Human metabolism produces two billion tonnes of CO₂ a year, global study finds

A University of Bath-led study has estimated that the carbon dioxide produced by humans as our bodies turn food into energy rose by 74% between 1985 and 2015, reaching 2.05 billion tonnes a year. 

Researchers from the University’s Centre for Exercise, Nutrition and Metabolism say the findings could help scientists better understand where carbon comes from, where it goes and how human activity fits into the bigger picture. 

Crucially, the CO₂ people breathe out is not the same as emissions from burning fossil fuels. The carbon we exhale comes from the food we eat and is largely carbon that plants have recently taken from the atmosphere. Fossil fuels, by contrast, release carbon that has been stored underground for millions of years.  

Published in The Lancet Planetary Health and led by Professor Javier Gonzalez at the University of Bath, the study found that the amount of CO₂ produced by people worldwide rose from 1.18 billion tonnes in 1985 to 2.05 billion tonnes in 2015. Population growth accounted for more than 60% of the increase. 

Professor Javier Gonzalez, lead author from the University of Bath’s Centre for Exercise, Nutrition and Metabolism, said: 

“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.” 

Measuring metabolism in everyday life 

The researchers used thousands of real-world measurements of people from infancy to old age, showing how much CO₂ their bodies produced during everyday life. 

They combined these with global population and body-weight data and found that population growth, rather than a major rise in the amount produced by each person, was the main reason for the increase worldwide. 

Getting this number right could help scientists build a more accurate picture of where carbon comes from and where it goes. Human metabolism is a sizeable part of that movement, particularly in heavily populated towns and cities. 

But the researchers stress that breathing is not the problem. The findings are about improving carbon accounting, not suggesting people should exercise less or change what they eat. 

Professor Gonzalez added: 

“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 programmes” 

ENDS 

Notes to editors:  

For more information, please contact Rebecca Tanswell at the University of Bath Press Office on press@bath.ac.uk 

Paper: Accounting for global carbon dioxide production from human metabolism between 1985 and 2015: a modelling study, The Lancet Planetary Health. DOI: 10.1016/j.lanplh.2026.101546. 

Authors: Javier T Gonzalez, Bruno Spellanzon, Dylan Thompson and James Betts, University of Bath; Lorenzo Lolli, Manchester Metropolitan University; Greg Atkinson, Liverpool John Moores University; Ed Maunder, Auckland University of Technology and Maurice Wilkins Centre for Molecular Biodiscovery; Alfonso Moreno-Cabañas, University of Castilla-La Mancha; and Jariya Buniam, Chulabhorn Royal Academy, on behalf of the IAEA Doubly Labelled Water Database Consortium writing group. 

Funding: The IAEA doubly labelled water database was financially supported by the International Atomic Energy Agency, Taiyo Nippon Sanso and SERCON. 

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