To Keep More Carbon on the Ground, Halting Farmland Expansion Is Key

The conversion of forests to farmland is recognized as a major contributor to rising levels of greenhouse gases.  And yet it hasn't been clear how to best minimize the loss of sequestered carbon into the atmosphere.  Is it better to maximize farm yields so as to use less land area over all?  Or should farms be operated so as to retain more carbon on site, even at the expense of crop yields?  Researchers reporting in Current Biology on July 26 say that, based on their extensive studies of agricultural operations in the humid tropics of Ghana, the dry tropical forest in Mexico, and temperate wetlands and forests in Poland, the best course in all cases is to limit the conversion of natural habitat to farmland, a strategy known as land sparing.  That means maximizing yields on existing agricultural lands.

"At one extreme, farmers can try to produce all their food from as small an area of farmland as possible, by having very high yields," says David Williams from the University of California, Santa Barbara.  "This will probably reduce the amount of carbon stored on their farmland, but allows policy makers and farmers to free up more space to conserve natural habitats, where there is likely to be a lot of carbon stored.  At the other extreme, farmers can try to use lower yield farming practices to increase the carbon stored on farmland, which will reduce the area of natural habitats available for conservation.  And then there are all the in-between strategies that use a mix of high and low yield farmland.

"We found that the first strategy -- what we call 'land sparing' -- resulted in a greater amount of carbon being stored than any other.  So, slightly counter-intuitively, trying to conserve carbon on farmland resulted in less carbon being stored across the landscape as a whole.  This was because it resulted in lower yields and so required larger areas to produce the same amount of food, and therefore meant less land could be spared for natural habitats."

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