Every now and again, I find someone who believes that carbon dioxide (CO₂) emitted by the breathing of humans accounts for the rise of CO₂ in Earth’s atmosphere.  After all, there is a direct and powerful correlation between the rise of the human population and the rise of CO₂ in the atmosphere during the past few decades.  And human numbers are still increasing rapidly, so it makes sense that more CO₂ is emitted from humans each year.  Maybe fossil fuels are the wrong culprit.

Others believe that human bodies must provide a significant storage (or sink) for atmospheric CO₂ following the same reasoning—more people, more human mass.

Let’s look at the facts:

There are now about 7 billion people on Earth, with an average weight of 70 kilograms. Multiplying these two values yields about 500 billion kilograms, or about a half a billion metric tons of human body mass globally. The human body is about 65% water, so the dry mass of humans is about 0.2 billion metric tons.  About 50% of the dry mass of a human is carbon, so the carbon content of all humans is about 0.1 billion metric tons.

By comparison, all the vegetation on Earth contains about 560 billion metric tons of carbon, so humans are a relatively small reservoir of carbon. We have enormous impacts on the biomass and productivity of vegetation, but the rise of the human population, and thus its mass, stores only about 0.01% of the carbon dioxide released  each year from fossil fuel combustion.

The average human respires about 93 kilograms of carbon, as CO₂, each year.  That amounts to about 0.65 billion metric tons of carbon returned to the atmosphere from the global human population.  Nearly all of that carbon is obtained from foodstuffs, which removed the CO₂ from the atmosphere when they grew.  The rise in the human population each year increases the CO₂ released from human respiration by 0.007 billion metric tons—about 0.01% of the release from fossil fuel combustion.  So human breathing has essentially no direct impact on the rise of CO₂ in Earth’s atmosphere, versus the 10 billion metric tons of carbon that we extract from the Earth’s crust and burn, to power modern society.

In sum, it is tempting to think that human respiration—not fossil fuels—is responsible for the ongoing changes in our atmosphere that lead to global warming.   But, there is no scientific basis for thinking so.

References

Hofmann, D.J., J.H. Butler, and P.P. Tans. 2008. A new look at atmospheric carbon dioxide. Atmospheric Environment 43: 2084-2086.

Prairie, Y.T. and C.M. Duarte. 2006. Direct and indirect metabolic CO₂ release by humanity.  Biogeosciences Discussion 3: 1781-1789.

Schlesinger, W.H. and E.S. Bernhardt. 2013. Biogeochemistry: An analysis of global change. 3^rd. ed.   Academic Press/Elsevier, San Diego.

Schramski, J.R., D.K. Gattie, and J.H. Brown. 2015. Human domination of the biosphere: rapid discharge of the earth-space battery foretells the future of humankind. Proceedings of the National Academy of Sciences 112: 9511–9517,

Wolf, J., et al., 2015. Biogenic carbon fluxes from global agricultural production and consumption. Global Biogeochemical Cycles. Doi: 10.1002.2015GB005119