We review the major mechanisms proposed to cause extinctions at the Cretaceous - Tertiary geological boundary following an asteroid impact. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of colliding objects over a range of energies and compositions; but we also stress that there are many uncertainties. We conclude that, for impact energies below about 10⁴ Mt (i.e. impact frequencies less than one in 6x10⁴ years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m respectively), blast damage, earthquakes and fires should be important on a scale of 10⁴ or 10⁵ km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 10⁴ to 10⁵ Mt (intervals up to 3x10⁵ yrs; comets and asteroids with sizes up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized earth reaches the stratosphere than we think likely, stratospheric dust (causing global cooling) would also be important in this energy range. This energy range is a conservative lower limit where damage might occur beyond the experience of human history. The energy range from 10⁵ to 10⁶ Mt (intervals up to 2X10⁶ years; comets and asteroid up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wildfires sparked by thermal radiation from the impact can produce climatologically significant global optical depths on the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock-heated air to destroy the ozone shield. Between 10⁶ and 10⁷ Mt (intervals up to 1.5x10⁷ years; comet and asteroid diameters up to 4 and 6.5 km respectively) dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 10⁷ km², and the resulting smoke would reduce light levels even further. At energies above 10⁷ Mt. blast and earthquake damage reach the regional scale (10⁶ km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world's ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust and sulfate that vision is not possible. At energies approaching 10⁹ Mt (>10⁸ years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous-Tertiary impact in particular struck evaporite substrates that very likely generated a dense widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.

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