Carbon dioxide is the gas at the center of the greenhouse effect, and the carbon in it is always on the move: through the air, the ocean, living things and rock. You met fossil fuels as resources that took millions of years to form; here you follow their carbon.
Key idea
Carbon is stored in reservoirs: the air (about 900 billion tonnes of carbon, GtC, today), plants (about 450 GtC), soils (about 1,700 GtC), the ocean (about 38,000 GtC, mostly deep) and rocks (tens of millions of GtC). It moves between them in fluxes. The fast cycle runs through photosynthesis, respiration and the ocean surface, roughly a hundred GtC each way every year. The slow cycle runs through rock weathering, burial and volcanoes, well under 1 GtC a year. Burning fossil fuels moves carbon from the slow cycle into the fast one, about 10 GtC a year.
Why it works
Photosynthesis takes carbon dioxide from the air and builds sugar from it: $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$. Respiration and decay run the reaction backward. These two huge fluxes nearly cancel each year, as do the ocean's uptake and release, so the amount in the air stayed nearly steady for thousands of years. The slow cycle was balanced too: volcanoes add a little carbon, and weathering and burial of shells remove a little. Fossil fuels are ancient plants and plankton whose carbon was buried out of the fast cycle for millions of years. Burning them returns that carbon in decades, much faster than the slow cycle can take it back.
Worked example: How long does a carbon atom stay in the air?
The air holds about 900 GtC. About 120 GtC a year go into plants and about 80 GtC a year into the ocean. On average, how long does a carbon atom stay in the air before it is swapped out?
- Total flux out: $120 + 80 = 200$ GtC a year.
- Average stay: $900 \div 200 = 4.5$ years.
- But that is a swap: similar amounts come back each year. The extra carbon we add is removed far more slowly, over centuries to many thousands of years.
Answer: About 4.5 years
Worked example: Where does a tree's mass come from?
In glucose, $\text{C}_6\text{H}_{12}\text{O}_6$ (molar mass 180 g/mol), how many grams of every 180 g are carbon, and where did that carbon come from?
- Six carbon atoms at 12 g/mol: $6 \times 12 = 72$ g of the 180 g, which is 40%.
- All of that carbon came from carbon dioxide in the air, through the leaves.
Answer: 72 g, from CO₂ in the air
Common mistake
Thinking plants get their mass from the soil. Nearly all of a plant's dry mass is built from carbon dioxide taken from the air, plus water; minerals from the soil are a few percent. Check: a tree that grows by a tonne does not leave a tonne-sized hole in the ground around it.
Volcanoes add only about 0.1 GtC a year. People now add roughly a hundred times as much (USGS).