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Public Radio's Environmental News Magazine (follow us on Google News)

An Ice Age Averted?

Air Date: Week of

Modern advances like autos and power plants have mostly been to blame for causing climate change. But a University of Virginia professor claims our ancestors had a hand in warming the planet. Host Steve Curwood talks with William Ruddiman, who says that human activity 8,000 years ago may have put off an Ice Age.


CURWOOD: When most of us think about how people might be disrupting the global climate, we tend to blame greenhouse gas emissions from cars, power plants and factories--all products of an industrial society not even 300 years old. But, William Ruddiman, a marine geologist and professor emeritus of environmental sciences at the University of Virginia, has another idea. He thinks our ancestors may have had a hand in climate change, as far back as 8,000 years ago.

In a cover story for the March issue of the magazine Scientific American, Professor Ruddiman says early human activity caused atmospheric levels of methane and carbon dioxide to jump at a time when they should have been falling. Up until then, the earth had regularly alternated between ice ages and warming periods, due to the wobbles in our orbit around the sun. And right now, the earth should be on the verge of an ice age. But, since the climate has by and large stayed warm for the past 8,000 years, Professor Ruddiman wanted to know why. He joins me from member station WMRA in Harrisonburg, Virginia. Hello, sir.


CURWOOD: So, what exactly were early humans doing 8,000 years ago that could have brought on global climate change?

RUDDIMAN: Well, basically they were farming. They were clearing forests across the southern tier of Eurasia in order to open up land so that the sun could get to the plants. Agriculture had been discovered in a couple of locations in the Middle East and in China 11,000 years ago, but it began to spread into forested areas around 8,000 years ago. So, that's one part of it. The other part is that by 5,000 years ago, people began to flood wetlands in Southeast Asia to irrigate the land to grow rice. So, clearing the forest generates carbon dioxide; irrigating generates methane.

CURWOOD: So, your hypothesis is that we would be in an ice age if it weren't for human activity. Do I have that right?

RUDDIMAN: That's basically right, although it's easy to overstate it. What I said was that ice sheets of some size would now be growing in the Northern Hemisphere, probably in far northeastern Canada. Now, this should not be understood to mean that there would have been massive ice sheets down to Toronto and Chicago and New York the way there were 20,000 years ago. These would be small, but growing ice sheets.

CURWOOD: What did you find in your research that led to your theory of ancient global warming?

RUDDIMAN: Well, basically part of my hypothesis is that if greenhouse gases had done what they normally do, what they naturally did in the past for the last three or 400,000 years, they would have decreased during the last several thousand years. But instead they started that kind of decrease but then they turned around and went the other way; they increased, they went the wrong way. Humans were putting greenhouse gases in the atmosphere that warmed up the atmosphere. And, in effect, that warming stopped a natural cooling. It kept the earth from cooling off.

CURWOOD: Now, some of your colleagues are skeptical of your hypothesis and the conclusions that you have from the data. We talked to a couple; Professor William Pelletier of the University of Toronto, for example, questions if there really could be, could have been enough deforestation and crop irrigation with the resulting releases of carbon dioxide and methane, to cause the increases that you cite here. How do you respond to this criticism?

RUDDIMAN: It is at first blush a valid criticism if you go back say to just before the beginning of the industrial era when I say these greenhouse gas anomalies had reached these very large sizes that I had mentioned earlier. There were about 500 million people, 600 million people around and that's only a tenth of the number of people that are alive today. So, if you look at how much methane or CO2 farming and deforestation generated today, from six billion people, and then you think, oh well, there were only 500 million people alive then, you have to wonder if, indeed, if there is enough farming activity to do that. But, I think there's an underlying fallacy to that point of view and it's basically this: we don't live, the average person today does not live the way the average person lived in 1700 or a 1,000 years ago or 2,000 years ago. Back then, almost everyone was a farmer and so everyone that was farming needed cleared land and therefore deforestation to do the farming. Today, most of us are not farmers and so the relationship is not a one-for-one relationship between population and greenhouse gas emissions.

CURWOOD: What are the implications for the future here? If climate change has been an ongoing phenomena for the last 8,000 years from human agricultural activity, what does it mean now that we have all these industrial emissions of greenhouse gases and much larger proportions than our farming ancestors emitted these gases?

RUDDIMAN: There is a fundamental difference between the early warming that I claim to have detected and the present day industrial-era warming. The early warming came on very slowly, but it also did not carry the greenhouse gas concentrations or the climate beyond the natural balance that had been varying over the last several 100,000 years. That's not the case for the current warming. The greenhouse gas concentrations are now well outside, well above their natural, the natural range that they have been varying at and the global temperature is just at the point of exceeding the natural range of temperature as well. So, we're heading into terra incognita. We are right now at the warm limit of how warm the earth has been in the last several 100,000 years and we're heading fairly quickly toward something a good deal warmer.

CURWOOD: Professor William Ruddiman is author of the article, "Did Humans Stop an Ice Age?" in the March issue of the Scientific American. Thanks for taking this time with me today, Bill.

RUDDIMAN: It's good talking to you, Steve.



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