Warming is becoming a major problem. "A change in our climate," writes one deservedly famous American naturalist, "is taking place very sensibly." Snowfall, he notes, has become "less frequent and less deep." Rivers that once "seldom failed to freeze over in the course of the winter, scarcely ever do so now."

And it's having an especially worrisome effect on the food supply: "This change has produced an unfortunate fluctuation between heat and cold, in the spring of the year, which...