There's evidence that human activities have been driving up greenhouse gas levels for some seven thousand years. The concept was first posited over a decade ago by Dr. Bill Ruddiman, professor emeritus at University of Virginia. ... Carbon dioxide levels started rising between seven and eight thousand years ago, around the same time that the spread of agriculture would have led to pervasive deforestation. Methane didn't start rising until about five thousand years ago; that corresponds to the period when rice farmers in China, and then the rest of Asia, began irrigating rice paddies, c...