Breathing Out 2.05 Billion Tonnes of CO2 Is Not Warming the Planet
People exhaled 2.05 billion tonnes of CO2 in 2015, about 5.8% of fossil output. Most of it is carbon plants recently took from the air, so breathing is a recycled flux, not a warming one.
The average person on Earth exhaled about 274 kilograms of CO2 in 2015, a figure averaged over every person, children included. The fossil fuel and industry CO2 released that year works out to about 4.7 tonnes per person.
So no, human breathing is not warming the planet. The carbon in a breath was in the air not long ago: plants took it in, food carried it, and the body returned it. Fossil carbon comes out of rock where it sat for millions of years, and when it burns it adds to what is in the air.
That is the short answer to a headline that began circulating after the University of Bath said on Wednesday, Oct. 28 that people's metabolism produced 2.05 billion tonnes of CO2 in 2015. The number is a real estimate, but it measures a loop, not a leak.
That is about 0.75 kilograms a day. Adults alone averaged about 346 kilograms that year, roughly 0.95 kilograms a day, according to Earth.com's write-up of the study.
Carbon that plants recently took from the air
Follow one atom of carbon. A plant pulls it from the air through photosynthesis. A person eats the plant, or an animal that ate it, and the body burns that food for energy. Out comes CO2, in a breath, ready for the next plant.
NASA's Earth Observatory calls this the fast carbon cycle. It is, in the agency's words, "largely the movement of carbon through life forms on Earth, or the biosphere," and the time carbon takes to move through it "is measured in a lifespan." MIT's Climate Portal describes the same closed loop, one that, biology professor Penny Chisholm says, adds no net carbon to the atmosphere.
The slow cycle moves carbon through rock over 100 to 200 million years. Fossil fuels are that rock carbon, and NASA is blunt about the consequence: "The burning of fossil fuels is the primary source of increased carbon dioxide in the atmosphere today." This is also why fossil emissions, not breath, are what moves the temperature, the subject of Daybreak's explainer on what climate overshoot means.
The University of Bath release puts the distinction in two sentences:
"The carbon we exhale comes from the food we eat and is largely carbon that plants have recently taken from the atmosphere. Fossil fuels, by contrast, release carbon that has been stored underground for millions of years."
University of Bath press release, Oct. 28, 2026
NASA scientist Peter Griffith teaches the idea with "just a banana and a piece of coal." His conclusion, as the agency's blog puts it: by burning fossil fuels, we move carbon from the slow, old carbon cycle to the fast, young carbon cycle.
One word in that Bath sentence matters: "largely." Growing, shipping and cooking food takes energy and causes emissions. That cost sits in the food system, not in the lungs, and more on that below. It is also why "recycled" is a fairer word for exhaled CO2 than "carbon neutral."
A model built on 6,756 people, applied to the world
The study, published in The Lancet Planetary Health online on Monday, Oct. 26, comes from a team led by Prof. Javier Gonzalez of Bath's Centre for Exercise, Nutrition and Metabolism, with co-authors at several universities and the IAEA doubly labelled water consortium. It estimates that human metabolic CO2 rose from 1.18 billion tonnes in 1985 to 2.05 billion tonnes in 2015, a 74% increase. The abstract says production "approximately doubled"; nearly is the honest word.
Gonzalez said the paper fills a gap: 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.
To fill it, the team used the largest available database of doubly labelled water measurements, hosted by the International Atomic Energy Agency: 9,006 measurements from 6,756 people, aged 0 to 101, weighing from 2.4 to 101.9 kilograms. Earth.com explains the method. A participant drinks water made with rare, heavier forms of hydrogen and oxygen. The body loses the two at different rates over about two weeks, and the difference reveals how much CO2 was produced. Athletes were excluded, since hard training can push output well above typical.
Earlier estimates, Earth.com reports, started from food intake or small lab tests, sometimes on fewer than 30 young adults. Here the researchers modelled CO2 production by body mass and sex, then combined the result with population and body-size data from the World Health Organization, the UN and NCD-RisC, using statistical models to trace the 30-year path.
One quiet surprise: a person twice as heavy produces about 50% more CO2, not twice as much.
The share is the same in 1985 and 2015
Here is the comparison the headlines skipped. Set beside fossil fuel and industry CO2 for the same years, the human number looks like this.
| 1985 | 2015 | Change | |
|---|---|---|---|
| CO2 exhaled by people (billion tonnes) | 1.18 | 2.05 | +74% |
| Fossil fuel and industry CO2 (billion tonnes) | 20.1 | 35.4 | +76% |
| People's share of the fossil total | 5.9% | 5.8% | about level |
| World population (billions) | 4.87 | 7.47 | +53% |
Sources: Gonzalez et al., The Lancet Planetary Health (2026); Global Carbon Budget 2025 via Our World in Data; UN population data. Shares and changes computed by Daybreak Wire.
