The Earth is constantly producing carbon dioxide through natural processes like respiration and decomposition. These natural levels are managed and balanced by the Earth. Humans contribute to CO2 levels by actions like burning fossil fuels (combining Oxygen with Carbon to produce energy). The contribution from humans is beyond what nature can handle.
September, 2016 marked a milestone in CO2 levels, 400 parts per million (Kahn 2016). Even though this is an arbitrary milestone (NOAA 2016), this still shows how much humans may be contributing to CO2 levels. September is significant when it comes to this reading because typically September is the month when CO2 levels are at their lowest in the Northern hemisphere (NOAA 2016). This is due to the growing season just ending and plants consuming the most CO2 of the year.
To put this in perspective, the last time CO2 levels were this high was in the mid-Pliocene, about three million years ago. To go along with the high level of CO2, the rate of CO2 increase is more than 100 times faster than observations in the ice core record going back 800,000 years. This will continue as long as fossil fuel consumption remains at its current high level worldwide. (NOAA 2016).
More information in the link below:
Thank you to OCEAN Researcher Erich Dietterle