How long has climate change been around?

It is an odd-sounding idea, because the problem is usually assumed to be a modern one, the product of a world created by the Industrial Revolution and powered by high-polluting fossil fuels. But a professor emeritus at the University of Virginia has suggested that people began altering the climate thousands of years ago, as primitive farmers burned forests and built methane-bubbling rice paddies. The practices produced enough greenhouse gases, he says, to warm the world by a degree or more.

Other scientists, however, have said the idea is deeply flawed and might be used to dampen modern alarms over climate change. Understanding the debate requires a tour through polar ice sheets, the inner workings of the carbon molecule, the farming habits of 5,000-year-old Europeans and trapped air bubbles more ancient than Rome.

“The greenhouse gases went up, and they should have gone down” many thousands of years ago, said William Ruddiman. “Why did that happen?” His answer is based on circumstantial evidence. Ruddiman said two events in world history—an apparent shift in the composition of the atmosphere and the first explosion of human agriculture—took place at nearly the same time.