While we agonize over the future of human civilization in the face of climate change, a decade-old theory raises a different question: Would civilization ever have arisen without global warming?
Global warming. Climate change. Whatever you call it, here's what you've probably heard about it, in a nutshell: For a bit more than a century, humans have been burning fossil fuels, emitting carbon dioxide and other greenhouse gases at an unprecedented rate. As a result, the global average temperature has risen approximately one and a half degrees since 1880, sea level has risen an average of close to eight inches, and weather patterns everywhere are in flux.
That seems like enough, but there's another part to the story you probably haven't heard ... and you may want to sit down for it. There's evidence that human activities have been driving up greenhouse gas levels for some seven thousand years. That's millenia, not centuries. And, if the theory bears out, it means that no one alive today - for that matter, no one in recorded history - has experienced a climate unaltered by humans.
The concept was first posited over a decade ago by Dr. Bill Ruddiman, professor emeritus at University of Virginia. A climate scientist himself, he was staring at long-term trends in greenhouse gas concentrations derived from bubbles trapped in ice sheets when he noticed what he calls the "wrong-way trend." In a period of time when all other indicators suggested greenhouse gases should be declining, carbon dioxide and then methane turned around and started rising.
The only explanation Ruddiman could find for what he was observing was human activity - agriculture, to be specific. 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, creating man-made wetlands that would emit methane just like natural ones do.
All told, Ruddiman estimates the amount of greenhouse gases added to the atmosphere prior to the Industrial Revolution totals nearly twice what we've emitted since then - enough to have averted an impending ice age.
It was a highly controversial idea at the time, and remains the subject of debate. That doesn't bother Bill Ruddiman.
"This is a real, honest-to-God climate debate," says Ruddiman, "as opposed to the modern anthropogenic greenhouse gas warming, which is not a debate. That's settled science."
Ruddiman has spent much of the past several years gathering evidence in support of his theory. Most of it comes from archaeologists and anthropologists, not climate scientists. And he says he finds greater acceptance of his ideas in those fields, as well. But he says the tide seems to be shifting in his favor.
At this point, Ruddiman says, estimates of rice farming and livestock domestication fully explain the rise in methane. The carbon dioxide story still needs some work, but new models of population growth and resource use in the early days of agriculture come closer to matching the geological record of carbon dioxide levels.
It all raises one lingering, uncomfortable question: Would human civilization have been possible without human-caused global warming?