Human Bodies Are Producing Two Billion Tons of CO2 a Year

A new study reveals that the amount of carbon dioxide produced by human metabolism has increased dramatically over three decades.

The carbon dioxide produced by human bodies as they convert food into energy increased by 74% over three decades, according to a University of Bath-led study. By 2015, the global total had reached an estimated 2.05 billion metric tons (2.26 billion U.S. tons) annually, highlighting a substantial component of the Earth’s carbon cycle that scientists had not previously quantified on this scale.

Unlike emissions from burning fossil fuels, the CO₂humans exhale largely comes from carbon recently absorbed from the atmosphere by plants and transferred through food. Burning coal, oil, and natural gas releases carbon that has been stored underground for millions of years, adding it to the active carbon cycle.

Published in The Lancet Planetary Health, the study found that annual CO₂ production from human metabolism rose from 1.18 billion metric tons (1.30 billion U.S. tons) in 1985 to 2.05 billion metric tons (2.26 billion U.S. tons) in 2015. More than 60% of that increase was attributable to population growth rather than a substantial rise in the amount produced by each individual.

To estimate global emissions from metabolism, the researchers analyzed thousands of real-world measurements of CO₂ production in people ranging from infants to older adults. They combined these findings with worldwide population and body weight data to calculate how human metabolic activity changed over time.

Professor Javier Gonzalez, who led the research at the University of Bath’s Centre for Exercise, Nutrition and Metabolism, explained:

“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.

“What we’ve found is that the increase is mainly because there are more of us. This gives scientists one more piece of the puzzle when working out where carbon comes from and where it goes. Ultimately, this information can help scientists more accurately predict how effective carbon reduction strategies may be.”

Human metabolism contributes to the continuous movement of carbon through the environment, particularly in densely populated cities. Accounting for that activity could help researchers develop more accurate carbon budgets and better assess the effects of strategies designed to reduce emissions.

The researchers emphasize that their findings do not suggest people should exercise less, change their diets, or attempt to reduce the carbon dioxide they breathe out. The purpose is to improve scientific measurements of carbon movement, not identify respiration as a contributor to fossil fuel emissions.

Gonzalez stressed the distinction: “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.

“What this study does is give us a much clearer picture of how much CO₂ people produce around the world, and how that is changing as the population grows. This understanding can help improve the accuracy predictions of carbon reduction programs.”