Accounting for Global Carbon Dioxide Production From Human Metabolism Between 1985 and 2015: A Modelling Study

Background: Human activity is a dominant driver of global carbon emissions, yet one universal source–human metabolism–has never been quantified using free-living measurements of carbon dioxide (CO₂) production at scale. We aimed to describe the contribution of human metabolism to global CO₂ emissions between 1985 and 2015. Methods: In this modelling study, we used the largest available database of doubly labelled water (DLW) measurements from 1986 to 2019. The other data sources comprised WHO Global Health Observatory, United Nations Department of Economic and Social Affairs, and Non-communicable Disease Risk Factor Collaboration. We first allometrically modelled human CO₂ production (kg per year) using measurements available for 4479 female and 2277 male participants from the International Atomic Energy Agency DLW database to derive predicted estimates of human CO₂ production adjusted for differences in body mass and sex. Second, by combining predicted estimates based on allometric scaling of CO₂ production for body mass with available population demographic information, generalised additive models were used to describe the 30-year progression of human body mass and per capita and global CO₂ production. We used the time-series country-specific measurements from 1985 to 2015 with population-level global human metabolic CO₂ production summarised as cumulative sum for the progression analyses. Findings: Data were collected between 1986 and 2019. Mean ± SD (range) participant age was 40 ± 26 (0 to 101) and body mass was 63.3 ± 27.8 (2.4 to 101.9). 9006 measurements from 6756 participants (4479 women and 2277 men) from the DLW database were used to derive estimates of human CO₂ production adjusted for differences in body mass and sex.We observed that human metabolic CO₂ production approximately doubled over a 30-year period, from 1·18 Gt per year to 2·05 Gt per year. This increase was driven mostly by population growth (>60%) with a minor contribution from increases in body mass. Per capita sex differences equalised at the population level, and continental differences reflect population shifts rather than physiological differences, with Asia accounting for the majority of human metabolic CO₂ production among all continents. Interpretation: Our findings suggest that human metabolism is a quantitatively important component of the fast carbon cycle and should be accounted for in carbon system modelling to enhance precision and fidelity. With a projected peak global population in the mid-to-late 21st century, metabolic CO₂ production will increasingly contribute to global carbon fluxes. Funding: The the DLW database was funded by The International Atomic Energy Agency, Taiyo Nippon Sanso, and SERCON.