Every human body produces carbon dioxide simply by converting food into energy. Although the amount released by one person may seem small, the combined output of billions of people adds up to an enormous quantity.
A new study led by the University of Bath estimates that carbon dioxide production from human metabolism increased by 74% between 1985 and 2015, reaching 2.05 billion metric tons annually. The researchers found that the dramatic rise was primarily the result of population growth, rather than a substantial increase in how much CO2 each person produces.
The findings could help scientists better understand the global carbon cycle, which describes how carbon moves between the atmosphere, living organisms, oceans, and land. By measuring humanity’s contribution more accurately, researchers hope to improve predictions about how well efforts to reduce carbon emissions will work.
Why Human CO2 Emissions Are Different From Fossil Fuels
Despite the enormous numbers, the researchers emphasize that the carbon dioxide produced by human metabolism is fundamentally different from the emissions generated by burning fossil fuels.
When people breathe out CO2, most of that carbon originally came from the food they consumed. Plants absorbed it from the atmosphere relatively recently, meaning it is already circulating through the natural carbon cycle.
Burning coal, oil, and natural gas is different. These fuels contain carbon that has remained trapped underground for millions of years. Burning them releases that long-stored carbon into the atmosphere, adding to the amount of CO2 circulating through the Earth’s climate system.
The new research, published in The Lancet Planetary Health, was led by Professor Javier Gonzalez of the University of Bath’s Centre for Exercise, Nutrition and Metabolism.
The team calculated that global CO2 production from human metabolism climbed from 1.18 billion metric tons in 1985 to 2.05 billion metric tons in 2015. More than 60% of that increase could be attributed to the growing number of people worldwide.
Professor Gonzalez 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.”
How Scientists Measured Humanity’s CO2 Production
To estimate how much carbon dioxide people generate, the researchers analyzed thousands of measurements collected from individuals across the human lifespan, from infants to older adults.
These measurements provided information about the CO2 produced by the body during ordinary daily activities. The team then combined those findings with worldwide population figures and body weight data to calculate how human metabolic CO2 production had changed over time.
The results showed that population growth was the dominant factor behind the 74% increase. Changes in the amount of carbon dioxide generated by individual people played a much smaller role.
Accurately measuring this process is important because human metabolism contributes to the continuous movement of carbon through the environment. Its influence on local carbon measurements can be especially noticeable in densely populated cities and urban areas, where large numbers of people live and work close together.
Understanding these contributions can help researchers develop more complete carbon budgets, which track the sources, movements, and destinations of carbon within the Earth’s systems.
Scientists Stress That Breathing Is Not Driving Climate Change
Although the study reveals a surprisingly large amount of CO2 production, its findings should not be interpreted as evidence that human breathing is a major cause of global warming.
The researchers are not recommending that people reduce their physical activity or change their diets to lower the carbon dioxide produced by their bodies. Instead, the goal is to make scientific measurements and calculations of the global carbon cycle more accurate.
Distinguishing between carbon dioxide produced through natural biological processes and carbon released by burning fossil fuels is essential for understanding the planet’s changing climate.
Professor Gonzalez emphasized this 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.”
By establishing a clearer picture of humanity’s metabolic CO2 production, the researchers have provided another piece of information scientists can use to track global carbon movements and evaluate strategies for reducing emissions.
Funding: The IAEA doubly labeled water database was financially supported by the International Atomic Energy Agency, Taiyo Nippon Sanso, and SERCON.
