Dynamical studies of the asteroid belt reveal it to be an inadequate source of terrestrial impactors of more than a few kilometers in diameter. A more promising source for large impactors is an unstable reservoir of comets orbiting between Jupiter and Neptune. Comets 100–300 km across leak from this reservoir into potentially hazardous orbits on relatively short time scales. With a mass typically 10 3 –10 4 times that of a Chicxulub-sized impactor, the fragmentation of a giant comet yields a highly enhanced impact hazard at all scales, with a prodigious dust influx into the stratosphere over the duration of its breakup, which could be anywhere from a few thousand to a few hundred thousand years. Repeated fireball storms of a few hours' duration, occurring while the comet is fragmenting, may destroy stratospheric ozone and enhance incident ultraviolet light. These storms, as much as large impacts, may be major contributors to biological trauma. Thus, the debris from such comets has the potential to create mass extinctions by way of prolonged stress. Large impact craters are expected to occur in episodes rather than at random, and this is seen in the record of well-dated impact craters of the past 500 m.y. There is a strong correlation between these bombardment episodes and mass extinctions of marine genera.

The impact of a large body in the oceans would inject large quantities of water through the tropopause cold trap into the stratosphere and lower mesosophere. We consider the consequences of enhanced water vapor concentrations on the middle atmosphere (50–100 km) chemistry and heat budget. The increased mixing ratio of hydrogen dramatically decreases the ozone concentration above 60 km. Catalytic reactions with odd hydrogen are the main sink of ozone in this region. The ozone reduction causes a lowering of the average height of the mesopause, as well as a lowering of the average temperature. The lower colder mesopause and the creation of saturation conditions over much of the upper mesosphere would have resulted in a permanent layer of mesospheric ice clouds of nearly world-wide extent. (At present, these exist only at high latitudes and are observed in summer as “noctilucent clouds.”) The globally-averaged albedo resulting from these clouds is dependent on the particulate size and shape, and can be as high as several percent, preferentially covering the summer hemisphere. This could have important implications for the short-term climate following a large-body impact. Similar effects would also result from an encounter with a more extended object such as a swarm of cosmic debris or a dense interstellar cloud.