Early Earth, Page 23: [Previous Page] [Next Page] [Syllabus] Textbook pages 54 - 71

The Carbon Cycle on Earth

Atmospheric CO₂ comes from Volcanism. It is released along subduction zones. Early on, there was much volcanism, thus a lot of CO₂ was put into the early Earth atmosphere. CO₂ is removed from the atmosphere via rain, and weathering of rocks, driving the CO₂ to the ocean floor where carbonate sediments are the greatest reservoir for CO₂. Moving CO₂ back and forth between the Atmosphere and in the form of Carbonates and Rock is controlled mostly by the current temperature of the Earth.


This cycle has been going on since the beginning. Most important, the cycle has a negative feedback loop. Changing the temperature conditions in one direction tends to cause the system to move back in the other direction.
If the Earth is very warm, there is additional evaporation, cloud formation and rain, increasing the weathering and the removal of CO₂ from the surface rocks and atmosphere. Sedementation rates are higher, too, but subduction (and volcanic outgassing) does not increase. In the end, the effective removal of CO₂ from the atmosphere, without a corresponding increase in volcanism to replace it, reduces the greenhouse warming. The Earth will cool off.
If the Earth is very cold, most of the water and oceans are frozen over. There will be relatively little weathering or corrosive rain to wash the CO₂ out of the atmosphere and rocks and onto the ocean floor. However, subduction zones and volcanism would continue to supply additional CO₂ to the atmosphere, until the build up created such a large greenhouse effect, the Earth would warm up and its water would melt.

In this way, the oceans and volcanism kept the Earth at a very similar temperature, allowing for liquid water (not too hot, not too cool), even in the first billion years when the Sun was not as bright. Burning of fossil fuels has changed this balance in recent years, and may be leading to a heighten recent warming. See the American Geophysical Unions recent position on this issue here.