University of Bath models global human metabolic CO2 emissions at 2.05 billion tonnes

Global carbon dioxide production from human metabolism reached 2.05 billion metric tons per year in 2015, representing a 74% increase from 1.18 billion metric tons in 1985, according to a study led by the University of Bath published in The Lancet Planetary Health and reported by sciencedaily.com sciencedaily.comsciencedaily.comsciencedaily.comOpen source ↗.

The research team, led by Professor Javier Gonzalez from the University of Bath's Centre for Exercise, Nutrition and Metabolism, determined that more than 60% of this increase was directly caused by worldwide population growth rather than major shifts in per capita emission rates.

Data synthesis and modeling

To calculate global biological emissions, the investigators evaluated thousands of metabolic measurements spanning individuals from infancy to old age during routine daily activities. The researchers integrated these physiological parameters with worldwide demographic registries and body weight statistics to quantify long-term shifts in metabolic output. The analysis confirmed that demographic expansion served as the primary driver of the upward trend, while fluctuations in individual metabolic rates exerted a minor impact.

The authors noted that localized metabolic releases are particularly pronounced in high-density metropolitan areas, where population concentrations alter local atmospheric measurements sciencedaily.comsciencedaily.comsciencedaily.comOpen source ↗. Incorporating biological respiration data helps scientists build more detailed carbon budgets and assess how carbon shifts between terrestrial systems, oceans, and the atmosphere.

Biological cycle versus fossil fuel emissions

The study authors emphasized that carbon dioxide generated by human metabolism does not drive anthropogenic climate change. Metabolic CO2 represents carbon recently fixed by plants via photosynthesis and consumed through food, meaning it circulates within the existing short-term natural carbon cycle.

In contrast, emissions derived from coal, oil, and natural gas release carbon that remained sequestered underground for millions of years, introducing net new carbon into the active Earth system. The researchers noted that their findings are intended to refine environmental models and evaluate the baseline efficiency of global carbon mitigation strategies, rather than suggest dietary or physical restrictions for individuals.