcanic eruptions cannot be

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180 substantiated from current data. Very little study has been devoted to the environmental effects of giant volcanic eruptions. Clearly, the environmental effects may have been severe even thousands of kilometers from the volcanoes since substantial quantities of ash were deposited at such distances. The greatest volcanic eruptions, even if they do put as much as 1016 g into the atmosphere, would not be expected to cause effects similar to those of the much more powerful meteorite posited by Alvarez et al. as discussed by Toon et al. (1982~. Major biological extinctions due to volcanic eruptions are then neither expected nor detected in the geologic record. Severe global climatic changes that would pose problems for modern society can neither be substantiated nor excluded on the basis of our current limited knowledge of prehistoric volcanic eruptions. In summary, large explosive volcanic eruptions may be reasonable analogs for some atmospheric effects of a nuclear war, but not enough is known about these eruptions to provide useful guidelines. Clearly, studies of the amount of fine dust and aerosols that actually reaches the stratosphere are warranted. Likewise, a better knowledge of the environmental impact of previous eruptions is needed. EXTRATERRESTRIAL IMPACTS Alvarez et al. (1980) suggested that the abrupt extinction of many species of marine plankton and other organisms at the end of the Cretaceous period {about 65 million years ago) was a consequence of the impact of an extraterrestrial body of about 10 km in diameter that lofted quantities of cratering dust particles into the atmosphere. Subsequent work by Alvarez and others has led to the discovery at the Cretaceous-Tertiary boundary of a characteristic claystone layer enriched in certain noble metals at about 60 additional localities around the globe. Between one-fourth and one-half of the late Cretaceous plant taxa recognizable from pollen became extinct at the claystone layer, where it has been observed in North America. These new observations greatly strengthen the initial hypothesis of Alvarez et al. that a body of about 10 km in diameter did strike the earth at the end of the Cretaceous and that this impact may have caused the extinction of species. The extinction may have been a consequence of the darkening of most of the earth's surface by cratering debris suspended in the atmosphere (Alvarez et al., 1982; Toon et al., 1982), of an increase in surface temperature after the debris had settled due to an increase in the water vapor content of the atmosphere (Emiliani et al., 1981), of the production of large quantities of NOX in the impact fireball (Lewis et al., 1982), or of the interaction of physical and biological effects. The suggestion that a global veil of dust was the cause of the extinction of species at the end of the Cretaceous spurred the present concern that the dust and soot produced in a nuclear war might have similarly deleterious effects. Shoemaker (1983) estimates that the cumulative frequency of impacts by extraterrestrial bodies with radii >r varies approximately as r~2, at least for bodies in the kilometer size range. At a representative velocity of 20 km/s, the impact of a 10-km body would

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181 release an amount of energy of the order of 108 Mt. The Cretaceous- Tertiary claystone layer, where it is recognized, is about 2 cm thick on average, and the total mass of a global deposit with this thickness is about 1019 g. Impacts of bodies of 5-km diameter or larger occur about 4 times as frequently as impacts of 10-km bodies, or at a rate of 10-7 yr~l, with impact energies of 107 Mt; for 2-km-diameter bodies or larger the rate is 6 x 10-7 yr~1 and the energy is 106 Mt. The masses of ejecta scale approximately with yield; scaling from the Cretaceous-Tertiary boundary claystone layer gives 1018 g and 1017 g (not necessarily all of it submicron sizes) globally dispersed ejecta for the 5-km and 2-km bodies, respectively. The resultant clay layers would be roughly 0.2 and 0.02 cm thick, respectively, and would not have been detected in the geologic record. Although the particle size distribution in impact-generated dust clouds is unknown and the clouds could have had much smaller submicron fractions than the nuclear clouds, estimates of impact-produced dust could be from one to several orders of magnitude larger than the 2 x 1014 g of smoke and 2 x 1013 g of submicron dust generated in the baseline nuclear war. The impact energies are also orders of magnitude greater than the 6500-Mt yield of the baseline nuclear war. Impacts roughly comparable to the baseline war in energy release and in dust lofted require objects of about 500-m diameter, and occur roughly once every 105 yr. Mass extinction events comparable to that at the end of the Cretaceous are fairly rare events in the history of the earth. Global mass extinctions that have been recognized at the taxonomic level of families of organisms have recurred at a mean interval of about 30 million years (Raup and Sepkoski, 1984~. There is some evidence that mass extinctions might have been produced by the impact of bodies as small as 5 km in diameter, but the impact of 2-km bodies appears not to have left an easily recognized imprint on the succession of life forms recorded by tOSSllS. It is clear that traceable catastrophes or the magnitude of the Cretaceous-Tertiary extinction are only produced by impacts with energy releases substantially exceeding those of a possible near-term global nuclear exchange. On the other hand, quite severe perturbations of the environment that did not succeed in producing extinction of many species--and with durations of a few years or less--cannot be easily detected in the stratigraphic record. Therefore it is not, at present, known whether impacts with energies in the range of 104 to 106 Mt had atmospheric effects similar to or even more severe than those projected for the baseline nuclear war. Further studies of the environmental effects of large asteroidal impacts as well as studies of the debris lofted by asteroid impacts may be valuable for establishing analogs for the nuclear war case. Toon et al. {1982) have shown that impact events that produce masses of dust as small as 1017 g should have produced light levels and low temperatures very similar to those of 101 -g impacts. Moreover, light levels low enough to cause failure of photosynthesis may occur with injections of no more than 1016 or 1017 ~ of dust. Hence the impacts of bodies 2 km in diameter or somewhat smaller may have produced both physical and biological effects that would be detectable in the stratigraphic record by means of a directed intensive search.

