Fall 2022

Natural Selections

A colorful bowl of various food items, including peapods and onions.
Photo by Mauricio


Every choice we make as consumers has a climate impact. It’s often measured in terms of a “carbon footprint” — that is, the amount of greenhouse gases emitted in the process of producing a good or providing a service. This includes our choices related to food.

“Food accounts for 10 to 30% of a household’s carbon footprint, with the higher proportions more typical of lower income households,” says Rob Anex, a professor of biological systems engineering.

Globally, food systems contribute about one-quarter of all human-caused greenhouse gas emissions, says Clare Bassi, a recently graduated master’s student in environmental studies who worked with Anex. That includes emissions associated with food production, processing, transportation, cooking, and waste. And different foods have very different environmental impacts.

“Meat products have large carbon footprints compared to vegetable and grain products,” Anex says. “This is because of the inefficient conversion of plant material into animal energy. It also stems from the methane released from the management of animal manure and the enteric fermentation that takes place in the digestive systems of ruminant animals like cattle.”

However, according to a recent study led by Bassi and coauthored by Anex, changing dietary patterns in the U.S. are leading to lower emissions of food-related climate-warming gases. And much of the reduction can be attributed to eating less meat.

The study explored how Americans’ eating patterns — and their associated carbon footprints — have changed in recent decades. In just 15 years, the carbon footprint of the U.S. diet fell by more than 35%, mostly due to Americans eating less meat and other carbon-intensive foods. Lower consumption of beef, dairy, chicken, pork, and eggs accounted for more than 75% of the observed diet-related carbon dioxide savings during the study period; beef alone was responsible for nearly half of the drop.

“The trend is quite exciting,” Bassi says. “Over the study period, national greenhouse gas savings from dietary changes alone is roughly equivalent to offsetting emissions from every single passenger vehicle in the country for nearly two years.”

As an individual, sometimes it feels like you don’t have much power to make positive change, Bassi notes. But the findings show that “our collective behavior changes are making a difference,” she says. By choosing foods with a smaller carbon footprint, “you can feel empowered that you can reduce your impact in a significant way.”

The researchers calculated greenhouse gas emissions based on individual daily diets reported by more than 39,000 U.S. adults in the National Health and Nutrition Examination Survey between 2003 and 2018. They looked at how the averages changed over time and also examined trends based on demographic factors, such as sex, age, household income, and race/ethnicity.

Every demographic subgroup analyzed showed a 30–50% reduction in diet-related greenhouse gas emissions during the study years. In general, females ate lower-impact diets than males. Females had an average food-related carbon footprint of about two kilograms of carbon dioxide emissions per person per day in 2018; for males, it was about three kilograms per person per day.

When the data were grouped by race/ethnicity, average carbon footprint was slightly higher among Hispanics than among non-Hispanic whites, and it was lowest among Blacks. In a breakdown by income level, diet-related carbon footprint was greatest in the highest income group (annual household income more than 1.84 times the federal poverty level, or about $46,000 for a family of four in 2018) and lowest in the lowest income group (annual income less than 1.3 times the poverty level, or $32,600 in 2018).

The lowest income group also showed the largest percentage reduction between 2004 and 2018 (46.4%), compared with 39.3% in the highest income group. When analyzed by age group, the youngest eaters showed the largest reduction in diet-related carbon emissions, with a 15-year drop of 47.2%.

“All of that saving is essentially from people eating less greenhouse gas-intensive foods,” Bassi says. Calorie intake stayed steady over the years of the survey, and the analysis used constant values for emissions related to production and other systemic factors to focus just on changes due to eating patterns.

These positive trends are encouraging, she notes, but Americans are still exceeding their fair share of food-related emissions compared to other parts of the world. A 2019 scientific report from the international EAT-Lancet Commission identified global thresholds for diet-related greenhouse gases that would adequately feed the world’s population while keeping global warming below 2° C by 2050. The average U.S. diet-related carbon footprint in 2018 was still nearly twice as high as the global targets.

“People’s actions are making a difference,” Bassi says, “but we still have a long way to go.”

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