Human metabolism adds 2.05 billion tons of CO2 a year, study finds

Carbon dioxide produced by human metabolism reached 2.05 billion metric tons a year in 2015, up 74% from 1.18 billion metric tons in 1985, according to a study led by the University of Bath.

The researchers found the rise came mainly from population growth rather than a large increase in how much CO2 each person produces. More than 60% of the increase could be attributed to the growing number of people worldwide, they said. The findings could help scientists better understand the global carbon cycle, which describes how carbon moves between the atmosphere, living organisms, oceans and land.

Why human CO2 differs from fossil fuels

The researchers stress that CO2 from human metabolism is fundamentally different from emissions from burning fossil fuels. Most of the carbon people breathe out came from the food they ate; plants absorbed it from the atmosphere relatively recently, so it is already circulating through the natural carbon cycle. Coal, oil and natural gas hold carbon trapped underground for millions of years, and burning them releases that long-stored carbon into the atmosphere.

The 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. Gonzalez 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.”

How the researchers measured it

To estimate human CO2 production, the team analyzed thousands of measurements collected from individuals across the lifespan, from infants to older adults, covering the CO2 the body produces during ordinary daily activities. They combined those findings with worldwide population figures and body weight data to calculate how metabolic CO2 production had changed over time. Changes in the amount of CO2 generated by individual people played a much smaller role than population growth, the results showed.

Human metabolism’s 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.

Breathing is not driving climate change

The researchers say the findings should not be read as evidence that human breathing is a major cause of global warming. They are not recommending that people reduce physical activity or change their diets to lower the CO2 their bodies produce. The goal is to make measurements and calculations of the global carbon cycle more accurate.

Gonzalez emphasized the 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.”

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