Human metabolism generated an estimated 2.05 billion metric tons of carbon dioxide in 2015, marking a 74 percent increase from 1985 levels. Researchers point to population growth rather than individual metabolic changes as the primary driver behind the rising global output.
Every human body releases carbon dioxide as it converts food into energy. A study published in The Lancet Planetary Health estimates that this global output climbed from 1.18 billion metric tons in 1985 to 2.05 billion metric tons by 2015.
More than 60 percent of that increase stems from a growing worldwide population.
That finding comes from Professor Javier Gonzalez of the University of Bath’s Centre for Exercise, Nutrition and Metabolism, according to Sciencedaily.
Researchers Track Decades of Metabolic Carbon Output
To quantify these global figures, researchers analyzed thousands of measurements collected from individuals ranging from infants to older adults during routine daily activities. The team combined those readings with worldwide population figures and body weight data to construct a model tracking metabolic carbon dioxide production over time. The calculations revealed that individual output changed far less than population totals, with an average adult’s annual output rising by roughly 8 percent over the three decades.
Between 1985 and 2015, the estimated global total rose from about 1.18 billion metric tons (1.30 billion U.S. tons) to 2.05 billion metric tons (2.26 billion U.S. tons). Each adult’s average output shifted from about 707 pounds (321 kilograms) a year to about 763 pounds (346 kilograms). The modeling predicted that someone who weighs twice as much would produce about 50 percent more carbon dioxide, rather than a full doubling of output. Even after adjusting for body weight changes, the global total still grew by 48 percent over the study period.
If each person’s output stayed at its 2015 level, the yearly total could reach about 3.2 billion tons around 2084.
That year is around the time the global population is projected to peak.
Exhaled Carbon Differs From Fossil Fuel Emissions
Scientists emphasize that the carbon dioxide exhaled by humans is fundamentally different from the emissions generated by burning fossil fuels. When people breathe, the carbon originates primarily from food, having been recently removed from the atmosphere by plants as part of the fast carbon cycle. Conversely, burning coal, oil, and natural gas releases carbon that has remained trapped underground for millions of years, adding long-term accumulated carbon to the active carbon cycle.
Incorporating human metabolism into climate models helps scientists develop more complete carbon budgets that track the sources, movements, and destinations of carbon within Earth’s systems. Accounting for that activity helps researchers better assess the effects of strategies designed to reduce emissions, particularly in densely populated urban centers where local metabolic output is most pronounced. In 2015, human breathing produced an amount equal to roughly 5 percent of the carbon dioxide released by fossil fuels and land-use changes.
China and India Generate Highest Regional Output
Geographic distribution of metabolic carbon output mirrors global population trends. China and India together accounted for about 744 million metric tons (820 million tons) in 2015. The next eight countries combined produced about 458 million metric tons (505 million tons): Bangladesh, Brazil, Indonesia, Japan, Nigeria, Pakistan, Russia, and the United States. Only the United States remained in the top ten when researchers ranked countries by output per person. During the study period, Africa passed Europe to become the second-largest contributor among continents.
The research was led by Javier Gonzalez of the University of Bath. The underlying data for the models drew from the IAEA doubly labeled water database, which received financial support from the International Atomic Energy Agency, Taiyo Nippon Sanso, and SERCON.
Earlier coverage: Researchers estimate human metabolism produces 2.05 billion metric tons of CO₂ annually