Dietary Carbon Intensity

Introduction

Fig. 1: Carbon intensity of common protein sources. [3-6] (Image Credit: H. Moise.)

In the year 2022, human activities contributed to the release of roughly 37 gigatons (GT) of carbon dioxide (CO₂) into the atmosphere. This substantial anthropogenic surplus of greenhouse gas (GHG) emissions poses profound and detrimental consequences for our environment, prompting a worldwide initiative to address and mitigate these emissions. Efforts have been focused on finding new forms of energy that can compete with fossil fuels and new chemical processes to produce the materials crucial to human activity such as steel, concrete, polymers, and ammonia. As necessary as these efforts are, progress has yet to meet the necessary benchmarks for reducing carbon emissions adequately to align with the net-zero objectives established by global governments. Efforts in these areas are largely restricted to those in our society who can participate in science, economics, and policy meaning a vast majority of our population is not being utilized in this campaign to mitigate carbon emissions. But average citizens collectively hold a tremendous amount of power in combatting climate change. They have the potential to reduce GHG significantly by simply changing our diets.

Protein Sources and Their Emissions

The global food system may account for up to 37% of all anthropogenic GHG emissions. [1] Total emissions encompass various sources such as the production of the food, fuel and fertilizer usage, processing and distribution, losses and waste, and land usage and practices. Government agencies and the private sector have made significant efforts to reduce emissions in each of these respective areas and improve overall efficiencies, but one area in particular has tremendous potential for curbing carbon emissions - dietary choice. Numerous metrics are available to gauge the environmental friendliness of different food choices, including water usage, GHG emissions, land use, impact on biodiversity, and pesticide use, among others. A comprehensive evaluation of food choices should consider all these factors. However, we will specifically concentrate on GHG emissions, expressing carbon footprints in units of CO_2eq and normalizing these emissions with the protein content of the food, so as to fairly compare each choice.

The United Nations Food and Agriculture Organization (FAO) estimated that animal agriculture represented around 7.1 Gt CO_2eq in 2013, amounting to 14.5% of annual anthropogenic greenhouse gas emissions for that year. [2] Surveying recent academic publications on the carbon footprints of different protein sources reveals a prevailing pattern: animal-based proteins generally exhibit higher carbon intensity compared to plant-based proteins, as illustrated in Fig. 1. [3-6] Since carbon emissions cannot be directly measured across these massive industries, these studies rely on assumptions that will vary between each study. For this reason, it would be more helpful, and accurate, to focus on the trends and not the specific values. From Fig. 1, it would be challenging to determine whether poultry is more carbon intensive then grains or beans, but one could claim with some confidence that all three are less carbon intensive than beef and lamb. Along with the indirect GHG emissions associated with raising livestock in general, ruminant mammals like Bovidae also directly emit GHGs like methane through enteric fermentation. This means beef and lamb can emit five times as many GHGs per unit protein as pork, making it a greater outlier among protein sources than coal is among energy sources; coal can emit anywhere from 20-35% more GHG than oil.

Putting it in Perspective

The USDA's Dietary Guidelines for Americans 2020-2025 recommends that we consume about 0.8 grams of protein per kilograms of bodyweight per day. [7] For an individual weighing 68 kilograms (150 pounds), they would need to consume at least 55 grams of protein per day to meet these requirements. This means they would emit 8.7 kg CO_2eq/day if they met this protein requirement by consuming beef as opposed to 1.0 kg CO_2eq/day from poultry or 0.8 kg CO_2eq/day from beans. To put this in perspective, the average GHG emission per passenger-mile for personal vehicles is 0.21 CO_2eq/mile. [8] This means that the 8.7 kg CO_2eq/day emitted from beef consumption is equivalent to driving 40.6 miles/day, a distance further than driving to Oakland from Stanford Campus. The average California household consumes 6,800 kWh of electricity in a single year, which emits 4.4 kg CO_2eq/day . [9,10] This means that acquiring your daily protein from beef is roughly equivalent to powering two California homes for that day.

Conclusion

We have focused here solely on GHG emissions, which by itself is a partial measure in assessing the overall environmental impact of food choices. The ascending order of protein sources displayed in Fig. 1 would likely shift if we instead considered water consumption or land usage per gram of protein. While this limits our assessment of these protein sources, it is enough information to help guide some of our daily dietary choices in a society that is increasingly concerned about GHGs and energizing the public to help aid in mitigating our emission of them.

© Henry Moise. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "FAO Strategy on Climate Change; 2021-2022," Food and Agriculture Organization of the United Nations, 2022.

[2] P. J. Gerber et al., "Tackling Climate Change Through Livestock: A Global Assessment of Emissions and Mitigation Opportunities," Food and Agriculture Organization of the United Nations; 2013.

[3] M.C. Heller and G. A. Keoleian, "Greenhouse Gas Emission Estimates of U.S. Dietary Choices and Food Loss," J. Ind. Ecol. 19, 391 (2014).

[4] D. Nijdam,R. Rood and W. Westhoek, "The Price of Protein: Review of Land Use and Carbon Footprints From Life Cycle Assessments of Animal Food Products and Their Substitutes," Food Policy 37, 760 (2012).

[5] S. Clune, E. Crossin, and K. Verghese, "Systematic Review of Greenhouse Gas Emissions For Different Fresh Food Categories," J. Clean. Prod. 140, 766 (2017).

[6] J. Poore and T. Nemecek, "Reducing Food's Environmental Impacts Through Producers and Consumers," Science 360, 987 (2018).

[7] "Dietary Guidelines for Americans, 2020-2025, 9th Ed.," U.S. Department of Agriculture, December 2020.

[8] "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990- 2020," U.S. Environmental Protection Agency, EPA 430-R-22-003, April 2022.

[9] "Household Energy Use in California," U.S. Energy Information Administration, 2009.

[10] "PG&E Climate Strategy Report," Pacific Gas and Electric, June 2022.