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Forecast: How disease relates to carbon dioxide

Part 2: The spread of disease

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.