Did Climate Change Kill the Roman Empire?

Rainfall data suggest climate change may have partly caused the empire's fall.

December 9, 2008, 6:38 PM

Dec. 10, 2008— -- Scientists have discovered extraordinarily precise data on rainfall in the Mediterranean region from 200 B.C. to 1100 A.D. which suggests that the decline of the Roman and Byzantine empires may have been partly caused by climate change.

It is not likely to end the debate among historians, some of whom believe the fall was more of a transformation than a collapse, but it is a tantalizing bit of evidence. And the way it was collected is as intriguing as the fact that researchers can now analyze rainfall on a year-to-year basis, season to season, even many thousands of years ago.

For more than 15 years scientists at the Geological Survey of Israel and Hebrew University have been studying stalactites from a cave near Jerusalem. Recently,Ian Orland and geology professor John Valley of the University of Wisconsin-Madison began the study of oxygen isotopes in these samples. The Israeli scientists have dated some of the stalactites to about 185,000 years ago, and they have reconstructed broad climate fluctuations over many years because the formation of the calcite deposits depends partly on rainfall.

But those records indicate averages over long periods of up to a century, and Valley wanted more precise data because of the current interest in global climate change. He can do that now, thanks to a new instrument that he has been trying to develop for over 20 years.

The first in a new generation of "ion microprobes" in the world arrived in his lab two years ago, just as an enterprising graduate student, Ian Orland, started working on his master's degree in geology.

"The advantage of the ion microprobe is it allows us to analyze samples that are a million to a billion times smaller than we could in the past," Valley said in a telephone interview. It the old days, say a couple of years ago, scientists used dental drills to collect samples from the stalactites, but those samples were about the size of a pencil point. A pencil point, in the growth of stalactites, can cover an entire century.

The ion microprobe uses a tightly focused beam of ions to analyze microscopic points in a cross section of a stalactite. These samples were first imaged with a laser to show annual bands that look somewhat like tree rings. Orland developed techniques to use the laser to image those points, even separating a single year down to individual seasons. The age is determined by measuring isotopes of uranium and thorium that decay at a known rate.

"We're measuring samples that are a million times smaller than you could measure in the past," Orland said in an interview.

That has enabled the Wisconsin team to reconstruct the climate record, year by year, during the time when both the Roman and Byzantine empires were struggling to survive. That's not what the scientists had set out to do, but the coincidence was just too great to pass up.

"The downfall of the Byzantine empire lies right in the middle of one of the samples," Orland said.

The various samples reveal gradual reduction in rainfall all the way back to 100 A.D., according to a study to be published in the journal Quaternary Research. Orland, who is now a doctoral candidate, is the lead author of that study, which notes that a number of historians have suggested that drought may have contributed to the decline of both empires.

"We are not able to say for sure that climate change was definitely the reason that the Roman Empire and the Byzantine Empire declined, but rather that we now have data that correlates with the hypothesis historians have put forward," Orland said. "We're just adding data to the big pile that's already there."

Scholars have debated for centuries over the causes of those declines, and no doubt much more was at work than just climate change. But some records indicate that trade and agriculture both declined, which could have been caused by droughts. However, many historians also point to high taxes on goods and farmland as a probable cause for those declines.

If weather does turn out to be a significant player, it will not be an isolated incident. North America's Anasazi Indians created impressive "cities" throughout what is now the U.S. Southwest, including six-story apartment complexes that predate the arrival of the first Europeans, and wide roads stretching 200 miles, even though they had no vehicles and no beasts of burden.

But about 1,000 years ago, the Anasazi abandoned their homes, and historians have argued for years over why. Tree rings analyzed at the University of Arizona indicate that the four corners region where the Anasazi maintained their cultural centers, went into prolonged drought, and thus, the Indians fled south to join the Hopis and an easier life.

But that, according to a number of scholars, is probably only part of the reason. Big monuments, built by years of hard labor more than 1,000 years ago, were probably demanded by leaders who perhaps had become more worrisome than drought. The answer as to why civilizations collapse is never as simple as a single cause, although drought has been linked to other collapses in Asia and elsewhere around the world.

The Wisconsin researchers said they weren't trying to resolve that controversy. It just kind of fell into their laps.

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