Humans are Primarily Responsible for the Rise in CO2

Humans are primarily responsible for the rise in CO₂

Bookkeeping by itself shows that humans are responsible. We produce roughly 100 times more CO₂ than volcanoes do (maybe only 50 times, maybe closer to 200 times, if you include the uncertainties, but something like 100). Nature was producing its CO₂ for a long time, but humans have increased from being a very small source to being much more important than volcanoes.

Furthermore, several tracers in the atmosphere confirm the bookkeeping. These include:

• Dropping oxygen. The level of oxygen in the air is dropping as CO₂ rises. You'll still be able to breathe with no trouble, but this shows that the rise in CO₂ is caused by fire, which uses oxygen, and not by melting beneath a volcano or CO₂ coming out of the ocean, which don't use oxygen.
• Falling carbon-13. Plants preferentially use CO₂ containing the lighter and faster-diffusing carbon-12 rather than the heavier carbon-13, so CO₂ formed by burning living plants or fossil fuels is more enriched in carbon-12 than is CO₂ emitted by volcanoes or coming from chemicals dissolved in the ocean. The CO₂ in the air is becoming more enriched in carbon-12 and less enriched in carbon-13, which shows that the extra CO₂ comes from living or dead plants.
• Falling carbon-14. Carbon-14 is made in the atmosphere by cosmic rays, then taken quickly into living plants, but decays radioactively over about 50,000 years. (Half of the carbon-14 decays in just about 5730 years, half of the remainder in the next 5730 years, and by 50,000 years or so almost none remains.) Thus, living plants contain carbon-14 but fossil fuels don't. Carbon-14 is becoming less common in the atmospheric CO₂, much more rapidly than can be explained by radioactive decay, showing that the extra CO₂ in the air is coming from plants that have been dead a long time.

Taken together, bookkeeping says that the rise in atmospheric CO₂ is coming from human burning of fossil fuels. And, the atmosphere says that the rise in CO₂ is coming from burning of plants that have been dead a long time. The agreement is beautiful, confirming that we are responsible for what is occurring.

There is a bit more complexity to this, linked to our burning of forests, but also letting some forests grow back and fertilizing others, and linked to us releasing some CO₂ while making cement. But overall, the biggest source of CO₂ is our fossil fuels, and this will become more and more important in the future if we continue on our present path.