A study led by the University of Bath estimates that carbon dioxide produced by human metabolism increased by 74% between 1985 and 2015, reaching 2.05 billion tonnes per year. The researchers say population growth, rather than a substantial rise in the amount of carbon dioxide produced by each person, was the main reason for the increase. The findings could help improve scientists’ understanding of the global carbon cycle and the accuracy of carbon-emissions models.
Why do humans emit carbon dioxide?
Human bodies produce carbon dioxide as they convert food into energy. Although the amount produced by one person is relatively small, the total from billions of people is substantial.
Researchers emphasize that carbon dioxide from human metabolism is fundamentally different from emissions caused by burning fossil fuels.
When a person exhales CO2, most of that carbon originally came from food. Plants absorbed the carbon from the atmosphere relatively recently, so it is already circulating through the natural carbon cycle.
Coal, oil and natural gas contain carbon that has been trapped underground for millions of years. Burning these fuels releases that long-stored carbon into the atmosphere, increasing the amount of carbon dioxide circulating through Earth’s climate system.
The research, published in The Lancet Planetary Health, was led by Professor Javier Gonzalez of the Center for Exercise, Nutrition and Metabolism at the University of Bath.
The team calculated that global carbon dioxide production from human metabolism increased from 1.18 billion tonnes in 1985 to 2.05 billion tonnes in 2015. More than 60% of the increase was attributed to population growth worldwide.
Professor Gonzalez explained:
“We all generate carbon dioxide just by converting food into energy, but until now we didn’t really understand how much carbon dioxide we produce around the world.
“What we found is that this increase is primarily due to our growing population. This gives scientists one more piece of the puzzle when figuring out where carbon comes from and where it goes. Ultimately, this information will help scientists better predict how effective carbon reduction strategies will be.”
How scientists measured human carbon dioxide production
To estimate the amount of carbon dioxide produced by humans, the researchers analyzed thousands of measurements collected from people across the lifespan, from infants to older adults.
These measurements showed how much CO2 the body produces during normal daily life. The team combined the results with global demographic and body-weight data to calculate how human metabolic CO2 production changed over time.
The results showed that population growth was the main driver of the 74% increase. Changes in the amount of carbon dioxide produced by individuals played a much smaller role.
Human metabolism contributes to the continuous movement of carbon through the environment, making it important to measure this process accurately. Its effect on local carbon measurements can be particularly pronounced in densely populated cities and urban areas, where many people live and work close together.
Understanding these contributions could help researchers develop more complete carbon budgets that track the sources, movement and destinations of carbon within the Earth system.
Why breathing is not causing climate change
The finding that human metabolism produces a large amount of CO2 should not be interpreted as evidence that human breathing is the primary cause of global warming.
The researchers do not recommend reducing physical activity or changing diets to reduce the carbon dioxide produced by the body. Instead, the study aims to make scientific measurements and calculations of the global carbon cycle more accurate.
Distinguishing carbon dioxide produced by natural biological processes from carbon released through fossil-fuel combustion is essential to understanding changes in Earth’s climate.
Professor Gonzalez emphasized this distinction:
“Importantly, the carbon dioxide we breathe out is not the same as burning fossil fuels. We’re not saying that what we breathe is causing climate change.
“This study aims to provide a clearer picture of the carbon dioxide people are producing around the world and how that is changing as the population grows. This understanding can help improve the predictive accuracy of carbon reduction programs.”
By establishing a clearer picture of human metabolic CO2 production, the researchers have provided another source of information for tracking global carbon movements and evaluating emissions-reduction strategies.
Funding: The IAEA Dual Label Water Database received financial support from the International Atomic Energy Agency, Taiyo Nippon Sanso and SERCON.
Source: www.sciencedaily.com
