Human metabolism emits 2 billion tonnes of CO2 yearly

A large carbon figure that should not be confused with fossil emissions

Human bodies collectively produce more than 2 billion metric tons of carbon dioxide a year through metabolism, according to research led by the University of Bath. The figure is substantial, but the researchers stress that it does not mean breathing is driving climate change in the same way as burning coal, oil and gas.

The study, published in The Lancet Planetary Health, estimates that carbon dioxide production from human metabolism rose from 1.18 billion metric tons in 1985 to 2.05 billion metric tons in 2015. That is an increase of 74% over three decades.

The finding offers a useful corrective to an intuitive but misleading comparison. Carbon dioxide is chemically the same molecule whether it comes from exhalation or a power plant, but its role in the climate system depends on where the carbon originated and how long it had been stored. The researchers’ point is that metabolic emissions are generally part of a much shorter biological cycle.

Food links exhaled carbon to the living carbon cycle

Every person produces carbon dioxide while converting food into energy. Much of the carbon in that food was recently taken from the atmosphere by plants. When people exhale carbon dioxide, that carbon is largely returning to the atmosphere as part of the normal exchange among plants, food, people, animals, soils, oceans and air.

Fossil fuels work differently. Coal, oil and natural gas contain carbon that was locked underground for millions of years. Combustion transfers that long-stored carbon into the active atmosphere-ocean-land system, increasing the amount of carbon dioxide circulating in it. That is why reducing fossil-fuel use is central to climate mitigation while human respiration is not treated as a comparable source of net new atmospheric carbon.

The distinction does not make metabolic carbon dioxide irrelevant to science. A global carbon-cycle model is a ledger of many connected flows. Improving estimates for biological flows can help researchers describe that ledger more accurately, even when those flows are not the same as fossil carbon emissions in climate-policy accounting.

Population growth drove most of the rise

The study attributes the large increase primarily to population growth rather than a major increase in the amount of carbon dioxide produced per person. More than 60% of the rise was associated with the growth in the number of people worldwide, according to the supplied research summary.

That result is significant because it shifts attention away from a simplistic idea that changing individual metabolism is the main explanation. Aggregate metabolic output grows as population rises, and it is also shaped by the amount of food converted into energy. The reported estimate is therefore a global measure of human biological activity, not a measure of household energy use, transportation emissions or industrial pollution.

Framing the result correctly is essential. The 2.05-billion-tonne estimate may sound comparable to national emissions totals, but comparing those quantities without specifying the carbon source risks mixing two different concepts: carbon circulating through recent biological processes and carbon newly released from geological storage.

Why the estimate can improve climate understanding

The researchers say a better measurement of humanity’s metabolic contribution could improve climate models and projections of the global carbon cycle. Such models aim to track how carbon moves between the atmosphere, living organisms, the oceans and land. Small uncertainties across several large flows can affect how confidently scientists can assess the balance of the system.

Human metabolism is unusual in public discussion because it is both universal and often omitted from broad emissions conversations. The research puts a numerical estimate on that flow while explicitly explaining its context. Rather than creating a new target for emissions cuts, the work may help prevent a category error: treating all carbon dioxide releases as climatologically identical regardless of their source.

The finding also highlights the importance of keeping carbon accounting intelligible. Public debate can turn a single striking statistic into an alarmist claim or a dismissal of climate science. Neither response follows from the study. The research says that metabolic carbon dioxide is large in gross terms, rises with population, and belongs mainly to a natural, relatively short-term cycle. Fossil-fuel emissions remain different because they add ancient carbon to that cycle.

What the research does and does not change

The new estimate does not alter the basic case for cutting fossil-fuel emissions. It instead fills in part of the scientific picture of how carbon moves through human societies and ecosystems. People must eat, and biological metabolism will continue to produce carbon dioxide. The relevant climate question is whether the carbon in food is drawn from, and returned to, the recently active carbon cycle or whether additional long-stored carbon is being extracted and burned.

For policymakers and communicators, that difference is more than a technical footnote. Policies designed to curb climate warming focus on avoiding net additions of carbon dioxide, especially from fossil sources. Counting exhalation as though it were equivalent to burning petroleum would obscure the role of energy systems, industry and land use.

The University of Bath-led analysis provides a scale for a previously under-quantified human carbon flow. Its core message is plain: human metabolism produces a vast amount of carbon dioxide, and measuring it can improve carbon-cycle science. But respiration should not be mistaken for a fossil-emissions source, because the carbon people exhale generally entered the food system from the atmosphere only recently.

This article is based on reporting by Science Daily. Read the original article.