Growing Greener: The Carbon Footprint of Food

The following was published as part of my regular “Growing Greener” column in Public Garden magazine, Vol. 23 No. 2 (2008). Public Garden is the flagship publication of the American Public Gardens Association. In “Growing Greener” I answer sustainability-related questions from public garden staff.

Q: How does the carbon footprint of homegrown fruits and vegetables compare with that of imported produce?

A: In the past few years the carbon footprint of food has become one of the hottest issues in the western world. A number of luminaries have weighed in on the subject in the U.S. alone, from best-selling novelist Barbara Kingsolver (in Animal, Vegetable, Miracle) to ethnobotanist Gary Nabhan (in Coming Home to Eat). All this discussion has generated its own jargon, including such terms as “food miles” (the distance any item of produce travels from farm to table) and “locavore” (a person who makes a point of eating food grown within 100 miles, give or take).

At first glance comparing, say, a tomato grown 30 feet from your back door with one cultivated half a continent away would seem to be a no-brainer. But there is no hard data to back up claims because “the research really has not yet been done,” says Gail Feenstra, Food Systems Analyst at the UC Davis Agricultural Sustainability Institute. Nevertheless, based on the research that does exist on the carbon footprint of food it is possible to extrapolate some conclusions.

One of the most popularly cited studies to emerge from academia in recent times is the 2003 Iowa State University report that found most produce travels almost 1,500 miles before reaching plates in the Hawkeye State. Not surprisingly then, says Feenstra, “food miles is the first thing people think of” when calculating the greenhouse gas emissions that result from what they eat. Homegrown fruits and vegetables don’t have to be shipped from farm to supermarket—you don’t even have to drive to the grocery store or farmer’s market to get them—so there is no transportation-related energy consumption and ensuing emissions of carbon dioxide, the greenhouse gas most widely implicated in global climate change. The carbon footprint of homegrown food is even more minimal when it is started from seed, instead of plants in cell packs that have been shipped to a local nursery.

The number of food miles isn’t the only measure of an item’s global warming potential, however. How the food is grown also matters. Most gardeners are surprised to learn that often their biggest contribution to greenhouse emissions arises from the use of nitrogen fertilizers, says David Wolfe, Professor of Plant and Soil Ecology at Cornell University. The manufacture of synthetic fertilizer is extremely energy intensive. Manures and other organic sources are better because the CO2 emissions associated with manufacture are mostly eliminated. But using either synthetic or organic fertilizers releases nitrous oxide gas, which in Wolfe’s words “has 300 times more global warming potential per molecule than carbon dioxide” and is the third most important of the greenhouse gases, after CO2 and methane. Chemical pesticides are also energy intensive to produce, and they are toxic to boot.

The energy involved in pumping and distributing the water we use for irrigation can be another major source of CO2. Generally, the more arid the area, the higher the water’s “embodied energy,” the technical term for this indirect form of energy consumption. A study commissioned by the city of Irvine, California, for example, found that the energy used to deliver water was second only to the fuel consumed by the service vehicles of municipal landscapers—and far more water is lavished on private gardens than these public landscapes.

Growing fruits and veggies in a heated greenhouse adds to their impact on the atmosphere. So does using rototillers or other power equipment.

Whether a homegrown carrot or an imported New Zealand kiwi, much of a foodstuff’s carbon footprint is associated with what happens once it reaches the kitchen. Waste is a particularly big issue, Feenstra points out, because some of the energy used to grow, transport, and package a fruit or vegetable is wasted when part of it is thrown away. Using the plant part—carrot top soup anyone?—or composting it and enriching your soil with the compost in place of synthetic or organic fertilizers are better options. (In fact, studies of which fruits and vegetables can thrive in an area with a minimum of supplemental water and solely with applications of compost, not fertilizers of any kind, would be great citizen science projects for public gardens.)

Beyond planting a vegetable patch and composting food scraps, the Agricultural Sustainability Institute recommends the following steps toward a “low-carbon diet:”

• Eat less meat, especially beef, and more minimally processed and packaged whole grains, fruits, and vegetables. Growing feed for the cattle, not to mention their methane emissions, adds to beef’s large burden on the atmosphere.

• Support local organic farmers who use a minimum of water, fertilizer, and pesticide.

• Avoid foods produced in heated greenhouses.

• Avoid foods shipped by air.

• Walk, bike, or take public transportation to do your shopping, and limit your food shopping excursions to once a week.

Taking such steps isn’t just easier on the atmosphere. Buying local also supports local farmers and helps preserve open space. And the fruits and vegetables on your dinner plate are a lot fresher and taste better too.

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One Response to “Growing Greener: The Carbon Footprint of Food”

  1. 1russell Says: