Tropical agriculture "double-whammy": high emissions, low yields

Jeremy Hance
November 02, 2010

Food produced in the tropics comes with high carbon emissions and low crop yields, according to a new study in the Proceedings of the National Academy of Sciences (PNAS). In the most comprehensive and detailed study to date looking at carbon emissions versus crop yields, researchers found that food produced in the tropics releases almost double the amount of carbon while producing half the yield as food produced in temperate regions. In other words, temperate food production is three times more efficient in terms of yield and carbon emissions.

"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," explains lead author Paul C. West, a graduate student at the University of Wisconsin-Madison, in a press release.

The researchers found that one ton of food emitted approximately over 75 tons of carbon in the tropics, whereas a ton of food grown in temperate regions released just less than 27 tons of carbon.

The tradeoffs between the release of carbon to the atmosphere and agricultural production are markedly different between the world's temperate and tropical regions. In this representation, for each hectare of land cleared for agriculture, each rail car is equivalent to 68 tons of carbon released to the atmosphere and each bushel represents 3.9 tons of maize produced. Image courtesy of Paul West, University of Wisconsin-Madison Center for Sustainability and the Global Environment and University of Wisconsin-Madison Center for Limnology.
"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 co-author Jonathan Foley, director of the University of Minnesota's Institute on the Environment. "In terms of balancing the needs of food production and slowing carbon dioxide emissions, this is a tough tradeoff."

Rising human population, increasing consumption of meat (which requires more grain per area), the demand for biofuels, high commodity prices, and economic development plans have pushed many tropical nations to pursue large-scale agriculture over forest protection. However, the authors say the realities of carbon loss in the tropics makes a strong argument for intensifying agriculture on already cleared land, rather than more deforestation.

"Our results corroborate recommendations to concentrate reforestation and avoid deforestation in the tropics to have the greatest worldwide impact," the authors write.

But, West admits, "the realty is there will be some of both [agriculture intensification and deforestation]."

The authors explain in the paper that "despite the clear benefits of concentrating reforestation and forest conservation efforts in the tropics, several local and regional factors influence implementation. […] Choices are made locally and are influenced by local and regional food security, transportation costs, labor, poverty, and technology rather than global atmospheric carbon. Thus, local and global outcomes must be coupled to manage ecosystem services and assess their tradeoffs."

The study also highlight that agriculture comes with additional tradeoffs on top of carbon including impacting ecosystem services such as "soil and groundwater recharge, runoff, and nutrient regulation as well as ecosystems, species, and genome diversity of landscapes."

The broad study looked at 175 different crops worldwide using government data and satellite imagery.

"We have a very fine resolution of both what the carbon stocks and the yields are globally," says West. "Spatially, it is much more explicit than anything that has been produced before."

Approximately 20% of the temperate region is used for crops, as opposed to 10.5% of the tropics. In all, deforestation contributes more greenhouse gases to the atmosphere than global transportation: 12-20% of the total greenhouse gas emissions are due to the loss of forests. Scientists say that such emissions are driving global climate change.

CITATION: Paul C. West, Holly K. Gibbs, Chad Monfreda, John Wagner, Carol C. Barford, Stephen R. Carpenter, and Jonathan A. Foley. Trading carbon for food: Global comparison of carbon stocks vs. crop yields on agricultural land. PNAS. DOI: 10.1073/pnas.1011078107.

Crop types and yields between 1998 and 2002. Highest crop yields are in temperate regions of Western Europe and North America. Map courtesy of Paul West, University of Wisconsin-Madison Center for Sustainability and the Global Environment and University of Wisconsin-Madison Center for Limnology. Click to enlarge.

Map shows changes in carbon stocks due to cropland conversion. Blue is less carbon emissions due to land conversion, while red is more. Tropical areas with high conversion of croplands, such as Southeast Asia, have the highest emissions due to land conversion. Map courtesy of Paul West, University of Wisconsin-Madison Center for Sustainability and the Global Environment and University of Wisconsin-Madison Center for Limnology.Click to enlarge.

Combining cropland distribution and changes in carbon stocks due to crop expansion into native ecosystems shows that carbon loss per ton of annual crop yield is nearly three times as high in the tropics compared to temperate regions. Map courtesy of Paul West, University of Wisconsin-Madison Center for Sustainability and the Global Environment and University of Wisconsin-Madison Center for Limnology.Click to enlarge.

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Jeremy Hance (November 02, 2010).

Tropical agriculture "double-whammy": high emissions, low yields.