AGRICULTURE:

SUMMARY: Clearing more land in the tropics for agriculture is highly inefficient, from a carbon utilization standpoint.

Civilization clearly needs agricultural land, e.g. for a continually increasing population and for biofuels. Land is not unlimited, and clearing it for agriculture has been highly destructive.

Agricultural land has expanded by almost 25 million acres each year from 1980 to 2007. Furthermore, deforestation (in substantial part due to agricultural clearance) is responsible for roughly 12% to 20% of carbon emissions in the atmosphere, a major contributor to global warming.

Such statistics emphasize that it's important to understand where to establish new agricultural land. It's equally if not more important to learn how to make the most out of the agricultural land currently in use.

Towards such goals, Paul West (University of Wisconsin, United States) and coworkers have quantitated the efficiency (from a carbon standpoint) of agricultural clearance on a global scale. Previous studies of this general type have not been carried out on such a large scale.

A global analysis is necessary, in that regional or transnational trade policies may shift the burden of agriculture to different locations, and the consequences will therefore be felt on a global scale. West and coworkers' findings will be very useful for making scientifically sound land use decisions in the coming years, as long as they are properly disseminated to and appreciated by relevant actors.

Their findings are complimentary to the vast body of research aimed at making the most out of agricultural land, e.g. increasing biodiversity and facilitating transit of bird species in farmland at minimal cost, the findings that water conservation efforts can actually increase water utilization and that of increased pesticide resistance among insects feeding on improperly developed and applied transgenic corn, and research towards insecticides that are nontoxic to people.

Counting carbon.

The scientists first evaluated average crop yields in different climate zones (tropics, subtropics, temperate, subpolar, and polar), and the percentage of agricultural land within these zones. They used this data to arrive at average changes in carbon stock as a consequence of conversion to agriculture.

Temperate regions have the largest percentage land converted to agricultural uses (20%), and feature the highest average crop yield (1.5 tons of crops per acre per year). However, the average change in carbon stock within temperate zones is a loss of 25 tons of carbon per acre, less than all climate zones except polar regions.

That of tropical regions is far more, a loss of nearly 49 tons of carbon per acre (e.g. twice as much as within temperate climates). This makes sense, in that tropical regions typically have a lot more carbon stored within them (extensive and densely packed vegetation) than temperate climates.

Perhaps the most informative parameter calculated by the scientists is the average tradeoff between carbon stocks and crop yields. Except for polar climates, the smallest tradeoff is within temperate and subtropical regions (both a loss of roughly 27 tons of carbon per acre, per ton of crops per acre per year), and by far the largest is within the tropics (a loss of roughly 77), e.g. nearly three times as much.

Implications.

Even doubling crop yields within the tropics, a dramatic increase, would still not even come close to reaching the carbon tradeoff seen within temperate and subtropical locations. Although this study does not include the carbon usage necessary to achieve high yields in temperate regions (e.g. fertilizer production), it's hard to envision this omission qualitatively affecting the scientists' results.

Local, regional, and transnational agricultural efforts should focus on restoring tropical ecosystems and enhancing agricultural land currently in use. Clearly, other factors need to be considered for such challenges in addition to carbon costs (e.g. social and water needs), but the hard data reported herein is an extremely useful starting point for discussions regarding the urgent need to render agricultural production maximally sustainable in the coming years.

NOTE: The scientists' research was funded by the National Aeronautics and Space Administration, the Wisconsin Space Grant Consortium, the United States Department of Energy, and The Nature Conservancy.

West, P. C., Gibbs, H. K., Monfreda, C., Wagner, J., Barford, C. C., Carpenter, S. R., & Foley, J. A. (2010). Special Feature: Trading carbon for food: Global comparison of carbon stocks vs. crop yields on agricultural land Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1011078107