IMPACT LETHALITY AND RISKS IN TODAY'S WORLD: LESSONS FOR INTERPRETING EARTH HISTORY

Abstract

There is a modern-day hazard, threatening the existence of civilization, from impacts of comets and asteroids larger than 1.5 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.