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This PDF File Is Subject to the Following Conditions and Restrictions Geological Society of America 3300 Penrose Place P.O. Box 9140 Boulder, CO 80301 (303) 447-2020 • fax 303-357-1073 www.geosociety.org This PDF file is subject to the following conditions and restrictions: Copyright © 2002, The Geological Society of America, Inc. (GSA). All rights reserved. Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited copies for noncommercial use in classrooms to further education and science. For any other use, contact Copyright Permissions, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, fax 303-357-1073, [email protected]. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Geological Society of America Special Paper 356 2002 Impact lethality and risks in today’s world: Lessons for interpreting Earth history Clark R. Chapman* Southwest Research Institute, Suite 426, 1050 Walnut Street Boulder, Colorado 80302, USA ABSTRACT There is a modern-day hazard, threatening the existence of civilization, from impacts of comets and asteroids larger than ϳ1.5 km diameter. The average annual world fatality rate is similar to that due to significant accidents (e.g., airplane crashes) and natural disasters (e.g., floods), although impact events are much more rare and the deaths per impact event are much greater. (Smaller, more frequent impacts can cause regional catastrophes from tsunamis of unprecedented scale at intervals similar to the duration of recorded human history.) As the telescopic Spaceguard Survey census of Near Earth Asteroids advances, numerical simulations of the dynamic and collisional evolution of asteroids and comets have become robust, defining unambig- uously past rates of impacts of larger, more dangerous cosmic bodies on Earth. What are very tiny risks for impacts during a human lifetime become certainties on geologic time scales. Widely reported errors in predictions of possible impacts during the next century have no bearing on the certainty that enormous impacts have happened in the past. The magnitudes and qualitative features of environmental consequences of impacts of objects of various sizes are increasingly well understood. Prime attributes of impacts, not duplicated by any other natural processes, are (1) extreme suddenness, providing little opportunity for escape and no chance for adaptation, (2) globally pervasive, and (3) unlimited potential (for Cretaceous-Tertiary [K-T] boundary-scale impacts and larger) for overwhelming destruction of the life-sustaining characteristics of the fragile ecosphere, notwithstanding the rather puny evidence for impacts in the geologic record. A civilization-ending impact would be an environmental and human catastrophe of unprecedented proportions. The K-T-scale impacts, of which there must have been at least several during the Phanerozoic (past 0.5 b.y.), are 1000 times more destructive. No other plausible, known natural (or human) processes can ap- proach such catastrophic potential. The largest impacts must have caused mass ex- tinctions in the fossil record; other natural processes could not have done so. Per- spectives concerning both the potential modern-day destructive potential of impacts and conceivable, almost miraculous refugia in our own world provide a new gestalt for thinking about past cataclysms. *E-mail: [email protected] Chapman, C. R., 2002, Impact lethality and risks in today’s world: Lessons for interpreting Earth history, in Koeberl, C., and MacLeod, K.G., eds., Catastrophic Events and Mass Extinctions: Impacts and Beyond: Boulder, Colorado, Geological Society of America Special Paper 356, p. 7–19. 7 8 C.R. Chapman INTRODUCTION onstrate that they have no bearing on the certainty that impacts with unparalleled ecological consequences happened in the The idea that cosmic impacts on the Earth have played a past. I then discuss the implications for the role of impacts in significant, or even dominant, role in mass extinctions (and sub- Earth history from lessons learned in the study of the impact sequent explosive radiation of new species) has evolved from hazard. widespread skepticism to substantial acceptance during the two The following assertions about past mass extinctions are decades since publication of the Alvarez et al. (1980a) hypoth- justified later herein. esis concerning the Cretaceous-Tertiary (K-T) boundary. How- It is virtually certain that several other impacts have oc- ever, there remain pockets of nonacceptance as well as a wide curred during the Phanerozoic (past 0.5 b.y.) that had at least spectrum of opinions about the degree to which impacts have the energy and potential ecological consequences of the Chi- influenced evolution. Even among those who fully accept a role cxulub K-T boundary impact, and that many other impacts have for impact in the K-T boundary extinctions, views range all the occurred with potential