Human metabolism generated an estimated 2.05 billion metric tons of carbon dioxide worldwide in 2015, marking a 74 percent increase from 1985 levels, according to a University of Bath-led study published in The Lancet Planetary Health on 26 October 2026.
Researchers calculated that the jump in metabolic output was driven primarily by global population growth rather than major changes in individual respiration rates. The findings quantify a natural component of the global carbon cycle that computer models have largely overlooked.
Population Growth and Body Mass Drive Total Emissions
Global carbon dioxide production from human metabolism climbed from 1.18 billion metric tons in 1985 to 2.05 billion metric tons in 2015. More than 60 percent of that increase was attributed directly to the expanding world population during those three decades.
The remaining increase stemmed largely from rising average body weight. Average adult weight rose about 13 percent for both sexes over the study period, with men’s average weight increasing from about 148 to 168 pounds.
Spread across the global population, the 2015 total averages out to roughly 275 kilograms per person per year, or about three-quarters of a kilogram a day when factoring in infants, children, and older adults. In 1985, that per-capita figure stood closer to 240 kilograms.
Researchers Measure Human Metabolism Using Isotopes
To estimate humanity’s total metabolic output without relying solely on lab tests of small groups, the research team analyzed 9,006 measurements from 6,756 individuals ranging from newborns to 101-year-olds. The data came from a database hosted by the International Atomic Energy Agency, with financial support from the IAEA, Taiyo Nippon Sanso, and SERCON.
The underlying field measurements used a technique called doubly labeled water. Participants consumed water tagged with heavy forms of hydrogen and oxygen, and researchers tracked how quickly those elements left the body over roughly two weeks. The difference between the two elimination rates revealed the exact amount of carbon dioxide produced during ordinary daily activities.
The team gathered individual readings. They used those readings to build a mathematical formula linking yearly carbon dioxide production to body weight and sex. They subsequently applied that formula to worldwide population and body-weight datasets for every country.
Human Breath Differs From Fossil Fuel Emissions
The study’s authors emphasized that human metabolic carbon dioxide is fundamentally different from emissions generated by burning coal, oil, and natural gas. While fossil fuels release carbon trapped underground for millions of years, the carbon humans exhale comes from food and was recently absorbed from the atmosphere by plants.
In 2015, human metabolism added up to nearly 6 percent of the 35.4 billion tonnes of carbon dioxide released globally by fossil fuels, industrial processes, and land-use changes combined. In raw tonnage, human breath exceeded the individual fossil-fuel emissions of nations like Japan at 1.2 billion tonnes and Russia at 1.6 billion tonnes, while sitting below India’s 2.2 billion tonnes.
Accounting for Metabolism Improves Carbon Budgets
Accounting for human metabolism helps researchers build more complete carbon budgets, which track the sources and movements of carbon within the Earth’s systems. This metabolic contribution is especially pronounced in densely populated urban areas where large numbers of people live and work in close proximity.
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.
United Nations projections place the peak of the global population at approximately 10.3 billion people in the mid-2080s. The researchers note that metabolic carbon dioxide production will continue to expand alongside demographic changes.
Earlier coverage: Researchers estimate human metabolism produces 2.05 billion metric tons of CO₂ annually