Climate meddling dates back 8,000 years

Cutting down trees put carbon in the atmosphere long before the industrial revolution

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Caption: By the year 1000, humans had already cleared much land across the globe, for agriculture and other uses. In this computer simulation, darker greens signify higher levels of natural vegetation. Kaplan et al/The Holocene 2011

SANTA FE, N.M. — People started influencing their home planet’s climate millennia before the industrial revolution’s fossil fuel–burning machines began spewing carbon dioxide and other heat-trapping gases into the atmosphere, a new study suggests.

Clearing land — first to hunt and gather, and then to farm — removed trees that otherwise would have soaked up carbon dioxide. The new work suggests that humans working the land put nearly 350 billion metric tons of carbon — many times other estimates — into the atmosphere by the year 1850. (For comparison, between 1850 and 2000 people added 440 billion tons of carbon, mostly from burning fossil fuels — surpassing in a century and a half what had previously taken humankind eight millennia.)

“Our data show very substantial amounts of human impact on the environment over thousands of years,” says team leader Jed Kaplan of the Federal Polytechnic School in Lausanne, Switzerland. “That impact really needs to be taken into account when we think about the carbon cycle and greenhouse gases.”

Kaplan reported the work on March 25 at an American Geophysical Union conference on past civilizations and climate. He, Lausanne colleague Kristen Krumhardt and others also describe the findings in an upcoming issue of The Holocene.

Climate scientists often select 1850 as the putative start of the industrial revolution. But the world in 1850 was not a pristine globe untouched by human hands. “I call it the virgin continent myth,” says Kaplan.

Rather, people cut down forests and cleared land early on. Agriculture, for instance, arose in the Fertile Crescent some 8,000 to 10,000 years ago

Previous research often assumed that as the world’s population grew, the proportion of cleared land grew as well. But the more people crowded onto a landscape, the more efficient they became at extracting dinner from it, says William Ruddiman, a retired geologist from the University of Virginia in Charlottesville. Irrigation, fertilizer, multicropping and new tools allowed farmers to increase crop yields, and per-capita land use began to drop.

Kaplan and Krumhardt looked at how growing population and changing land-use trends affected carbon emissions. The scientists gathered data on how many people lived in each population center for the past 8,000 years, then cataloged how land use changed over time. The result: a dramatic video showing a green-forested world giving way to a brown spread of deforestation, up until the modern era.

The researchers then used a computer simulation to calculate the amount of carbon put into the preindustrial atmosphere. The 350-billion-ton estimate is roughly twice that the scientists found with a different, widely used computer simulation, and five times that reported by a University of Bern team in Biogeosciences in January. The differences, Kaplan says, trace in part to assumptions over how carbon is stored in the soil when forests give way to grassland.

Kaplan’s new, higher estimates support a theory Ruddiman has been putting forward for years — that people were responsible for far more carbon emissions, far earlier in history, than most researchers have thought.

Critics have pointed to evidence like the chemical composition of carbon dioxide bubbles trapped in an ancient ice core, which don’t show the change in types of carbon that would be expected if trees had been chopped down. (Growing plants like to use the carbon-12 isotope rather than carbon-13, and thus the ratio of the two forms can be used as a chemical signature of vegetation’s presence or absence.) But other factors, like rapidly expanding peatlands, could have sucked down lots of carbon-12 and counterbalanced some human emissions, says Ruddiman.

Julia Pongratz, a geographer at the Carnegie Institution for Science in Stanford, Calif., has done a more detailed study of how people’s land use varied from place to place during just the last millennium. For instance, when people first began growing rice in paddies in China, it took fewer acres to feed the same number of people than in Mesopotamia, because wet-grown rice produces more calories per acre than a cereal crop.

Taking these regional patterns one step further, Pongratz is now calculating how prehistoric land use contributed to countries’ total carbon emissions. “Surprisingly, we find that when we include preindustrial emissions, the attribution of today’s CO2 increases goes up for countries like China and India and goes down for industrialized countries,” she says.

Because of their long agricultural traditions, India and China emitted a lot of carbon millennia ago when early farmers cleared land there. In contrast, the United States remained relatively pristine forest until the past couple of hundred years. And in Europe, the fact that land was cleared thousands of years ago is swamped by the sheer size of its modern-day carbon emissions, Pongratz says.

She will present her work on April 5 in Vienna, at a meeting of the European Geosciences Union. “It’s important to turn to the past so we know what our actions mean for present-day and future climate,” she says.

Global land cover change from 8,000 to 50 years ago An animation shows how human land use has changed forested areas over the last eight millennia.
Credit: ARVE/ Federal Polytechnic School, Lausanne

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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