Carbon cycle

The carbon cycle is the way carbon is stored and replaced on Earth. Some of the main events take hundreds of millions of years, others happen annually.

The main ways that carbon gets into the carbon cycle are volcanoes, and the burning of fossil fuels like coal and gas. Through most of history, volcanoes were the biggest source of carbon to the carbon cycle, but in the last hundred years, people burning fossil fuels have been adding about a hundred times more CO₂ to the air than volcanoes. That is, for every ton of CO₂ added to the air by volcanoes, about 100 tons of CO₂ have been added to the air by people through combustion.

The main way carbon gets taken out of the atmosphere is by photosynthesis by living organisms. Some of this gets released as they die and decompose, but a proportion gets buried in sediment. This is shown in the diagram. Sediment turns to rock, and it is the carbonate rocks like limestone which contain the now-solid CO₂. Some of the carbon from plants also becomes part of the soil, where it can stay for a long time before decomposing.

Another process takes CO₂ out of the air. Weathering by rain washes out CO₂ in the form of dilute carbonic acid. This reacts with rock, helping to dissolve and destroy it. This also ends up as sediment.

"Weathering is a large consumer of the atmospheric carbon dioxide essential for dissolving rocks".^[1]

Some CO₂ is also dissolved in the ocean. Right now, the oceans are taking in more CO₂ than they are releasing, every year. However, this is making the oceans more acidic.

The store of carbon in sedimentary rock is far greater than the CO₂ in the atmosphere (this is not shown in the diagram). Eventually it returns to the air as oceanic plates subduct in tectonic plates. At the margins of plate boundaries (and some other places) volcanoes form and spew out CO₂. This completes the cycle.

1. ↑ Van Andel T.H. 1994. New views on an old planet:a history of global change. Cambridge University Press. p233