Human Activities Triggering "Global Soil Change"

Scott Norris, National Geographic News 5 Feb 08;

"Humans are rapidly consuming the global soil reservoir. In light of the growing global population, this is obviously a very serious change."

Earth's climate and biodiversity aren't the only things being dramatically affected by humans—the world's soils are also shifting beneath our feet, a new report says.

"Global soil change" due to human activities is a major component of what some experts say should be recognized as a new period of geologic time: the Anthropocene, or human-made, age.

This new era will be defined by the pervasiveness of human environmental impacts, including changes to Earth's soils and surface geology, proponents of the theory say.

"Unquestionably we are entering the Anthropocene," said Daniel Richter of Duke University, who authored the new study of Earth's changing soils.

In the December 2007 issue of the journal Soil Science, Richter warns that Earth's soils already show a reduced capacity to support biodiversity and agricultural production.

As the amount of depleted and damaged soils increases, global cycles of water, carbon, nitrogen, and other materials are also being affected.

Richter's report supports an independent proposal in the current issue of the journal GSA Today that calls for official recognition of the Anthropocene epoch.

In that paper, Jan Zalaseiwicz of the University of Leicester in England and colleagues argue that the fossil and geologic record of our time will leave distinct signatures that will be apparent far into the future.

To future geologists, Zalaseiwicz said, "the Anthropocene will appear about as suddenly as [the transition] triggered by the meteorite impact at the end of the Cretaceous" 65.5 million years ago, when the dinosaurs became extinct.

Overworked Earth

Today about 50 percent of the world's soils are subject to direct management by humans.

But global soil change is also occurring in more remote areas due to the spread of contaminants and alterations in climate, Richter's report says.

Worldwide, soils are being transformed by human activities in ways that we poorly understand, with possibly dire implications.

"Properties and processes in the soil are more dynamic and susceptible to change than we previously thought," Richter said.

"Only recently are we documenting how [many aspects of soil chemistry and composition] are all highly responsive to human activities."

Rattan Lal, of Ohio State University in Columbus, is a past president of the Soil Science Society of America.

He said that severe soil degradation is increasing globally at a rate of 12.4 million to 24.7 million acres (5 million to 10 million hectares) annually.

In parts of Africa and Asia where the problem is most severe, soils are simply put to too many uses, Lal said.

"Crop residue is taken away for competing uses, animal dung is used as cooking fuel rather than as soil amendment, topsoil is used for brick making, and nutrients are harvested and not replaced," he said.

Such local impacts are causing global problems. Soil degradation plays much a larger role in climate change, for example, than was previously suspected.

That's because organic matter in soils store vast amounts of carbon—more than is present in the atmosphere and in all land vegetation combined.

But heavily cultivated and degraded soils lose their carbon-storing ability as exposed organic matter breaks down, noted geologist Bruce Wilkinson of Syracuse University in New York.

"Over the past half century or so, global soils have lost approximately a hundred billion tons of carbon [in the form of carbon dioxide] to the atmosphere through such exposure," said Wilkinson, who was not involved in Richter's study.

Recent studies by Wilkinson and others also show that humans are now the predominant geological force operating on the planet.

Rates of sedimentation and erosion caused by human activities—mainly agriculture—are ten times higher those attributable to natural processes.

And on agricultural land, he says, soil is being lost ten times faster than it is being replaced.

"Humans are rapidly consuming the global soil reservoir," Wilkinson said. "In light of the growing global population, this is obviously a very serious change."

Human Legacy

The idea that these and other human impacts on the environment could represent a new geologic age was first proposed in 2002 by Nobel Prize-winning chemist Paul Crutzen.

Scientists who embraced the Anthropocene idea say that the Holocene epoch—the period from the last ice age 11,5000 years ago up to modern times—has already ended.

Our legacy in geologic time will be marked by global soil changes now under way, Zalaseiwicz and colleague note in their proposal.

Other factors include the decline of coral reefs and changes in the fossil record brought about by climate change and accelerated rates of extinction.

"The changes we highlight [in the proposal] are inherently geological in nature, in that they are leaving a clear signal in the sediments accumulating today, and thus also in the strata of the far future," Zalaseiwicz said.

"Some will be long-lived even on geological timescales."

But official recognition of the Holocene-Anthropocene transition would require approval by a commission of the International Union of Geological Sciences, and that may still be some years away.