Some Recalculations
Pollan's book is generally well-researched and very interesting, but I decided to check on a few of the numbers that get inserted in his narratives. In particular, Joe Salatin of Polyface Farm suggests that converting the 16 million acres of hybrid corn production in the U.S. to well-managed pasture, "would remove fourteen billion pounds of carbon from the atmosphere each year, the equivalent of taking four million cars off the road." I have studied and taught about plant growth and carbon fluxes, so I was interested in scrutinizing these numbers. I had two
questions: (1) do they add up? (2) how would this scenario compare with replacement of industrial corn fields by forests?
For the first question, let's take Joe's assumption about the amount of carbon drawn from the atmosphere (and stored in the soil as decaying
roots) at face value. It takes time before a perennial grassland reaches that level of productivity, and carbon fixation is going to vary a great deal with geography and climate, but let's allow that within about ten years the new pastures would be storing carbon at levels approaching his
14 billion pounds per year total. How close is he to a reasonable estimate that this is the equivalent of 4 million cars operating the same year? One gallon of gas contains 5.4 lb. of carbon (the rest is hydrogen and impurities), and combusting that with air adds two molecules of oxygen to every carbon. Oxygen is heavier than carbon, so we pump out 19.8 lb.
of carbon dioxide when burning one gallon of standard gasoline. So we've got the equivalent of 707 million gallons of gas. If we assume a fleet average of 25 mpg and 15,000 annual miles driven per car, we get compensation for 1.2 (not 4) million cars. He's off by quite a bit, but the figures are still worthy of attention.
Second, if we converted all that corn to forest, how much CO2 would get removed from the atmosphere? It takes about two acres of healthy, maturing forest to compensate for a year's driving. This calculation assumes about 200 trees per acre, each removing about 30 lb. of carbon dioxide per year, and storing that carbon in soils and wood. This 1,200 lb. of forest-fixed carbon is (after rounding off) the equivalent of 600 gallons of gas, or 15,000 miles of driving at 25 mpg. Conversion of 16 million acres of corn to forest would compensate for 8 million cars, almost seven times the value for pasture.
I am not arguing for implementing these large-scale conversions, but it helps to get a more accurate perspective. Pollan and several of his characters are exactly right in noting that we are treating the atmosphere as a dumping ground for pollution, and now that carbon dioxide is out of balance, we all should bear the costs that have been hidden. The petroleum/corn/industrial agriculture dilemma is particularly compelling and needs to be met with creative solutions. True costs must be calculated for food and fuels. Scientific progress already has tools that make these calculations possible under all kinds of circumstances, but it appears that we will need better political and moral leadership to bring them to bear.
George Robinson
Associate Professor
Department of Biological Sciences