A study led by the University of Bath estimates that the world's population produces about 2.05 billion metric tons of carbon dioxide each year through ordinary metabolism — the process of turning food into energy. According to Science Daily, the figure represents a 74% rise since 1985, when the estimated total was 1.18 billion metric tons. The research was published in The Lancet Planetary Health.
The work was led by Professor Javier Gonzalez of the university's Centre for Exercise, Nutrition and Metabolism. His team reports that more than 60% of the increase can be traced to a growing global population rather than to any large change in how much CO2 an individual person generates.
Why it matters
The numbers are large, but the researchers are explicit that they do not mean breathing is a driver of climate change. The study is intended to sharpen the accounting of the global carbon cycle — the system that moves carbon among the atmosphere, living things, oceans and land.
Gonzalez said in the published remarks that people produce carbon dioxide simply by converting food into energy, but that until now there had been no solid global estimate of the total. He said the finding gives scientists another piece of the puzzle when working out where carbon comes from and where it goes, and that this can help them predict more accurately how effective carbon reduction strategies will be.
The distinction the authors stress is one of origin. Most of the carbon in the CO2 people exhale came from food, and the plants behind that food drew it out of the atmosphere recently. That carbon was already moving through natural cycles. Coal, oil and natural gas are different: their carbon has been locked underground for millions of years, and burning those fuels pushes long-stored carbon into the air, adding to what the climate system must hold.
How the estimate was built
To reach the global figure, the researchers worked from thousands of measurements taken from individuals across the human lifespan, from infants to older adults. Those measurements described how much CO2 the body produces during normal daily activity. The team combined them with worldwide population figures and body weight data to track how metabolic CO2 output changed over the three decades studied.
Population growth emerged as the dominant factor. Shifts in the amount produced by the average person played a much smaller part in the 74% increase.
The authors note that human metabolism can noticeably affect carbon readings in crowded cities, where large numbers of people live and work in close quarters. Accounting for that contribution, they argue, supports more complete carbon budgets — inventories that track where carbon originates, how it moves and where it ends up.
Gonzalez also addressed how the results should be read. He said the important point is that the carbon dioxide people breathe out is not the same as burning fossil fuels, and that the study is not claiming breathing drives climate change. What it provides, he said, is a clearer picture of how much CO2 people around the world produce and how that total is shifting as the population grows, which can improve the accuracy of predictions about carbon reduction programs.
The team does not recommend that anyone cut back on physical activity or alter their diet to lower their body's CO2 output. The aim is measurement, not behavioral change.
The IAEA doubly labeled water database used in the work received financial support from the International Atomic Energy Agency, Taiyo Nippon Sanso and SERCON, according to the materials provided by the University of Bath.
Watch for how these metabolic figures are folded into future carbon budget models and whether other research groups adopt the same method for estimating biological contributions to regional CO2 readings.
