Human breathing CO2 adds up to far more than most people would guess. A study led by the University of Bath estimates that the carbon dioxide people produce as their bodies turn food into energy reached 2.05 billion tonnes a year in 2015, up 74% from 1.18 billion tonnes in 1985. The researchers are clear on one point from the start: this is not the same as burning fossil fuels, and breathing is not driving climate change.
The study was published in The Lancet Planetary Health and announced by the university on 28 October. It was led by Professor Javier Gonzalez of Bath’s Centre for Exercise, Nutrition and Metabolism.
How much CO2 human breathing produces
Every person produces carbon dioxide as a by-product of metabolism, the chemistry that turns the food we eat into energy. That CO2 leaves the body when we breathe out. Until now, there has not been a good global estimate of how much it adds up to.
“We all produce carbon dioxide simply by turning food into energy, but until now we haven’t had a good idea how much that adds up to worldwide,” Gonzalez said.
According to the study, the global total rose from 1.18 billion tonnes in 1985 to 2.05 billion tonnes in 2015, an increase of 74% over 30 years.
Population growth drives most of the rise
The team used thousands of real-world measurements showing how much CO2 people produce during everyday life, from infancy to old age. They combined those measurements with global population and body-weight data.
The result: population growth, rather than a major rise in how much each person produces, was the main reason for the increase. Population growth accounted for more than 60% of it.
“What we’ve found is that the increase is mainly because there are more of us,” Gonzalez said. “This gives scientists one more piece of the puzzle when working out where carbon comes from and where it goes.”
Why human breathing CO2 is not like fossil fuel emissions
The key difference is where the carbon comes from. The carbon we exhale comes from our food and is largely carbon that plants recently took out of the atmosphere. When we breathe it out, it returns to the air as part of a short cycle.
Fossil fuels work differently. Coal, oil and gas release carbon that has been stored underground for millions of years, adding new carbon to today’s atmosphere.
“The important thing is that the carbon dioxide we breathe out is not the same as burning fossil fuels,” Gonzalez said. “We’re not saying that breathing is driving climate change.”
The researchers also stress that the findings are not a reason for anyone to exercise less or change what they eat.
Better carbon accounting, especially in cities
So why measure it at all? The answer is accounting. Scientists track carbon flowing between the land, the oceans, living things and the atmosphere. Human metabolism is a sizable part of that movement, particularly in heavily populated towns and cities, where many people are concentrated in a small area.
Getting the number right helps scientists build a more accurate picture of where carbon comes from and where it goes. According to Gonzalez, that understanding can help improve the accuracy of predictions for carbon reduction programs.
From my perspective, the most useful part of this study is the clarity of its message. A figure like 2 billion tonnes could easily be misread, and the authors have taken care to explain what it does and does not mean. For more on how emissions are measured and cut, see our stories on how carbon capture fits India’s net zero 2070 goal and Trinity’s red mud concrete that cuts carbon by up to 79.8%.
FAQ
How much CO2 do humans breathe out each year?
The University of Bath-led study estimates that human metabolism produced 2.05 billion tonnes of CO2 in 2015, up from 1.18 billion tonnes in 1985.
Does human breathing CO2 cause climate change?
No. The researchers say exhaled carbon mostly comes from food made by plants that recently took it from the air, unlike fossil fuels, which release carbon stored underground for millions of years.
Why did human CO2 output rise by 74%?
Mainly because there are more people. Population growth accounted for more than 60% of the increase, rather than each person producing much more.