Climatic changes may have wiped out Mycenae and Unleashed the Vikings, this brief, highly suggestive book demonstrates, and since the overheated earth appears to be cooling off, we should look to the consequences: an average temperature change of only 6'C is sufficient to produce an ice age. Not that this is an alarmist tract. Bryson is a noted climatologist based at the University of Wisconsin, where much of the evidence for worldwide climatic patterns has been developed. With the aid of Murray, a science writer, he explains to the layman how the disappearance of Indian farming villages from the Great Plains after about 1200 (the result of drought) was correlated with the spread of St. Anthony's Fire, a fungus blight, in western Europe (the result of mild, wet weather) and the fatal isolation of Norse colonies in Greenland (because of increasingly heavy drift ice) to reveal a worldwide pattern of expanding westerlies--in effect, an expansion of the Arctic. The same panoply of climatic indicators--tree-ring counts, wine-harvest dates, glacier movement, the bone, potsherd, and pollen census at archaeological sites--permits the world's climate to be reconstructed for the past 1,000 years. From 1850 to 1950, the period generally considered ""normal,"" the northern hemisphere became--in the long view--exceptionally warm; but changes elsewhere since the early 1960s--notably the failure of the summer monsoon rains in the Sahel (the semi-arid zone south of the Sahara) and in northern India--point to another southward expansion of the westerlies: atmospheric evidence apart, famine often comes to India when Iceland cools. Man's role is unassayable but not negligible: dust is a rain-retardant, whether over deserts or over cities. Of particular concern, however, is the possibility of sudden change. Anything which affects the amount of sun energy the earth receives, or its distribution, can change the How of westerlies and the number of whole loops they make: ""monsoon rains can disappear from an area within a few years."" That the climate is not fixed, changes may be abrupt, food production will be affected: these are the critical lessons--reached via intriguing detection.