Human beings collectively produced about 2.05 billion metric tons of carbon dioxide through metabolism in 2015, up from 1.18 billion metric tons in 1985, according to a modelling study published in The Lancet Planetary Health.
The rise of 74 per cent over three decades was driven mainly by population growth, which the authors say accounted for more than 60 per cent of the increase, rather than by any large change in what each person produces.
The work was led by Professor Javier T. Gonzalez of the University of Bath’s Centre for Exercise, Nutrition and Metabolism, and is titled “Accounting for global carbon dioxide production from human metabolism between 1985 and 2015: a modelling study”. The announcement was written and provided by the University of Bath and carried by Phys.org.
The study combined thousands of real-world measurements of the carbon dioxide people produce, taken from infancy to old age during everyday life, with global population and body-weight data. Three age bands were used: neonates under one year, juveniles from one to seventeen, and adults from eighteen.
“We all produce carbon dioxide simply by turning food into energy,” Gonzalez said. “What we’ve found is that the increase is mainly because there are more of us.”
The authors are emphatic that the finding is an accounting exercise and not an accusation. “The important thing is that the carbon dioxide we breathe out is not the same as burning fossil fuels,” Gonzalez said. Exhaled carbon comes from recently eaten food, and so largely from carbon that plants have recently taken out of the atmosphere. Fossil fuel emissions release carbon that has been stored underground for millions of years.
“We’re not saying that breathing is driving climate change,” Gonzalez said.
The researchers say the figures are intended to improve carbon accounting, particularly in heavily populated towns and cities, where human metabolism makes up a sizeable part of the local movement of carbon. They do not suggest that anyone should exercise less or change their diet. As a modelling study, its figures are estimates.
This newspaper accepts every word of that caveat and proposes to set it aside, because the number has now been published, and numbers, once published, are available for use.
Two billion metric tons is a large quantity. It belongs to an emitter with no registered office, no chief executive, no annual report and no address at which correspondence could be served. It is the only source of that scale that cannot be visited.
The authors’ defence is that the carbon was recently in the air and is merely passing through. Our analysts note that the same is true of a bonfire of this autumn’s leaves, and of a good deal else that is counted anyway. The distinction is chemically sound and administratively hopeless.
The accounting consequence is then unavoidable. If metabolism is to be entered in the ledger, as the authors propose, the ledger acquires a line that grows with population and shrinks with nothing. Efficiency gains are not available. The only lever is the number of people.
The Department of Unnecessary Research has worked through the mitigation options and reports that they are limited. You cannot insulate a person. You cannot retrofit a child. There exists no scheme under which a juvenile aged one to seventeen is replaced with a more modern unit.
The study’s own conclusion is the sensible one and deserves quoting exactly: “We’re not saying that breathing is driving climate change.” The figure of 2.05 billion metric tons is offered purely for the completeness of the record. It will now be quoted without that sentence.