CO2 Sources

I was at dinner the other night with a friend who is a retired chemist and chemical engineer. He started talking about the CO2 we breathe out and how it compared to the amount put in the air by automobiles. He seemed to think that production by cars was relatively insignificant. I'm sorry to say that I didn't have any numbers to compare. I've since tried to check that out.

Our breathing produces somewhat less than about 300 kg per year per person or something like 2.1 x 10^9 metric tons for all 7 billion of us = 2.1 GT/yr.

One barrel of average crude oil is responsible for about the same amount as a person breathes out in a year - about 315 kg of CO2. the world currently uses about 70 million barrels per day x 365 days = 8 GT per year.

If we combine all fossil fuels and cement production, we add 30 GT of CO2 per year.

So our breathing is a relatively small part of our contribution of CO2 to the atmosphere, but humans, numerous as we are, are a small part of the total biosphere. If we take all respiration, from all living things, about 700 GT of CO2 are produced each year. About half of that comes from autotrophs (plants and other photosynthesizers) and half from heterotrophs - the rest of life. Photosynthetic organisms consume nearly twice as much CO2 as they produce, so respiration and photosynthesis are roughly balanced. Cutting down tropical forests and other efficient carbon consumers changes the balance in favor of respiration however, and deforestation contributes about 7 GT/yr additional CO2 to the atmosphere.

All told, the human effects contribution to annual CO2 production is only about 6% of the total. It's not dominant, but it has been enough to shift the balance and lead to a slow but continuing buildup of CO2 in the atmosphere - roughly one half of a per cent per year.