Comet disintegration proceeds through both sublimation and discrete splitting events. The cross-sectional area of material ejected by a comet may, within days, become many times greater than that of the Earth, making encounters with such debris much more likely than collisions with the nucleus itself. The hierarchic fragmentation and sublimation of a large comet in a short-period orbit may yield many hundreds of such short-lived clusters. We model this evolution with a view to assessing the probability of an encounter that might have significant terrestrial effects, through atmospheric dusting or multiple impacts. Such an encounter may have contributed to the large animal extinctions and sudden climatic cooling of 12 900 yr ago, and the near-simultaneous collapse of civilisations around 2350 BC.