History of carbon dioxide in Earth's atmosphere

What influenced the abundance of carbon dioxide in Earth’s atmosphere?

The concentration of atmospheric carbon dioxide has changed a lot throughout Earth's history. Theories based on geological evidence suggest the processes listed below are responsible for these changes.

Volcanic activity

In Earth’s early history, between 4.5 to 4.2 byabillion years ago, geological evidence indicates high volcanic activity. Modern-day volcanoes emit CO2 and water vapor, among other gasses. Thus, the early atmosphere probably contained lots of CO2 and water vapor. About 4.2 byabillion years ago, a decrease in volcanic activity, alongside other processes, caused atmospheric CO2 abundance to decrease.

Formation of the oceans

After the Earth cooled down, water vapor condensed and formed the oceans around 4.2 to 3.8 byabillion years ago. This directly reduced the amount of water vapor in the atmosphere. Over time, atmospheric CO2 dissolved in the oceans to form carbonate compounds which precipitated as sediments on the seafloor. This process reduced the abundance of CO2 in the atmosphere. Thus, the formation of the oceans eventually caused atmospheric CO2 levels to decrease between 3.5 to 2.5 byabillion years ago.

Evolution of life

In the oceans, life evolved and eventually became part of the carbon cycle. Some organisms use CO2 to build their shells. When these organisms died, they collected on the seafloor, eventually forming rocks that 'locked up' the carbon they contained (via sedimentation). This contributed to decreasing atmospheric CO2 levels and is an example of a carbon sink. Fossil evidence for the first anaerobic organisms goes back to around 3.7 byabillion years ago. Photosynthetic organisms might only have evolved around 1 million years later and significantly sped up the process.

Photosynthesis and the carbon cycle

From 2.4 bya billion years agoonwards, photosynthetic organisms evolved and quickly decreased CO2 abundance in the atmosphere. Photosynthesis, sedimentation, formation of fossil fuels (oil, gas, and coal), and weathering of rock are all part of the carbon cycle. These processes combined significantly decreased CO2 levels in Earth’s atmosphere to make it fit for life as we know it. Today the atmosphere contains less than 1% CO2.

Line graph showing changes in atmospheric carbon dioxide abundance over Earth's history from 4.5 billion years ago until today. Bar graph showing major factors affecting carbon dioxide abundance. Early carbon dioxide levels were high due to high volcanic activity, however, the data shows some uncertainty with regard to initial carbon dioxide abundance. After the oceans formed, carbon dioxide dissolved in the oceans, and atmospheric levels decreased steeply from 3.8 until around 2.5 billion years ago. From then, Photosynthesis and the Carbon Cycle further reduced carbon dioxide abundance to reach levels below 1% atmospheric carbon dioxide as of today.
Figure 1: Atmospheric carbon dioxide abundance over Earth’s history: Theories on historic CO2 abundance differ regarding early Earth's atmosphere (dashed line and yellow shaded area show uncertainty), yet initial CO2 levels were high due to high volcanic activity. The following long-term decrease in CO2 levels (solid line) is linked to the formation of the oceans, evolution of life, photosynthesis, and the carbon cycle, resulting in today's levels below 1% atmospheric CO2.