Expanding Croplands Chipping Away at World’s Carbon Stocks

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Todd Reubold, Institute on the Environment, reub0002@umn.edu, (612) 624-6140
Jeff Falk, University News Service, jfalk@umn.edu, (612) 626-1720

MINNEAPOLIS / ST. PAUL (11/1/2010) – According to a study published online today in the Proceedings of the National Academy of Sciences (PNAS), nature’s capacity to store carbon, the element at the heart of global climate woes, is steadily eroding as the world’s farmers expand croplands at the expense of native ecosystem such as forests.
The tradeoff between agricultural production and maintaining nature’s carbon reservoirs – native trees, plants and their carbon-rich detritus in the soil – is becoming more pronounced as more and more of the world’s natural ecosystems succumb to the plow. The problem, experts say, is most acute in the tropics, where expanding agriculture often comes at the expense of the tropical forests that act as massive carbon sinks because of their rich diversity and abundance of plant life.

“This study is important, because it asks how we make tradeoffs between producing more food and sustaining key aspects of the environment, especially our tropical forests”, says Jonathan Foley, director of the University of Minnesota’s Institute on the Environment, and a co-author on the study.

The seriousness of the problem is documented in the most comprehensive and fine-grained analysis of the world’s existing carbon stocks and global crop yields. The PNAS study was prepared by a team of researchers from the University of Minnesota, the University of Wisconsin-Madison, Stanford University, Arizona State University and The Nature Conservancy. The article is part of a special PNAS feature on climate mitigation and agricultural productivity in the tropics.

“We analyzed the tradeoffs between carbon storage and crop production at a level of detail that has never been possible before,” according to Stephen Carpenter, one of the senior authors of the study and a professor at the Center for Limnology at UW-Madison. “The main news is that agricultural production by clearing land in the tropics releases a lot of greenhouse gases per unit of food produced.”

Compared to the world’s temperate regions, the tropics release nearly twice as much carbon to the atmosphere for each unit of land cleared, explains Paul C. West, the lead author of the study. “Tropical forests store a tremendous amount of carbon, and when a forest is cleared, not only do you lose more carbon, but crop yields are not nearly as high as they are in temperate areas.”

“This creates a kind of ‘double whammy’ for a lot of tropical agriculture: we have to clear carbon-rich ecosystems to create tropical croplands, and unfortunately they often have lower yields than temperate systems,” says Foley.  “In terms of balancing the needs of food production and slowing carbon dioxide emissions, this is a tough tradeoff.”

In the tropics, for example, it is estimated that for every ton of crop yield, carbon stocks are diminished by as much as 75 tons. Such attrition, say West and his colleagues, makes a strong case for intensifying agriculture on already-converted land instead of putting new fields into production.

“One path is to expand agricultural land,” says West. “The other path is to intensify agriculture on existing lands. The realty is there will be some of both.”

Today, about 20 percent of the land in temperate regions is in cropland. In the tropics, 11 percent of the land is farmed. However, in the tropics pressure to plant more land is growing fastest due to increasing human population, changing diets, food security concerns, and a rising demand for the raw materials of biofuels.

Carbon is one of the planet’s most abundant elements. It is present in all known life forms and moves naturally between the biosphere, oceans and atmosphere in a process that allows the element to be continuously recycled. Human processes, and in particular agriculture, accelerate the process by rapidly converting carbon stocks in trees, other plants and the soil to carbon dioxide, the primary greenhouse gas.

Global carbon stocks, notes West, can be analogous to a checking account: “The math is pretty simple. When you clear a forest, it is like making a big withdrawal from the checking account.”

Foley, Carpenter and West believe the new analysis will be a valuable tool for governments, non-profit organizations and businesses. Already, commercial carbon exchanges are beginning to emerge and detailed knowledge of where carbon stocks are preserved or could be expanded will be valuable information.

In addition to Foley, Carpenter and West, authors of the new study include Holly K. Gibbs of Stanford University, Chad Monfreda of Arizona State University, John Wagner of the Nature Conservancy, and Carol Barford of UW-Madison. NASA, The Nature Conservancy and the U.S. Department of Energy funded the study.