Humans began altering global climate thousands of years ago Ruddiman questions accepted ideas and challenges believed truths

Photo by Andrew Shurtleff

In his study, William Ruddiman says, “Humans got in the way of what nature was going to do. The climate is stable only because [we] have kept it that way.”

By Fariss Samarrai

Before humans built cities, developed writing or founded religions, they began altering the global climate. Populations grew, struggled to survive in a brutal world, and developed agriculture. The earth and climate responded. Heat-trapping gasses — carbon dioxide and methane — increased as forests were cleared and crops planted. The climate went into a long stable period of relative warmth that has continued to the present day.

Without early and continued alterations of the land by humans, the Earth could instead be in a naturally occurring ice age now.

But there is no large glaciation because humans inadvertently changed the direction of nature thousands of years ago, according to a new study by William Ruddiman, U.Va. professor emeritus of environmental sciences.

According to Ruddiman, civilization has flourished because the climate for the past 8,000 or so years has been relatively stable and moderate. This, he said, did not occur naturally, but happened because humans cut down enormous tracts of forest in Europe, China and India to make room for crops and pastures. This loss of forestlands greatly increased the carbon dioxide in the atmosphere.

Humans also irrigated lands for rice fields, increasing the release of methane over the past 5,000 years. Instead of cooling naturally, the climate stabilized. An ice age that should have begun 4,000 to 5,000 years ago never happened, Ruddiman said.

The study was published in the December issue of the journal Climatic Change. Ruddiman also presented his findings at the December meeting of the American Geophysical Union in San Francisco. The study has gained international media coverage, with stories in The New York Times, The Economist, The Guardian and numerous other publications.

“We humans got in the way of what nature was going to do,” Ruddiman said. “The climate is stable only because humans have kept it that way.”

Ruddiman said his method was to look at what should be natural climate trends, and then to take notice of “things that are going the wrong way.”

“We should be in a natural period of glaciation, but we are not,” he said. “I got suspicious that humans were the explanation.”

He discovered that an increase in greenhouse gasses, and the resulting warming trend, coincides with the advent of agriculture.

“The world would look very different than it does now if it wasn’t for the human activity that has been altering the global climate for thousands of years,” he said.

Only about half of the human-caused alteration of greenhouse gasses and global climate occurred during the last 150 years, Ruddiman said. The other half occurred during the 5,000- to 8,500-year period of population growth and agriculture prior to industrialization.

Ruddiman’s broad but highly detailed study is unique because it draws from numerous unrelated studies from disparate fields of inquiry. He examined studies of early human history and agricultural practices, population fluctuations (including epidemics that killed off populations), atmospheric chemistry from ice core samples — every piece of the puzzle that he could pull together to get the big picture of why the climate had gone in a direction different than it was inclined.

“Investigators in the various disciplines don’t talk much to each other,” he said. “For this reason, some findings and ideas fall through the cracks. It’s rare for people to step back from the details of what they are doing to look at what others are doing and see the broader implications of all these findings.”

Ruddiman said he was able to do this because he is “retired.” He has the time to question ideas that are taken as fact in the busy academic world of teaching, publishing and service.

“I have found continuity in retirement,” he said. “I’m focusing my efforts on bigger studies with broader implications. I’m able to spend more time trying to answer the scientific questions that have intrigued me throughout my career.”

He said he sometimes feels like Lt. Columbo, the inquisitive and masterful TV detective. He pauses often, thinks carefully and says, “There’s something bothering me. Just one more question...”