The share barely moved. Both totals climbed by about three-quarters, so the ratio held at roughly 5.8% to 5.9%. A full year of human exhaling equals about three weeks of 2015 fossil output.
The two lines rose for different reasons. Gonzalez puts the human one plainly: What we've found is that the increase is mainly because there are more of us.
Population growth explains more than 60% of the human increase, with what the abstract calls "a minor contribution from increases in body mass." Adult average output edged up from about 321 kilograms a year to about 346, roughly 8%. Fossil CO2 rose because the world burned more of it. One curve follows headcount. The other follows appetite for energy, and the second is the one governments are trying to bend, as in the Oct. 21 piece on UNEP's 18% emissions-gap target in tonnes.
Gonzalez does not want the number misread:
"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."
Prof. Javier Gonzalez, lead author, University of Bath
Where the people are decides where the breath is. Per Earth.com, Asia produced the largest share; China and India together accounted for about 744 million tonnes in 2015. The next eight countries added about 458 million tonnes combined, and ranked per person only the United States stays in the top ten. Africa passed Europe as the second-largest continent, driven by population growth rather than body size or diet. Bath says metabolism matters "particularly in heavily populated towns and cities."
What about diet and exercise? The authors tested deliberately extreme scenarios on the 2015 estimate, Earth.com reports. Cutting carbohydrates from 60% to 10% of food energy, close to a ketogenic diet, lowers the total about 12%, or roughly 0.25 billion tonnes. A 72% carbohydrate diet raises it about 5%. If everyone walked 150 minutes a week, the WHO guidance, the total rises about 6%. That is about 0.12 billion tonnes, a little over a day of 2015 fossil output.
None of these is advice. Bath says the findings are "about improving carbon accounting, not suggesting people should exercise less or change what they eat."
Looking forward, Earth.com says that if per-person output stayed at its 2015 level, the yearly total could reach about 2.9 billion tonnes around 2084, near when population is projected to peak.
The estimate stops in 2015
Read the headline number as a careful model, not a meter reading. The study itself says the global total "is an estimate, not a direct measurement of everyone's breath," according to Earth.com. The abstract, the Bath release and Earth.com's summary give no margin of error, and guessing one here would add nothing.
The database represents some regions better than others, so ethnic differences could not be tested. The team used total body weight, not fat versus lean. The scenarios leave out effects that might develop over time, such as exercise gradually lowering weight. And animals displaced by a growing human population also breathe out CO2, so more people need not mean an equal rise in total animal CO2.
One more boundary is simply the calendar. The measurements run from 1986 to 2019, but the estimate ends in 2015, and the series should not be stretched past it. Fossil CO2 has not paused: the Global Carbon Project's 2025 budget, released Nov. 13, 2025, projects 38.1 billion tonnes for 2025, up 1.1% to a record. Prof. Pierre Friedlingstein of the University of Exeter said: "With CO2 emissions still increasing, keeping global warming below 1.5°C is no longer plausible."
The footprint of eating sits on the farm
If it is recycled carbon, why count it? Because a model that leaves out a flux has a hole in it. Earth.com notes that scientists already count other sources as small as about 300 million tonnes a year. The abstract calls human metabolism "a quantitatively important component of the fast carbon cycle" that "should be accounted for in carbon system modelling," and with population projected to peak in the mid-to-late 21st century, the flux will increasingly contribute to global carbon fluxes, and Bath says it is sizeable particularly in heavily populated towns and cities. Gonzalez says it can help scientists predict how effective carbon reduction strategies may be.
The real climate cost of eating is elsewhere. A 2021 analysis in Nature Food by Crippa and colleagues found that food-system emissions amounted to 18 Gt CO2 equivalent per year in 2015, "representing 34% of total GHG emissions," with agriculture and land use and land-use change making up 71% of that. That counts all greenhouse gases, so it cannot be set against the exhaled number; it shows where the levers are: how food is grown, fertilised, moved and wasted.
That is also why the framing matters. When a December 2023 UK study put human breathing at about 0.013% of the country's greenhouse gas emissions, Science Feedback reported on Jan. 12, 2024 that a tabloid claimed 0.1%, about eight times the study's value, while its corresponding author, Dr. Nicholas Cowan of the UK Centre for Ecology & Hydrology, said the overwhelming issue is the fossil fuels we burn.
In 2015, people exhaled about 65 tonnes of CO2 every second. In the same second, fossil fuel and industry released about 1,123 tonnes. Both streams are real, and only one is carbon that took millions of years to bury.
