Humanity’s bodies now pump out about 2.05 billion metric tons of carbon dioxide a year simply by staying alive — a 74% jump since 1985, driven mostly by there being more of us. That is the finding of a modelling study led by Professor Javier Gonzalez of the University of Bath’s Centre for Exercise, Nutrition and Metabolism, published in The Lancet Planetary Health on 26 October 2026.
Why breath is not the same as burning coal
The researchers are emphatic that the figure should not be read as an emissions problem. The carbon people exhale comes from food, and the plants behind that food pulled it from the atmosphere recently — so breathing returns carbon that was already circulating through the fast carbon cycle. Fossil fuels, by contrast, release carbon trapped underground for millions of years.
How the team counted billions of breaths
Because nobody can fit a mask on humanity, the researchers used the doubly labelled water method: volunteers drink water tagged with heavy hydrogen and heavy oxygen, and urine samples over one to three weeks reveal how quickly each leaves the body. The difference between the two rates shows how much CO2 the person produced while working, sleeping and going about daily life.
Earth.com reports the team drew on 9,006 measurements from 6,756 people in a database hosted by the International Atomic Energy Agency — participants ranging from newborns to age 101. The team built a formula linking yearly CO2 production to body weight and sex, then applied it to population and body-weight data for every country. Earlier estimates relied on average calorie requirements or small lab groups of sometimes fewer than 30 young adults.
Between 1985 and 2015, the per-adult average rose only about 8%, from roughly 321 to 346 kilograms a year. The global total moved far faster because the world added about 2.6 billion people over the same stretch.
Two billion tonnes, set against national emissions
In 2015, fossil fuels and cement released about 35.4 billion tonnes of CO2 worldwide, meaning human metabolism accounted for almost 6% of that total, Martin Cid Magazine reports, setting the figure against Global Carbon Project data compiled by Our World in Data. The proportion barely moved across three decades — in 1985, metabolic CO2 was also close to 6% of a much smaller fossil total.
That same year, Russia emitted roughly 1.6 billion tonnes from fossil fuels and cement, while Japan’s figure was around 1.2 billion — each lower than what humanity collectively exhaled. India’s stood at about 2.2 billion tonnes. The geographic spread follows people rather than physiology: Asia produces most of humanity’s metabolic CO2 simply because most people live there.
SciTechDaily adds a comparison the other outlets do not: the 2.05 billion metric tons equals 2.26 billion U.S. tons, and scientists already count other carbon sources as small as about 300 million metric tons a year — an argument the researchers make for giving human breathing a place in carbon models.
What the model leaves out
The 6,756 volunteers joined studies run for other purposes, so they are not a random sample of humanity — women outnumber men roughly two to one. The model predicts output from body mass and sex, folding differences in population-wide physical activity into averages rather than measuring them country by country, and the series stops in 2015. Other gases fall outside its scope too: some people’s gut microbes produce methane, and earlier chamber experiments found roughly a third of volunteers were methane producers.
Researchers are now working to collect more doubly labelled water measurements from groups the database currently underrepresents, and the authors want human breathing included in carbon models for cities and the planet.
According to United Nations forecasts, the global population will top out near 10.3 billion around the mid-2080s, and with metabolic CO2 anticipated to rise alongside it, the authors contend that carbon accounting should tally every breath.