global consequences nearly as great. way from belief that the only substantiated case of an impact This is not a hypothesis: it is an inescapable fact derived from playing a role in mass extinction was the Chicxulub impact 65 robust knowledge of asteroids and comets. Ma, which is viewed as just hastening the demise of already There is no other plausible, known kind of natural calamity stressed populations, to the hypothesis of Raup (1991), that all that can possibly approach asteroid and/or comet impact in mass extinctions, large and small, could have been caused by terms of the suddenness of the onset of devastating global con- sudden environmental changes due to impacts. sequences. (I exclude human devastations such as nuclear war, As the Alvarez hypothesis was researched, awareness grew as well as other conceivable but unlikely disasters; e.g., a among scientists and the public of the modern-day risk to civ- nearby supernova.) I assert that this suddenness, ranging from ilization from cosmic impacts. Once the purview of science minutes to months, greatly magnifies the devastation compared fiction, it has become widely accepted (e.g., the report of an with any other equally profound geologic, oceanic, or meteor- independent Task Force set up by the British Government; At- ological catastrophe. kinson, 2000) that the threat of a calamitous impact ranks The largest impacts during the Phanerozoic must have among other hazards meriting national and international atten- caused mass extinctions and, conversely, no other known plau- tion and consideration of preventative measures. While the sible mechanism can approach the magnitude of consequences chances of impact with an asteroid larger than 1.5 km diameter of such impacts. Therefore, the largest mass extinctions must (deemed sufficient to threaten modern civilization; Chapman have been caused by impact. (Only if required evidence of such and Morrison, 1994) are very small, about one chance in several impacts is missing from the geologic record must one then turn hundred thousand per year, the potential consequences are so to the unlikely alternative explanations, e.g., a nearby super- enormous (perhaps the deaths of a quarter of the world’s popu- nova or explosion of an unexpectedly stupendous super- lation) that the annualized fatality rate is similar to fatality rates volcano.) associated with other natural hazards, such as floods and earth- What is commonly accepted among impact hazard re- quakes (Fig. 1). searchers as the threshold size of asteroid that could terminate Research on the impact hazard, especially during the civilization as we know it (1.5 km diameter) is more energetic 1990s, has yielded a voluminous literature on the numbers and than the explosive force of the world’s combined arsenals of physical traits of the impactors, on the physical and environ- nuclear weapons by a factor of ϳ20. Yet the magnitude of the mental effects of impacts, and even on the potential response K-T boundary impactor (10–15 km diameter), and each of the of human society to an imminent impact or to the aftermath of several other equivalent or larger impacts that must have oc- one. With the perspective from modern research on the impact curred since the Phanerozoic, is equivalent to a thousand hazard, the issues faced by historical geologists trying to un- civilization-ending impacts, all occurring simultaneously. The derstand the role of impacts during Earth history can be viewed miracle is that anything survived. Perhaps the best way to vi- from a new gestalt. sualize mass extinctions is to try to imagine the refugia that However, the modern-day impact hazard is not well un- might exist for us, and for various species of animals and plants, derstood by the public, by policy makers, and even by most in our modern world after it has been utterly devastated by an scientists. Both the extremely low probabilities and the ex- unimaginably colossal holocaust. tremely great consequences of impact tax our intuition and common sense, because they are so far beyond the realm of our THE POPULATION OF IMPACTORS personal, or even historical, experience. Therefore, before ap- plying insights from studies of the modern impact hazard to the Geologists have traditionally invoked the uniformitarian historical record (the subject of a following section), I first in- concept that continuous geologic processes observable today troduce the impactor population and what is known about the can account for what is observed in the geologic record of the consequences of impacts. I then discuss the issues of risk per- past. In recent decades, a reasonable balance has been achieved ception and uncertainties in impact prediction in order to dem- between this two-century-old tradition and the important role Impact lethality and risks in today’s world: Lessons for interpreting Earth history 9 Figure 1.
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