Human metabolism released an estimated 2.05 billion tonnes of CO2 in 2015, study finds

Human metabolism produced an estimated 2.05 billion metric tons of carbon dioxide in 2015, up 74% from 1.18 billion metric tons in 1985. A modelling study led by the University of Bath and published in The Lancet Planetary Health attributes more than 60% of that rise to population growth rather than to any large change in what each person produces.

Putting a number on a neglected flux

Every person releases CO2 as the body converts food into usable energy. The amount per individual is small, but until now there has been no robust global estimate of the total. The study, led by Professor Javier Gonzalez of the Centre for Exercise, Nutrition and Metabolism at the University of Bath, aims to fill that gap and give carbon-cycle researchers a more complete accounting of sources and sinks.

The paper lists a large international author group, with John Speakman among the senior contributors. The work drew on the IAEA doubly labeled water database, which was funded by the International Atomic Energy Agency, Taiyo Nippon Sanso and SERCON.

How the estimate was built

The team analysed thousands of measurements taken from people across the lifespan, from infants to older adults. These records describe the CO2 the body generates during ordinary daily life. The researchers then combined them with worldwide population figures and body weight data to model how total metabolic CO2 production changed over the 30-year period from 1985 to 2015.

Population growth emerged as the dominant driver of the 74% increase. Changes in per-person CO2 production played a considerably smaller part. As a modelling study, the results depend on how well the underlying measurements represent global populations, and the source material does not detail the uncertainty ranges.

Why this is not a fossil-fuel problem

The authors are explicit that metabolic CO2 differs in kind from emissions caused by burning coal, oil and gas. Most of the carbon people exhale originated in food, and the plants or animals that supplied it took up that carbon from the atmosphere relatively recently. It is therefore already part of the active carbon cycle.

Fossil fuels, by contrast, hold carbon that has been locked underground for millions of years. Burning them adds carbon to the atmosphere that was not previously circulating, which is what raises net atmospheric concentrations.

For this reason, the researchers say the findings do not show that breathing drives climate change. They are also not advising anyone to alter their diet or physical activity to reduce exhaled CO2.

Relevance for carbon budgets

The main value of the estimate lies in improving the precision of carbon budgets, which track where carbon originates, how it moves and where it ends up. Human metabolic CO2 may also affect local atmospheric measurements, particularly in densely populated cities where many people live and work in close proximity. Accounting for it could help separate biological signals from other sources in urban carbon monitoring.

Gonzalez said the work offers one more piece of the puzzle for working out where carbon comes from and where it goes, with the aim of helping scientists predict more accurately how well carbon reduction strategies will work.

The study covers only the period up to 2015. Extending the model to more recent years, and testing how sensitive the totals are to regional differences in body size, diet and age structure, would be natural next steps for researchers who want to integrate the figure into global carbon-cycle models.

Source: Human bodies produce 2 billion metric tons of CO2 a year just by staying alive (ScienceDaily: top science, 7 November 2026).

Image: AI-generated illustration.