Human metabolism now releases 2.05 billion tons of CO2 each year
Researchers say most of the rise comes from a larger global population, and the carbon differs from fossil fuel emissions.
Key points
- Human metabolic CO2 rose 74% between 1985 and 2015, according to a University of Bath study.
- Output reached 2.05 billion metric tons a year in 2015, up from 1.18 billion in 1985.
- More than 60% of the increase is linked to a growing number of people.
- Researchers say breathing is not driving climate change and do not advise people to change their habits.
- The study was published in The Lancet Planetary Health.
Researchers at the University of Bath have calculated that the carbon dioxide people release through metabolism has grown considerably over three decades. According to ScienceDaily, the global figure rose by 74% between 1985 and 2015. By the later year, it had reached 2.05 billion metric tons annually.
The team says the main driver is the number of people on Earth. Population growth accounts for more than 60% of the increase. The amount of CO2 that each individual produces changed much less over the period.
Where the carbon comes from
The work was led by Professor Javier Gonzalez of the university’s Centre for Exercise, Nutrition and Metabolism. He stresses that breathed-out carbon is different from carbon released by burning fuel. Most of the carbon in a person’s breath originally came from food. That food drew carbon from the air through plants fairly recently, so the carbon is already part of the natural cycle.
Fossil fuels such as coal, oil and natural gas work differently. Their carbon has been locked underground for millions of years. When these fuels are burned, that old carbon enters the atmosphere and adds to the total circulating in the climate system.
The study was published in The Lancet Planetary Health. Gonzalez said the team wanted a clearer estimate of how much CO2 people produce worldwide, something that had not been well measured before.
How the estimate was built
To reach their figures, the researchers analysed thousands of measurements taken from people at different stages of life, from infants to older adults. These readings showed how much CO2 the body gives off during ordinary daily activities. The team then combined this information with world population data and body weight figures.
By tracking these inputs over time, the scientists could see how human metabolic output had shifted. Their analysis indicated that the rise was chiefly driven by the growing number of people. Changes in individual output played a much smaller part.
The researchers note that human metabolism can affect local carbon readings. Its effect may be most visible in crowded cities, where many people live and work close together. A better estimate of this source can help scientists build fuller carbon budgets, which track where carbon comes from, how it moves and where it ends up.
What the findings do not mean
Even with such a large total, the team says the work does not prove that breathing contributes heavily to warming of the planet. The researchers are not asking people to cut back on exercise or alter their diets to reduce the CO2 their bodies produce. Their aim is simply to improve scientific measurements of the global carbon cycle.
Gonzalez said the work gives a clearer picture of how CO2 output is changing as the population grows. He added that it may also help make predictions about carbon reduction programmes more accurate. Separating natural biological carbon from fossil fuel carbon remains central to understanding the planet’s changing climate.
Funding came from the International Atomic Energy Agency, Taiyo Nippon Sanso and SERCON, which supported the doubly labelled water database used in the research.
Frequently asked questions
How much CO2 do humans produce each year?
According to a University of Bath study, human metabolism produced 2.05 billion metric tons of CO2 in 2015. That was up from 1.18 billion metric tons in 1985.
Is human breathing causing climate change?
The researchers say the findings do not prove that breathing plays a heavy role in warming the planet. Most of the carbon breathed out originally came from food and is already part of the natural carbon cycle.
What caused the rise in human CO2 output?
The researchers found that population growth was the main cause, accounting for more than 60% of the increase. Changes in how much CO2 each person produces played a much smaller role.
Sources
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