Humans exhaled approximately 2.05 billion metric tons of carbon dioxide in 2015, a 74% increase from 1985 levels. A University of Bath-led study published in The Lancet Planetary Health suggests that while this metabolic output is not a driver of climate change, tracking it is essential for more accurate global carbon cycle modeling.
Human Metabolism and the Global Carbon Cycle
While human activity is often associated with fossil fuel combustion, a new study led by Professor Javier Gonzalez at the University of Bath highlights a significant but often overlooked source of carbon dioxide: human metabolism. Every day, people release carbon dioxide as a byproduct of converting food into energy. Unlike the carbon released by burning fossil fuels—which has been trapped underground for millions of years—the carbon we exhale is part of a short-term cycle, primarily derived from plants that recently removed the gas from the atmosphere.
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.
Professor Javier Gonzalez, lead author from the University of Bath’s Centre for Exercise, Nutrition and Metabolism
The Impact of Population Growth Since 1985
Researchers analyzed thousands of real-world measurements from individuals ranging from newborns to centenarians to estimate the global total of exhaled carbon dioxide. The data reveals that the total human contribution rose from 1.18 billion tons in 1985 to 2.05 billion tons in 2015. According to the study, this surge is not primarily due to individual metabolic changes, but rather the sheer increase in the number of people on Earth.
The study found that population growth accounted for more than 60% of the observed increase. While body weight also rose during this period—with average adult weight increasing by roughly 13%—it was a secondary factor compared to the growing global population. What we’ve found is that the increase is mainly because there are more of us,
Gonzalez noted.
Regional Contributions and Carbon Modeling
The researchers mapped these findings to geographic regions, noting that Asia produced the largest share of exhaled carbon dioxide in 2015. China and India alone accounted for approximately 820 million tons of the global total. In contrast, when ranked by output per person, only the United States remained among the top ten contributors.
By integrating these figures into computer models, scientists may gain a more precise understanding of how carbon moves through the environment. The researchers emphasize that their goal is not to suggest lifestyle changes, but to refine carbon accounting, especially for densely populated urban centers where human metabolism represents a significant component of local gas exchange.
Ultimately, this information can help scientists more accurately predict how effective carbon reduction strategies may be.
Professor Javier Gonzalez, lead author from the University of Bath’s Centre for Exercise, Nutrition and Metabolism
The team, which utilized data hosted by the International Atomic Energy Agency, plans to continue refining their formulas. Future research aims to incorporate more measurements from underrepresented demographics to further improve the accuracy of global carbon reduction programs.
