<P> Most paleontologists now agree that an asteroid did hit the Earth about 66 Ma ago, but there is an ongoing dispute whether the impact was the sole cause of the Cretaceous--Paleogene extinction event . </P> <P> Sustained and significant global cooling could kill many polar and temperate species and force others to migrate towards the equator; reduce the area available for tropical species; often make the Earth's climate more arid on average, mainly by locking up more of the planet's water in ice and snow . The glaciation cycles of the current ice age are believed to have had only a very mild impact on biodiversity, so the mere existence of a significant cooling is not sufficient on its own to explain a mass extinction . </P> <P> It has been suggested that global cooling caused or contributed to the End - Ordovician, Permian--Triassic, Late Devonian extinctions, and possibly others . Sustained global cooling is distinguished from the temporary climatic effects of flood basalt events or impacts . </P> <P> This would have the opposite effects: expand the area available for tropical species; kill temperate species or force them to migrate towards the poles; possibly cause severe extinctions of polar species; often make the Earth's climate wetter on average, mainly by melting ice and snow and thus increasing the volume of the water cycle . It might also cause anoxic events in the oceans (see below). </P>

When did each of the 5 major mass extinctions occur