Carbon Cycle

Carbon is a building block for all life and plays a key role in regulating Earth’s climate. It shuttles throughout the planet in two major cycles.

The first is the biospheric carbon cycle. Plants convert carbon dioxide from the air into organic carbon compounds (biogenic carbon). Microbes decompose plants and animals, releasing carbon dioxide that is released back into the atmosphere over timescales of years. A small fraction of biogenic carbon, along with petrogenic carbon weathered from rocks, is swept into the sea by rivers and buried in marine sediments.

The second is the geologic carbon cycle, which is a much larger and slower. Tectonic forces subduct the carbon in marine sediments down into Earth’s crust and mantle. This carbon re-emerges millions to hundreds of millions of years later in volcanic eruptions or when tectonic forces thrust segments of the seafloor up onto dry land, where it intersects again with the biospheric cycle.

Much more carbon is locked up in the geologic carbon cycle than the biospheric cycle, but it leaks out into the atmosphere much more slowly. Any increase in heat-trapping carbon dioxide in the atmosphere would lead to a warmer climate. Conversely, any increase in carbon burial would reduce biospheric carbon, lower carbon dioxide levels in the atmosphere, and cool Earth’s climate.