A University of Bath-led study estimates human metabolism produces 2.05 billion tonnes of CO₂ annually, a 74% rise from 1.18 billion tonnes in 1985, driven mainly by population growth, not individual output, according to research published in The Lancet Planetary Health.
The study, led by Professor Javier Gonzalez, analyzed data from 9,006 individuals worldwide, revealing that the increase in CO₂ emissions from human respiration between 1985 and 2015 was largely due to more people rather than higher per-person output. Researchers emphasized that the carbon dioxide exhaled by humans originates from recently consumed food, distinct from fossil fuel emissions, which release ancient carbon stored underground.
Population Growth Drives Emissions Rise
Between 1985 and 2015, global CO₂ emissions from human metabolism rose from 1.18 billion to 2.05 billion tonnes, with over 60% of the increase attributed to population growth. Average adult body weight also increased by 13%, but this contributed less to the rise than demographic shifts. If current trends continue, annual emissions could reach 3.2 billion tonnes by 2084, the study predicts.
Professor Gonzalez noted, What we’ve found is that the increase is mainly because there are more of us.
The research combined real-world metabolic measurements with global population and body-weight data, highlighting the role of demographics in carbon cycling. This information can help scientists more accurately predict how effective carbon reduction strategies may be,
he added.
Regional Contributions and Carbon Cycle Insights
Asia accounted for the largest share of emissions, with China and India together producing 744 million tonnes in 2015. Africa surpassed Europe as the second-largest contributor due to population growth, while the United States remained the top per capita emitter. The study also found that men produced slightly more CO₂ than women, largely due to higher average body weights.
Unlike fossil fuel emissions, human respiration contributes about 5% of global CO₂ from fossil fuels and land-use changes. Researchers stress that while breathing is not a driver of climate change, quantifying this biological carbon flow improves models of the global carbon cycle. The important thing is that the carbon dioxide we breathe out is not the same as burning fossil fuels,
Gonzalez said.
Methodology and Limitations
The study used doubly labeled water measurements from the International Atomic Energy Agency’s database, tracking CO₂ production over two weeks. It included 4,479 females and 2,277 males aged 0–101, excluding athletes to avoid skewed results. Future work aims to refine models with more comprehensive datasets.
Ultimately, this information can help scientists more accurately predict how effective carbon reduction strategies may be,
Gonzalez said. The study’s findings, published in The Lancet Planetary Health, highlight the interplay between human biology and climate science.