We know, because they often say so, that those who think catastrophic global warming is probable and perhaps imminent are exemplary empiricists. They say those who disagree with them are "climate change deniers" disrespectful of science.

Actually, however, something about which everyone can agree is that of course the climate is changing — it always is. And if climate Cassandras are as conscientious as they claim to be about weighing evidence, how do they accommodate historical evidence of enormously consequential episodes of climate change not produced by human activity? Before wagering vast wealth and curtailments of liberty on correcting the climate, two recent books should be considered.

In "The Third Horseman: Climate Change and the Great Famine of the 14th Century," William Rosen explains how Europe's "most widespread and destructive famine" was the result of "an almost incomprehensibly complicated mixture of climate, commerce, and conflict, four centuries in gestation." Early in that century, 10 percent of the population from the Atlantic to the Urals died, partly because of the effect of climate change on "the incredible amalgam of molecules that comprises a few inches of soil that produces the world's food."