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.