It's ecological theory with a delightful Malthusian twist: the BBC reports on a study which suggests that the population decline following the 14th century plague pandemic known as the Black Death caused the climate cooling known as The Little Ice Age. Fewer people clearing farmland = more trees = less carbon dioxide in the atmosphere = cooler temperatures. Or so the theory goes. Wait until Zero Population Growth gets together with Greenpeace to strategize. Early advice: don't accept any suspicious letters or unsolicited Halloween candy from environmentalists.