How much greenhouse gas comes from food?
Perhaps the most widely-circulated estimate comes from Michael Pollan’s open letter to the presidential candidates, published in the New York Times last October under the title Farmer in Chief. As soon as the letter was published it started spreading through email and the blogosphere. At least four of the conferences I have attended since then have included it among the conference handouts. Here’s what it said:
After cars, the food system uses more fossil fuel than any other sector of the economy — 19 percent. And while the experts disagree about the exact amount, the way we feed ourselves contributes more greenhouse gases to the atmosphere than anything else we do — as much as 37 percent, according to one study.
That’s a lot of energy and greenhouse gas. But are Pollan’s numbers credible?
I don’t think so.
According to an energy analysis conducted at Iowa State University, US farms consumed about 1.8 exajoules in 2002 — about 1.7% of the nation’s total energy.
Farming accounts for about a fifth (~20%) of the energy used to get food to our plates, according to a University of Michigan study.
I did some complex mathematics (i.e. multiplied 1.7 by 5) to estimate that the food system accounts for about 9% of the energy consumed in the U.S.
The nice thing about this back-of-the-envelope estimate is that it agrees with another approach to estimating food system energy consumption. The graph above shows the food system using 7.3 calories to deliver each calorie of food consumed by Americans. I multiplied this factor (7.3) by the average per capita calorie consumption in the U.S. (2,775 kcal/day), the number of days in a year (365), and the number of people in the country (307 million). That comes to 9.5 exajoules (let Google do the math), or about 9% of the energy consumed in the U.S.
My estimate is about half of Pollan’s. If the food system really used 19% of the energy consumed in the U.S. — as Pollan says — then every calorie we consume would take about 15 calories to get to our plate (let Google do the math). That’s twice as much as the University of Michigan estimate, and 50% more than Pollan himself claims later in the same paragraph:
The 20th-century industrialization of agriculture has increased the amount of greenhouse gases emitted by the food system by an order of magnitude; chemical fertilizers (made from natural gas), pesticides (made from petroleum), farm machinery, modern food processing and packaging and transportation have together transformed a system that in 1940 produced 2.3 calories of food energy for every calorie of fossil-fuel energy it used into one that now takes 10 calories of fossil-fuel energy to produce a single calorie of modern supermarket food.
A greenhouse gas inventory released by the US-EPA yesterday shows that energy use accounts for the vast majority of emissions.
Farming generates additional emissions associated with land use practices, manure management, ruminant digestion, residue burning and other agricultural practices. These account for about 6% of US emissions. (The dark grey zone in the graph above is broken down for 2007 in the graph below).
If food system energy use makes up ~9% of U.S. greenhouse gas emissions, and agricultural practices not associated with energy use make up ~6%, then the food system is responsible for about 15% of emissions. That’s a far cry from 37%, but it’s still a lot of greenhouse gas.
Each year, Americans emit almost 20 tonnes of carbon dioxide equivalents per person. If 37% of that went to the food system then simply feeding each American would release the equivalent of 7.2 tonnes of carbon dioxide — more than a typical person in Sweden or France emits in a year, and more than 70% of the amount emitted by somebody in the United Kingdom. That seems improbable.
Analyses conducted in Sweden, France, and Britain show that these countries also have energy and greenhouse gas-intensive food systems, not unlike the US. A British study, for example, attributed 19% of the country’s greenhouse gas emissions to the food system:
Since per capita greenhouse gas emissions in the UK are approximately half that of the US, it follows that the UK food system would release a larger proportion of greenhouse gas than the US food system, even if the UK system is somewhat more efficient. I find it interesting that agriculture accounts for about 40% of food system emissions in the UK study, but only 20% in the US study.
My rough approaches to estimating US food system energy and greenhouse gas emissions suggest that the food system accounts for about 9% of US energy use and 15% of greenhouse gas emissions.
I agree wholeheartedly with Pollan’s message that our energy-intensive food system needs to be restructured, but I think that his numbers are off by a factor of two or more.