HELP SAVE THE PLANET: TAKE THE ½ CO2E CHALLENGE!
Author of Everyday Calculus: Discovering the Hidden Math All Around Us
You probably think about the health implications of your diet often—many of us do. What you may not think about is the implications your food choices have for the planet’s health. I’m not here to guilt-trip you into being a vegetarian—this is, after all, a book about reducetarian solutions. Instead, I’d like to help you understand how your food choices contribute to global greenhouse gas emissions (GHGEs), and then challenge you to cut your food-related GHGE in half. And I’ll help you do it.
Let’s start with the data. In 2011, the Environmental Working Group (EWG)—a nonprofit environmental research organization based in Washington, D.C.—published the results of a comprehensive study of GHGE organized by food category. Titled “Meat Eater’s Guide to Climate Change + Health,” the study draws on several data sources (including peer-reviewed journal articles, reports from the UN’s Intragovernmental Panel on Climate Change, and publications from several U.S. government agencies) to estimate the cradle-to-grave kilograms (kg) of the carbon dioxide equivalent (CO2e) of GHGEs for various foods. These emissions result from the production and postproduction activities (including transportation) required to deliver 1 kg of a particular food. Animal products, in particular meat, dominate the top of the list: lamb comes in first (39.2 kg CO2e per kg), followed by beef (27 kg CO2e per kg), cheese (13.5 kg CO2e per kg), and pork (12.1 kg CO2e per kg). But because we’re not used to thinking of our diet in terms of how many kilograms of food we eat, let’s convert the Environmental Working Group’s numbers to kilograms of CO2e per food calorie.
The first step is to divide EWG’s numbers by 10 to get kilograms of CO2e per 100 grams of food. Then, we divide the result by the number of calories in a 100-gram serving of whatever food we’re calculating for. This will give us an idea of the amount of emissions produced per calorie. For example, according to Google a 100-gram serving of 85 percent lean ground beef contains 250 calories, so 1 calorie of this lean beef requires 27 ÷ 10 ÷ 250 = 0.0108 kg of CO2e emissions to get to your plate. That’s a tiny number! Let’s make things easier by multiplying by 1,000 to convert to grams of CO2e emissions per calorie (arriving at 10.8 g CO2e per calorie of beef, for example). With all of EWG’s numbers converted to grams of CO2e per calorie, we can now calculate the food-related GHGEs of an entire diet.
Let’s suppose that, as a percentage of your daily calories, your diet consists of: b percent beef, c percent chicken, d percent dairy, g percent grains, l percent legumes, and v percent vegetables. Using the mathematical steps just outlined, your diet’s “carbon intensity”—the grams of CO2e per calorie associated with your diet—is approximately:
CID=10.8b+4.2c+3.5d+2g+0.5l+4.8v
(Note: CID in this equation stands for carbon intensity of the diet.) To make the calculation easier I’ve created a free, customizable CID calculator on my Web site: surroundedbymath.com/interactive -math. But let’s do another calculation by hand just for extra. Consider a hypothetical diet that’s 10 percent beef, 10 percent chicken, 15 percent dairy, 40 percent grains, 5 percent legumes, and 20 percent vegetables. Then:
CID=10.8(0.10)+4.2(0.10)+3.5(0.15)+2(0.4)+0.5(0.05)+4.8(0.2)
This translates to 3.81 grams CO2e per calorie. When adjusted for a 2,000 calorie daily intake, this diet will require 7,620 grams (7.62 kg) of CO2e emissions. That’s equivalent to burning 7.3 pounds of coal—and that’s only for one day and one person. Cue my ½ CO2e challenge.
I know what you’re thinking: “How in the world am I going to cut my CID number in half?” It’s going to take a lot of determination—and a little bit of math as well. Suppose we swapped 1 percent of our hypothetical diet’s beef calories for legumes, so that “b” decreases by 1 while “l” increases by 1. The decrease in b lowers CID by 10.8(0.01) = 0.108, while the increase in l raises CID by 0.5(0.01) = 0.005. The net effect is a CID number that’s lower by 0.103. That would lower the CID of the hypothetical diet to 3.81 − 0.103 = 3.707, which is about 2.7 percent lower. Now we’ve discovered that the coefficients of the variables in the CID equation tell us how much the CID will increase or decrease when the caloric intake of each food group is increased or decreased by 1 percent. Beef has the largest coefficient and legumes the smallest, so you get the biggest reduction in CID by swapping beef for legume calories. (Black bean burgers, here we come!) More generally, because I’ve calculated that the coefficients for lamb, turkey, and farmed salmon all turn out to be at least 9 (see the aforementioned Web site if you’re curious about my methodology), the math clearly shows that the best way to lower your diet’s carbon intensity is by eating less animal meat, be that red meat, poultry, or seafood (and, ideally, swapping those calories for legume calories).
I hope you now see how useful the CID equation is. Sure, it’s only an estimate of a diet’s carbon footprint. But the beef coefficient’s outsize influence is a clear indicator of the scale to which different food choices impact the environment. This isn’t speculation anymore; it’s arithmetic. That’s why I hope you’ll join the Reducetarian movement. You’ll be reducing your carbon footprint, and assuming you swap those meat-heavy calories for healthful foods, you’re probably adopting a healthier lifestyle as a bonus. So take the ½ CO2e challenge—your body, your math teachers, and your planet will thank you.