<p>People produce carbon dioxide as their bodies turn food into energy. Added together, that flow is large: a modelling study estimated that human metabolism produced 2.05 billion metric tons of CO₂ in 2015. That was 74% higher than the estimate of 1.18 billion metric tons for 1985. The study was led by Professor Javier Gonzalez of the University of Bath and published in <em>The Lancet Planetary Health</em>.</p>

<p>The increase was driven mainly by population growth. The researchers attributed more than 60% of the rise between 1985 and 2015 to there being more people worldwide. Changes in how much CO₂ each person produced played a much smaller role. The larger total, in other words, reflects the combined metabolism of a growing population more than a substantial change in per-person output.</p>

<p>The large total needs context: metabolic CO₂ is not equivalent to carbon dioxide from burning fossil fuels. Most of the carbon people breathe out came from food. Plants absorbed that carbon from the atmosphere relatively recently, before it moved through food and into human bodies. Its return to the air is part of the natural carbon cycle, which moves carbon among the atmosphere, living organisms, oceans and land.</p>

<p>Coal, oil and natural gas follow a different path. Their carbon remained underground for millions of years; burning these fuels releases it into the atmosphere. The report stresses that the study is not evidence that breathing is driving climate change. The two sources of CO₂ must be distinguished when assessing how carbon moves through the climate system.</p>

<p>To estimate human metabolic production, the researchers analyzed thousands of measurements from people across the lifespan, from infants to older adults. The measurements covered CO₂ produced during ordinary daily activities. The team combined them with global population figures and body-weight data to estimate how the total changed over time. The report describes a modelling estimate, rather than a direct count of everyone’s breath.</p>

<p>Accounting for this flow could help scientists build more complete carbon budgets: records of carbon’s sources, movements and destinations. The report says metabolic CO₂ can be especially noticeable in local measurements in densely populated cities, where many people live and work close together. Better accounting may help researchers interpret those measurements and improve predictions of how effective carbon-reduction efforts will be.</p>

<p>The study’s contribution is therefore a clearer estimate of one part of the carbon cycle, not a reason to treat biological CO₂ as interchangeable with fossil-fuel emissions. Its estimates cover 1985 to 2015 and show how population growth changed the global total. More accurate carbon accounting can help scientists track where carbon comes from and goes, while preserving the distinction between recently cycled biological carbon and carbon released from long-term underground stores.</p>

<p>Source: Human bodies produce 2 billion metric tons of CO₂ a year just by staying alive</p>