Ask Dr. Global Change
How does respiration by humans and animals affect carbon dioxide levels in atmosphere?
Last updated 14 July 2002
Originally answered 14 July 2002
How does respiration by humans and animals affect carbon dioxide levels in the atmosphere?
Humans exhale about 1 kg of carbon dioxide per day (http://cdiac.esd.ornl.gov/pns/faq.html). The exact amount depends on age, sex, size, and most importantly activity level. Multiply that by a world population of six billion and you get a very large number.
However, human exhalation of carbon dioxide is part of a closed system. There can be no net addition of carbon dioxide to the atmosphere because the amount of carbon dioxide we exhale can’t be greater than the carbon we put into our bodies by eating plants, or eating animals that eat plants. The plants got the carbon from the atmosphere via photosynthesis.
This closed system is true for any animal, not just humans. It is also true for a growing population. You simply can’t have more animals than there are plants to support those animals.
The reason why burning fossil fuels is a concern is because it is not a closed loop over human time scales. Extracting coal and oil and burning them puts carbon back into the atmosphere that plants removed millions of years ago.