Antonio Manaytay Every time people breathe out, they release carbon dioxide. It is a normal part of how the body turns food into energy. But when billions of people do it every day, the combined amount is far from insignificant.
A study led by the University of Bath estimates that humans produced 2.05 billion metric tons of carbon dioxide through metabolism in 2015. That was 74% more than the estimated 1.18 billion metric tons recorded in 1985.
The main reason was straightforward: there were more people on the planet.
Researchers found that population growth accounted for more than 60% of the increase. Changes in the amount of CO2 produced by individuals played a much smaller role.
Human Metabolic CO2 Production
Annual global production, billion metric tons
Key finding: More than 60% of the increase was attributed to global population growth, rather than a substantial rise in CO2 production per person.
Source: University of Bath-led study, The Lancet Planetary Health.
The findings, published in The Lancet Planetary Health, offer a more detailed estimate of how human activity contributes to the natural movement of carbon through the atmosphere, land, oceans and living organisms.
“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,” said Professor Javier Gonzalez, who led the research at the University of Bath’s Centre for Exercise, Nutrition and Metabolism.
How researchers calculated the figures
The research team examined thousands of measurements of carbon dioxide production from people at different stages of life, from infants to older adults.
They combined these measurements with global population figures and body-weight data to estimate how much CO2 humans produce through metabolism worldwide.
The results showed a steady increase over the 30-year period, with population growth emerging as the biggest factor.
The distinction matters because scientists need to account for different sources of carbon when studying the global carbon cycle. This includes carbon released by living organisms as well as carbon entering the atmosphere through human activities.
Why breathing is not the same as burning fossil fuels
The figures may sound alarming, but the researchers stressed that carbon dioxide released through breathing should not be confused with emissions from coal, oil and natural gas.
The difference lies in where the carbon comes from.
Plants absorb carbon dioxide from the atmosphere as they grow. People and animals consume plants, directly or indirectly, and release some of that carbon back into the atmosphere through respiration.
This is part of the natural carbon cycle. The carbon is already circulating through the environment.
Fossil fuels are different. Coal, oil and natural gas contain carbon that has been stored underground for millions of years. Burning them releases that long-stored carbon into the atmosphere, adding to the amount of carbon dioxide in the climate system.
For this reason, the study does not suggest that human breathing is a major driver of global warming. Nor does it recommend that people change their diets or reduce physical activity to limit metabolic CO2.
Instead, the researchers want scientists to have a more complete picture of where carbon comes from and how it moves through the environment.
The contribution of human metabolism may be particularly noticeable in densely populated cities, where large numbers of people live and work in relatively small areas.
A better understanding of these emissions could help improve carbon accounting and climate models, giving researchers more accurate information when assessing efforts to cut greenhouse gas emissions.
The study adds another dimension to the science of the global carbon cycle. Its central finding is not that breathing is worsening climate change, but that population growth has increased the amount of carbon moving through the human body and back into the natural environment.
Reference: University of Bath. (2026, November 7). Human bodies produce 2 billion metric tons of CO2 a year just by staying alive. ScienceDaily. Retrieved November 7, 2026 from www.sciencedaily.com/releases/2026/11/261104012305.htm
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