A study led by the University of Bath estimates that carbon dioxide produced by human metabolism rose by 74% between 1985 and 2015, reaching 2.05 billion tonnes per year. The researchers say that more than 60% of this increase was caused by population growth, rather than a major rise in the amount of carbon dioxide produced by each person.
The findings could help scientists develop a more complete understanding of the global carbon cycle, which describes how carbon moves among the atmosphere, living organisms, oceans and land. More precise measurements of humanity’s contribution may also improve predictions of how effective carbon-reduction efforts will be.
Why do humans produce carbon dioxide?
Researchers emphasize that carbon dioxide produced by human metabolism is fundamentally different from emissions generated by burning fossil fuels.
When people exhale CO2, most of the carbon originally comes from the food they consume. Plants have absorbed that 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.
Population growth drove most of the increase
The new research, published in The Lancet Planetary Health, was led by Professor Javier Gonzalez of the Centre for Exercise, Nutrition and Metabolism at the University of Bath.
The team calculated that global human metabolic CO2 production increased from 1.18 billion tonnes in 1985 to 2.05 billion tonnes in 2015. More than 60% of the increase was attributed to the growth of the global population.
Professor Gonzalez said:
“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 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 had 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 had a much smaller effect.
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 large numbers of 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.
Breathing is not the primary cause of climate change
The study’s findings about the large amount of CO2 produced by human metabolism 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 aim is 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 for 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 how much 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 data point 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