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182 REFERENCES Alvarez, L.W., F. Asaro, and H.V. Michel (1980) Extraterrestrial causes for the Cretaceous-Tertiary extinction. Science 208:1095-1108. Alvarez, W.L., F. Asaro, and H.V. Michel (1982) Current status of the impact theory for the terminal Cretaceous extinction. Geol. Soc. Am. Spec. Pap. 190:305-315. Cadle, R.D., C.S. Riang, and J.F. Louis (1976) The global scale dispersion of the eruption clouds from major volcanic eruptions. J. Geophys. Res. 81:3125-3132. Chuan, R.L., W.I. Rose, and D.C. Woods (1984) Size and chemistry of small particles in eruption clouds, In Clastic Particles: Scanning Electron Microscopy and Shape Analysis of Sedimentary and Volcanic Clasts, edited by J. Marshall. Stroudsburg, Pa.: Hutchinson Ross (in press). Chuan, R.L., D.C. Woods, and M.P. McCormick (1981) Characterization of aerosols from eruptions of Mount St. Helens. Science 211:830-832. Clanton, U.S., J.L. Gooding, and D.P. Blanchard (1982) Volcanic ash "clusters n in the stratosphere after the E1 Chichon, Mexico, eruption. Eos Trans. AGU 64:1139. Deirmend j fan, D. (1973) On volcanic and other particulate turbidity anomalies. J. Adv. Geophys. 16:267-297. Emiliani, C., E.B. Kraus, and E.M. Shoemaker (1981) Sudden death at the end of Mesozoic. Earth Planet. Sci. Lett. 55:317-334. Evans, W.F.J., and J.B. Kerr (1983) Estimates of the amount of sulfur dioxide injected into the stratosphere by the explosive volcanic eruptions--E] Chichon, Mystery Vulcan, Mount St. Helens. Geophys. Res. Lett. 10:1049-1052. Farlow, N.H., V.R. Overbeck, K.G. Snetsinger, G.V. Ferry, G. Polkowski, and D.M. Hayes (1981) Size distributions and mineralogy of ash particles in the stratosphere from eruptions of Mount St. Helens. Science 211:832-834. Francis, P. (1983) Giant volcanic calderas. Sci. Am. 248~6~:60-70. Gooding, J.L., U.S. Clanton, E.M. Gabel, and J.L. Warren (1983) E1 Chichon volcanic ash in the stratosphere--Particle abundances and size distributions after the 1982 eruption. Geophys. Res. Lett. 10:1033-1036. Hammer, C.U., H.B. Clausen, and W. Dansgaard (1980) Greenland ice sheet evidence of post-glacial volcanism and its climatic impact. Nature 228:230-235. Hirono, M., and T. Shibata (1983) Enormous increase in stratospheric aerosols over Fukooka due to volcanic eruption of E1 Chichon in 1982. Geophys. Res. Lett. 10:152-154. Hofmann, D.J., and J.M. Rosen (1983) Stratospheric sulfuric acid fraction and mass estimate for the 1982 volcanic eruption of E1 Chichon. Geophys. Res. Lett. 10:313-316. Humphreys, W.J. (1940) Physics of the Air. New York: McGraw-Hill. Iwasaki, Y., S. Hayashida, and A. Ono (1983} Increasing backscattered light from the stratospheric aerosol layer after Mt. E1 Chichon eruption. Geophys. Res. Lett. 10:440-442.

