July 30, 2019 — We hear a lot about how different food choices influence the climate crisis. But exactly how much does each bite you eat heat the planet? The answer depends on how you do the math.

With demand for plant-based foods on the rise, a study published in the journal Sustainability offers a new way to measure the climate cost of different foods. Like most estimates, the new math assesses greenhouse gas emissions throughout a meal’s life cycle, from cropland to landfill. But the method also looks at the quality of protein, and tries to rate environmental impact with serving size in mind.

In crunching the numbers, the study points to protein powders as one of the more climate-friendly ways to get the nutrient in your diet. Peanuts, tuna, salmon and grasshoppers also score well. Coming in with a big carbon footprint, however, are cheeses, grains, beef and white rice.

Compared with prior findings, the new ranking paints a more positive picture of some foods, like protein powder. Other foods, such as spaghetti, come out looking worse for the climate under this math.

Why the differences? After scientists estimate the emissions that come from every stage of making a given food, a big decisions remains: how to compare different items. Ranking foods based on the emissions per gram — or some other unit of mass — is one typical way, but the researchers say that weight doesn’t reflect the nuances of nutrition.

“For instance, comparing beef against broccoli on a per-kilogram basis would be inappropriate,” they write, “given that both are usually eaten in different amounts and for different reasons.”

To analyze the amounts people actually eat, the study compares climate impacts using the standard serving sizes developed by the U.S. Food and Drug Administration.

As for the reasons people eat certain foods? Getting nutrients like protein is a big one, particularly for climate-conscious consumers trying to avoid meat products.

But the nutritional point of consuming protein is to digest amino acids, molecules that make up proteins and help the body function. To get an accurate assessment, the researchers assess the amount of a particular amino acid that’s actually absorbed, a metric called the digestible indispensable amino acid score (DIAAS).

These new rankings add to our knowledge of how our diets influence climate change. But while the study’s results can be the basis for quick comparisons, the researchers note that their findings are just a draft, “subject to change.”