A University of Bath–led study estimates that carbon dioxide produced by human metabolism worldwide reached 2.05 billion metric tons in 2015, up 74% from 1.18 billion metric tons in 1985. The research, published in The Lancet Planetary Health and led by professor Javier Gonzalez, found that population growth accounted for more than 60% of this increase rather than a rise in CO₂ production per person.
Researchers used real-world measurements of people from infancy to old age combined with global population and body-weight data. Gonzalez, from the University of Bath's Center for Exercise, Nutrition and Metabolism, stated, "We all produce carbon dioxide simply by turning food into energy, but until now we haven't had a good idea how much that adds up to worldwide."
The study distinguishes between CO₂ from human respiration and emissions from burning fossil fuels. The carbon exhaled by people comes from recently eaten food and represents carbon that plants have recently taken from the atmosphere, whereas fossil fuels release carbon stored underground for millions of years.
Gonzalez emphasized that the findings are about improving carbon accounting rather than suggesting behavioral changes. He noted, "The important thing is that the carbon dioxide we breathe out is not the same as burning fossil fuels. We're not saying that breathing is driving climate change." The research aims to help scientists build more accurate models of carbon movement and predict the effectiveness of carbon reduction strategies.