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183 Kennett, J.P., and R.C. Thunnel (1977) On explosive Cenozoic volcanism and climatic implication. Science 196:1231-1234. Krueger, A.J. (1983) Sighting of E1 Chichon sulfur dioxide clouds with the Nimbus 7 total ozone mapping spectrometer. Science 220:1377-1379. Landsberg, H.E., and J.M. Albert (1974) The summer of 1816 and volcanism. Weatherwise 27:63-66. Lazrus, A.L., R.D. Cadle, B.W. Gandrud, J.P. Greenberg, B.J. Huebert, and W.I. Rose (1979) Sulfur and halogen chemistry of the stratosphere and of volcanic eruption plumes. J. Geophys. Res. 84:7869-7875. Lewis, J.S., G.H. Watkins, H. Hartman, and R.G. Prinn (1982) Chemical consequences of major impact events on earth. Geol. Soc. Am. Spec. Pap. 190:215-221. Mossop, S.C. (1963) Stratospheric particles at 20 km. Nature 199:325-327. Mossop, S.C. (1965) Stratospheric particles at 20 km altitude. Geochim. Cosmochim. Acta 29:201-207. Murrow, P.J., W.I. Rose, and S. Self (1980) Determination of the total grain size distribution in a volcanian eruption column, and its implications to stratospheric aerosol perturbation. Geophys. Res. Lett. 7:893-896. Newell, R.E., and A. Deepak (1982) Mount St. Helens eruptions of 1980: Atmospheric effects and potential climatic impact. NASA SP-458. 119 pp. Pollack, J.B., O.B. Toon, C. Sagan, A. Summers, B. Baldwin, and W. Van Camp (1976) Volcanic explosions and climatic change: A theoretical assessment. J. Geophys. Res. 81~6~:1071-1083. Rampino, M.R. , and S. Self (1982) Historic eruptions of Tambora (1825), Krakatau (1853), and Agung (19631: Their stratospheric aerosols, and climatic impact. Quat. Res. 18:127-143. Raup, D.M., and J.J. Sepkoski, Jr. (1984) Periodicity of extinctions in the geologic past. Proc. Nat. Acad. Sci. USA 81:801-805. Robock, A. (1978) Internally and externally caused climate change. J. Atmos Sci. 35:1111-1122. Rose, W.I., and M.F. Hoffman (1982) The May 18, 1980 eruption of Mount St. Helens: The nature of the eruption, with an atmospheric perspective. NASA CP-2240. Pages 1-14. Rose, W.I., A.T. Anderson, S. Bonis, and L.G. Woodruff (1978) The October 1974 basaltic tephra from Fuego Volcano, Guatemala: Description and history of the magma body. J. Volcanol. Geotherm. Res. 4:3-53. Rose, W.I., Jr., T.J. Bornhorst, S.P. Halsor, W.A. Capaul, and P.S. Plumley (1983a) Volcan E1 Chichon, Mexico: Pre-1982 S-rich eruptive activity. J. Volcanol. Geotherm. Res. 23:tin press). Rose, W.I., R.L. Wunderman, M.F. Hoffman, and L. Gale (1983b) A volcanologist's review of atmospheric hazards of volcanic activity. J. Volcanol. Geotherm. Res. 17:133-157. Sedlacek, W.A., E.J. Mroz, A.L. Lazrus, and B.W. Gandrud (1983) A decade of stratospheric sulfate measurements compared with observations of volcanic eruptions. J. Geophys. Res. 88:3741-3776.

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184 Self, S., M.R. Rampino, and J.J. Bar bera (1981) The possible effects of large 19th and 20th century volcanic eruptions on zonal and hemispheric surface temperatures. J. volcano!. Geotherm. Res. 11:41-60. Sepkoski, J.J., Jr. (1982) Mass extinctions in the Phaneozoic oceans: A review. Geol. Soc. Am. Spec. Pap. 190:283-289. Settle, M. (1978) Volcanic eruption clouds and the thermal power output of explosive eruptions. J. Volcanol. Geotherm. Res. 3:309-324. Shoemaker, E.M. (1983) Asteroid and comet bombardment of the earth. Ann. Rev. Earth Planet. Sci. 11:461-494. Sigurdsson, H., S. Carey, and J.M. Espinbola (1984) 1982 eruptions of E1 Chichon Volcano, Mexico: Stratigraphy of pyroclastic deposits. J. Volcanol. Geotherm. Res. 23:tin press). Simlcin, T. (1981) Volcanoes of the World. Smithsonian Institution. Stroudsburg, Pa.: Hutchinson Ross. Toon, O.B., and J.B. Pollock (1982) Stratospheric aerosols and climate. Pages 121-147 In The Stratospheric Aerosol Layer, edited by R.C. Whitten. Berlin: Springer Verlag. Toon, O.B., J.B. Pollack, T.P. Ackerman, R.P. Tur co, C.P. McKay, and M.S. Liu (1982) Evolution of an impact generated dust cloud and its effects on the atmosphere. Geol. Soc. Am. Spec. Pap. 190. Varekamp, J.C., J. Luhr, and K. Prestegaard (1984) The 1982 eruptions of E1 Chichon Volcano (Chiapas, Mexico): Character of the eruptions, ash-fall deposits, and gas phase. J. Volcanol. Geotherm. Res. 23:(in press). Wilson, L., R.S.J. Sparks, T.C. Huang, and N.D. Watkins (1978) The control of volcanic column heights by eruption energetics and dynamics. J. Geophys. Res. 83(B4~:1829-1836.