Cometary impacts and ice-ages

Cycles of glaciation with an average period of ∼ 100 kyr are mediatedby impacts of cometary bolides. Ice-age conditions are dry and dusty withlow rates of precipitation. Comets in the mass range 1015 -1016 gimpacting the oceans could release enough water vapour into the atmosphereto enhance a depleted greenhouse effect. The energy deposited in the oceanswould also warm the surface layers, thus starting up anevaporation-precipitation cycle which ushers in warmer interglacialinterludes. The latter have neutral stability and are necessarilyshort-lived, eventually drifting back to glacial conditions on timescalesof ∼ 10 kyr.