The surprising findings of one researcher raise an important question in the history of climate change: Could the depopulation of the Americas have caused Europe’s Little Ice Age?

ROME — I was living here and researching the book when I reached William Ruddiman, an American scientist who had come across a graph of historical methane levels that unlocked a fascinating new insight into the history of climate change, one that connected ancient Europe and the newly discovered Americas.

Ruddiman, a paleoclimatologist at the University of Virginia, was near retirement after spending his early career examining ocean sediments for hints of ancient temperatures. He focused his later research on the major drivers of the world’s climate.

But the data, gleaned from air bubbles trapped in the Antarctic ice sheet, surprised him.

“I had a very clear expectation of what those concentrations should have been doing,” he said, when I interviewed him over the phone from my office in Rome.

Monsoons in the tropics have been weakening for the past 10,000 years, shrinking the swamps and wetlands that create most of the world’s natural methane. The trend in the ice cores should have been unrelentingly in decline. “It does go down from 10,000 to 5,000 years ago,” Ruddiman said.

“But then it reverses direction and goes up. It goes so far up that by the time we get to the industrial era, it’s as if the monsoon is cranking away at full speed. And yet the tropics are drying out. The methane trend went the wrong way. It shouldn’t have done that. Why did it do that?”

Ruddiman’s hypothesis, published after his retirement, generated more controversy than anything he had done during his career.

“If the natural sources weren’t putting this extra methane in the atmosphere, then it must have been something else,” he said.

“Humans were the obvious possibility.”

The bend in the methane graph, he concluded, occurred when farmers in China first began to grow rice in large quantities.

“People started to irrigate,” he said.

“They were better fed. The population started to boom. Larger numbers of people kept larger numbers of livestock, and that’s a source of methane. The people themselves are a source of methane. They probably burned off their fields every year, and that’s another source of methane.”

Human civilization, Ruddiman was arguing, had been altering the global climate since the beginning of large-scale agriculture.

When he looked at carbon dioxide, Ruddiman found a similar trend.

“It should have been falling the last 10,000 years, up until the industrial era,” he said. “And it did fall for a couple of thousand years, and then it started to rise. It’s a parallel problem. Why did it go up when at similar times in the past it’s always gone down?

“Well, there’s really good data in Europe showing the beginnings of what would become major deforestation starting just about 8,000 or 7,500 years ago,” he said.

"And by the time you get to maybe a thousand years ago, most of Europe is completely deforested. There’s probably more forest now in most of Europe than there was then.”

Intriguingly, carbon levels didn’t rise steadily. Around the time of the Roman era, they wobbled, dipped, then swung back up around A.D. 1000. Shortly after A.D. 1500 — in a period coinciding with the Little Ice Age, when the northern hemisphere plunged through a series of bitterly cold winters — they dove again.

If the rise in carbon dioxide was due to population growth and the cutting of trees, could the wobbles and drops Ruddiman was seeing have been caused by depopulation and reforestation?

The early dips matched up with plagues during the Roman and medieval eras, but the dating from the ice core data was hazy.

Ruddiman concentrated on the biggest decline, the one that began after A.D. 1500 and lasted more than 200 years.

“That carbon dioxide drop correlates with the biggest pandemic of all of preindustrial history, the arrival of Europeans into the Americas,” he said.

Before the arrival of Christopher Columbus, the New World was home to between 50 and 60 million people. Two hundred years later, about 5 million native Americans remained.

“Entire villages that once lined the valleys of the lower Mississippi River system were abandoned, along with endless cornfields in between,” Ruddiman wrote in "Plows, Plagues, and Petroleum: How Humans Took Control of Climate."

“After the forests again took over, the only obvious evidence left of the former existence of these agricultural people was massive earthen mounds used for ceremonial purposes, and most of these mounds were plowed by settlers and flattened to create towns and cities. In the Amazon Basin and other rain forest regions, lush tropical vegetation swallowed up most evidence of former habitation.

"Many decades later, so little evidence remained of the former occupation of North America that scientists and historians in the 1800s and early 1900s assumed that populations had been relatively small.”

Smallpox, typhus, cholera, measles and a host of other diseases had killed roughly one-tenth of the earth’s population. The trees that reclaimed their farms and villages sucked the carbon out of the air. “How much did the American pandemic contribute to the Little Ice Age?” said Ruddiman.

“Conceivably all, but at least half, of the carbon dioxide drop that occurred is due to the pandemic.”  

Forecast: The global consequences of climate change

Part 1: Why America Should Care
The Florida Keys are sinking
Can insurance cover the costs of climate change?
Who will be able to afford to live on the coast?

Part 2: The Spread of Disease
Pathogens find new habitats
Countries could backslide into poverty
How disease relates to carbon dioxide

Part 3: The Arctic Melt
Easier passage through the Arctic
A scramble for control of the Arctic
Opening the Arctic to damage

(Stephan Faris is the GlobalPost environment correspondent. His new book is "Forecast: The Consequences of Climate Change, from the Amazon to the Arctic, from Darfur to Napa Valley.")

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