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GUY RAZ, host:

If you read any account of why the Roman Empire fell, you'll likely to come across stories of barbarian invasions, political intrigue, even economic collapse.

But add one more to the mix: climate change. That's what a new study published in the journal Science suggests. It was written by a team from the Swiss Federal Institute for Forest, Snow and Landscape Research. They analyze tree rings on pieces of ancient wood and scoured peat bogs and old Roman ruins.

Michael Mann is a professor of meteorology at Penn State University and he joins me now to explain this new research.

Welcome.

Professor MICHAEL MANN (Meteorology, Pennsylvania State University): Great to be here.

RAZ: We are talking about the period from 250 to about 550 A.D. These are the final centuries of Roman imperial dominance. Describe what was happening with respect to the climate.

Prof. MANN: Well, at the end of the period, it appears that the climate began to become more variable. Rainfall patterns and temperature patterns fluctuated more from year to year and from decade to decade. And the authors argue that that presented some real challenges for a civilization and may have had a substantial role in the downfall of the Roman Empire.

RAZ: This study went back 2,500 years. One of the ways they did this was to look at tree rings, right? I mean, they had about 9,000 samples. They looked at peat bogs and ruins. And I guess in years where the weather is good, the tree rings are bigger, and when the weather is bad, the rings are smaller. Is that about right?

Prof. MANN: Exactly. And so they were able to tease out two different pieces of information from these trees. They can get some idea of how warm the summers were and how wet the sort of late spring, early summer was.

RAZ: So, for example, would it be cold and wet, rainy one year and then really hot the next year? Was it that dramatic?

Prof. MANN: I think that's basically the conclusion that they're drawing.

RAZ: You're a meteorologist, and the scientists who worked on this are also not historians. But you have a basic understanding of what was happening on the ground at the time. I mean, what are some of the theories for how the fluctuations in the climate could have contributed to the collapse of the Roman Empire?

Prof. MANN: Well, you know, the Roman Empire was a large empire. And like any large civilization, including the civilization we have today, it was highly dependent on predictability of natural resources; the predictability of water supply, the predictability of food resources. It was very heavily adapted to the climate conditions that had persisted for centuries.

So whenever you have unpredictable and dramatic changes in climate, you're likely to see challenges such as those that led presumably to the fall of the Roman Empire.

RAZ: Michael Mann, do we know why this was happening? I mean, this is long before industrialization. So presumably, can we rule out human interference?

Prof. MANN: Well, there are a number of factors that we know are responsible for natural variability of the climate. The amount of energy put out by the sun varies from decade to decade and century to century enough to cause some degree of climate variability. We also know a volcanic eruption can cool the global climate for several years.

And so presumably, it was some combination of these external natural factors like solar variability and volcanic eruptions and just the pure sort of chaotic internal variability of the climate system like we see today. Right now, we are experiencing a very substantial La Nina event...

RAZ: Right.

Prof. MANN: ...in the tropical Pacific, and that's influencing weather patterns around the globe. That's a natural phenomenon. El Nino and La Nina come and go naturally.

So these are the sorts of concerns that would have played out in the past and are playing out today as we proceed with this uncontrolled experiment of changing our climate.

RAZ: That's Michael Mann. He's a professor of meteorology at Penn State University and the author of the book "Dire Predictions: Understanding Global Warming."

Michael Mann, thank you.

Prof. MANN: Thank you. It's my pleasure.

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