Past Climate Cycles: Ice Age Speculations

To understand climate change, the obvious first step would be to explain the colossal coming and going of ice ages. Scientists devised ingenious techniques to recover evidence from the distant past, first from deposits left on land, then also from sea floor sediments, and then still better by drilling deep into ice. These paleoclimatologists succeeded brilliantly, discovering a strangely regular pattern of glacial cycles. The pattern pointed to a surprising answer, so precise that some ventured to predict future changes. The timing of the cycles was apparently set by minor changes in sunlight caused by slow variations of the Earth's orbit. Just how that could govern the ice ages remained uncertain, for the climate system turned out to be dauntingly complex. One lesson was clear: the system is delicately poised, so that a little stimulus might drive a great change. (There is a separate essay on shorter-term climate fluctuations, lasting a few years to a century or so, possibly related to Variations of the Sun.)