Quantifying the Metabolic Footprint
Every day, human bodies convert food into energy, releasing carbon dioxide with every breath whether a person is walking to work, watching television, or sleeping. A University of Bath-led study published in The Lancet Planetary Health has quantified this collective global output, estimating that people breathed out approximately 2.05 billion metric tons (2.26 billion tons) of carbon dioxide worldwide in 2015.
According to the research, this global metabolic output surged by 74% over a three-decade span, rising from 1.18 billion metric tons (1.30 billion tons) in 1985. The investigation attributes more than 60% of that increase directly to population growth, while rising average body weight played a lesser role. Over the same period, an individual adult’s average output increased by roughly 8%, moving from about 321 kilograms (707 pounds) a year to roughly 346 kilograms (763 pounds), which works out to just over two pounds per day.
The Science Behind the Measurements
Led by Professor Javier Gonzalez at the University’s Centre for Exercise, Nutrition and Metabolism, the team analyzed 9,006 measurements taken from 6,756 individuals. The participants ranged in age from newborns to 101 years old and included 4,479 females and 2,277 males. Athletes were excluded from the dataset because intense physical training can elevate carbon dioxide production well above typical everyday levels.
To determine how much carbon dioxide individual bodies produced, researchers relied on a method called doubly labeled water. This technique involves participants consuming a drink containing rare, heavier isotopes of hydrogen and oxygen. By tracking the different rates at which the body eliminates these elements over approximately two weeks, scientists can calculate carbon dioxide production. The underlying data came from a database hosted by the International Atomic Energy Agency.
To scale these individual readings globally, the researchers built a formula linking yearly carbon dioxide production to body weight and sex, then applied it alongside global population and body-weight data for every country according to AZoCleantech. The model predicted that an individual weighing twice as much would produce about 50% more carbon dioxide rather than double the amount. Furthermore, totals for men and women came within 5% of each other, supported by women’s longer average lifespans. An average adult man produced approximately 358 kilograms (789 pounds) of carbon dioxide in 2015, compared to 334 kilograms (736 pounds) for an average adult woman, a difference primarily driven by typical variations in body weight.
Context and Climate Implications
The study’s authors emphasize that human exhalation is entirely distinct from the emissions generated by burning fossil fuels. The carbon released through human metabolism originates from the food people consume, representing carbon that plants recently removed from the atmosphere. In contrast, fossil fuels release carbon that has remained stored underground for millions of years.
We’re not saying that breathing is driving climate change,
Professor Gonzalez explained, stressing that the research is intended to improve carbon accounting rather than suggest altering diets or exercising less. In 2015, human metabolism produced an amount equal to roughly 5% of the carbon dioxide released by fossil fuels and land-use changes. Because scientists already account for other smaller sources down to about 300 million metric tons (330 million tons) a year, the researchers argue that human breathing is large enough to warrant a place in global calculations, particularly in heavily populated urban areas.
By refining these figures, researchers hope to give scientists an additional tool to understand where carbon originates and how it moves through the environment, ultimately helping to evaluate the effectiveness of future carbon reduction strategies.