Catastrophe theory applied to ice ages: by no means a new idea, but astronomer Hoyle's combination of scientific expertise and selective blindness has an allure of its own. According to Hoyle, available evidence indicates that the Pleistocene glacial epochs descended rapidly rather than by a gradual buildup of ice. And another ice age might occur in the same way, he warns. To explain how, he discusses the functioning of the Earth as a global heat engine which could--the ""greenhouse effect"" notwithstanding--cool catastrophically after formation of ""diamond dust"" (highly reflective ice crystals) in the upper atmosphere. The change might be almost instantaneous, however: a large stony meteorite colliding with the earth could produce a similar layer of reflective particles and thus induce a sudden ice age. (An opposite warming effect could occur after a collision with an iron meteorite, whose particles absorb heat rather than reflect it.) The danger is real and pressing, says Hoyle; So we must quickly warm up the global ocean to prevent disaster. Exhilarating, sometimes; but less fact than fanciful speculation.