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.

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.