Mitigating Climate Change Through Plant-Based Diets
Animal agriculture is a major contributor to emissions of the greenhouse gases (GHGs) methane and nitrous oxide. If we want to limit global warming to 1.5 degrees Celsius, we must make major cuts in agriculture-related emissions. While some experts suggest solutions such as changing how animals are farmed or increasing product yields, the best way to reduce agriculture-related emissions in the long term is for more people to transition to a plant-based diet.
Researchers in this study estimated how many GHGs are emitted by animal agriculture. They combined global data on how many farmed animals there are and how many emissions are caused by each kind of farmed animal. In 2019, the most recent year for which complete data are available, animal agriculture caused the emission of 1.6 gigatons of carbon dioxide, 120 megatons of methane, and seven megatons of nitrous oxide. This amounts to 4% of carbon dioxide emissions, 35% of methane emissions, and 66% of nitrous oxide emissions. Energy use, manure management, byproducts from the digestion process, and feed-crop fertilization are the main emissions culprits.
The researchers considered several different dietary scenarios: phasing out animal products immediately, phasing them out gradually over 15 years, replacing only certain animal products, and continuing the current rate of animal product consumption. They note that if no one farmed animals, we could return the land used in animal agriculture to its native state. Over thirty years, that would sequester 216 gigatons of carbon in plant and nonliving biomass.
In general, switching to a plant-based diet over 15 years (versus immediately) is the most realistic scenario considered in the research. Phasing out animal product consumption would have no effect during the transition period. Between 2030 and 2060, the rise of atmospheric carbon dioxide levels would slow. Methane, which has a half-life in the atmosphere of nine years, would approach a steady state by 2100. Nitrous oxide, which has a half-life of 115 years, would take much longer to reach a steady state.
The authors found that phasing out animal agriculture over the next fifteen years would have the same effect as eliminating 1,609 gigatons of carbon dioxide emissions. At current emission rates, that equals 47 years’ worth of carbon dioxide emitted by humans. Without any other changes, ending animal agriculture would take humanity 53% of the way to the goals laid out in the Paris Agreement on Climate Change. This is likely an underestimate, because as the world gets warmer, certain feedback loops further increase the global temperature. In a model that incorporates those feedback loops, phasing out animal products would compensate for 70% of the carbon dioxide humans emit each year.
Researchers also modeled the effects of phasing out various specific animal products. Cow meat alone accounts for 47% of the benefits, while cow milk accounts for 24% of the benefits. To put this into perspective, one pound of beef has the same climate impact as driving 537 miles. Ruminants (cows, sheep, goats, and buffalo) account for 90% of the climate impact related to animal agriculture. However, they provide only 19% of the protein in the human diet. The products with the least climate impact per gram of protein are chicken meat and eggs.
The study has some limitations. The authors didn’t include the seafood industry in their analyses, which is another source of GHG emissions. Their estimates are very uncertain, especially about how much carbon is sequestered on land reclaimed from animal agriculture. The researchers’ models are also simple and don’t account for many factors that influence climate and food-related emissions such as socioeconomic changes, expanding global populations, and per capita meat consumption. However, these limitations don’t change the overall conclusion: switching to a plant-based diet is one of the best ways to reduce greenhouse gas emissions.