Humans began altering global
climate thousands of years ago
Ruddiman questions accepted ideas and challenges
by Andrew Shurtleff
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.”
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
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
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
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
of Ruddiman’s study include:
• Beginning 8,000 years ago, humans reversed an expected
decrease in carbon dioxide by clearing forests in Europe,
China and India for croplands and pasture.
• Beginning 5,000 years ago, humans reversed an
expected decrease in methane by diverting water to irrigate
rice and by tending large herds of livestock.
• In the last few thousand years, the size of the
climatic warming caused by these early greenhouse emissions
may have grown large enough to prevent a glaciation that
climate models predict should have begun in northeast Canada.
• Abrupt reversals of the slow carbon-dioxide rise
caused by deforestation correlate with bubonic plague and
other pandemics near 200-600, 1300-1400 and 1500-1700 A.D.
Historical records show that high mortality rates caused
by plague led to massive abandonment of farms. Forest re-growth
on the untended farms pulled carbon dioxide out of the atmosphere
and caused carbon dioxide levels to fall. In time, the plagues
abated, the farms were reoccupied and the newly re-grown
forests were cut, returning the carbon dioxide to the atmosphere.
Ruddiman’s complete study is available online
as a PDF file at http://www.kluweronline.com/issn/