The Carbon Cycle
Every breath you exhale contains carbon that a plant pulled out of the air not long ago. Some of the carbon in a piece of coal was in the air 300 million years ago. How can the same element be in the air, in your body, and in a rock? And why does the speed of its journey matter so much today? By the end of this page you will be able to answer both questions.
Reservoirs and fluxes
Earth has four systems: the geosphere (rock, soil, and the solid planet), the hydrosphere (water), the atmosphere (air), and the biosphere (living things). Carbon atoms travel among all four. The atoms are not created or destroyed. They just move.
Think of a bathtub. The water in the tub is a reservoir. The faucet and the drain are fluxes. If inflow equals outflow, the level stays steady. If one flux gets bigger, the level changes.
List the four Earth systems. Then name one place where carbon is stored in each of the geosphere, the hydrosphere, and the atmosphere.
List the four Earth systems. Then name one place where carbon is stored in each of the geosphere, the hydrosphere, and the atmosphere.
The fast cycle: photosynthesis and respiration
Plants, algae, and some bacteria take carbon dioxide from air or water and use sunlight to build sugars. This is photosynthesis.
$$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$$Nearly all living things, including plants, break sugars down to release energy. This is cellular respiration, and it gives CO₂ back to the air or water. Decay of dead organisms does the same.
$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{energy}$$This loop is fast: carbon may spend days, years, or a few centuries in living things before returning to the air.
Name the process by which plants and algae take carbon dioxide from the air or water and convert it to sugars, and name the process by which organisms release carbon…
Name the process by which plants and algae take carbon dioxide from the air or water and convert it to sugars, and name the process by which organisms release carbon dioxide back as they break down food for energy.
The slow cycle: shells, coal, and rock
Some carbon leaves the fast loop and gets locked away for millions of years.
Carbonate rock. CO₂ dissolves in seawater. Marine organisms such as corals, clams, and plankton combine dissolved carbon with calcium to build shells of calcium carbonate (CaCO₃). When they die, shells sink and pile up on the seafloor. Over time, pressure and cementation turn them into limestone. Carbon moves from atmosphere to hydrosphere to biosphere to geosphere.
Fossil fuels. When plants or plankton die in places with little oxygen, such as swamps or deep sea mud, they do not fully decay. Burial, heat, and pressure over millions of years turn that organic carbon into coal, oil, and natural gas. These are fossil fuels. Much US coal formed in the swampy forests of the Carboniferous Period, about 300 million years ago.
Coal, oil, and natural gas are called fossil fuels. State what they are made from and what happens to their stored carbon when people burn them.
Coal, oil, and natural gas are called fossil fuels. State what they are made from and what happens to their stored carbon when people burn them.
A marine organism builds a shell of calcium carbonate ($CaCO_3$). After it dies, the shell sinks to the sea floor and is buried. Describe the path of carbon in this…
A marine organism builds a shell of calcium carbonate ($CaCO_3$). After it dies, the shell sinks to the sea floor and is buried. Describe the path of carbon in this example, naming the forms of carbon (for example, dissolved carbon dioxide, carbonate in shells, and carbonate rock) and the Earth systems it moves among. Explain why this path can store carbon for millions of years.
During the Carboniferous Period, huge swampy forests covered parts of what is now the eastern United States. Explain how the growth, death, and burial of these plants…
During the Carboniferous Period, huge swampy forests covered parts of what is now the eastern United States. Explain how the growth, death, and burial of these plants removed carbon dioxide from the atmosphere, and what form the carbon eventually took. Then state one way that process is related to the coal that Americans burn today.
Comparing reservoirs
Scientists measure reservoirs in gigatons of carbon (GtC). One gigaton is one billion metric tons.
| Reservoir | Approx. carbon (GtC) |
|---|---|
| Atmosphere | 850 |
| Living plants and soils | 2,500 |
| Fossil fuel deposits | 4,000 |
| Surface and deep ocean | 38,000 |
The ocean holds the most. To compare two reservoirs, divide:
Now look at the atmosphere. It is the smallest reservoir in the table, so moving a large amount of carbon into it changes its content by a large percentage. When people burn fossil fuels, carbon moves from the geosphere (fossil fuel deposits) to the atmosphere as CO₂. And because the ocean is so large, even a small percentage change in the ocean's carbon is a huge amount of carbon, enough to matter for the atmosphere and for ocean chemistry.
The table shows approximate carbon stored in several reservoirs. | Reservoir | Carbon stored (gigatons of carbon, GtC) | |---|---| | Atmosphere | 850 | | Living plants…
The table shows approximate carbon stored in several reservoirs.
| Reservoir | Carbon stored (gigatons of carbon, GtC) |
|---|---|
| Atmosphere | 850 |
| Living plants and soils | 2,500 |
| Surface and deep ocean | 38,000 |
| Fossil fuel deposits | 4,000 |
- a) Which reservoir holds the most carbon?
- b) How many times more carbon does that reservoir hold than the atmosphere? Round to the nearest whole number.
- c) Burning fossil fuels moves carbon from one reservoir to another. Identify the direction of the transfer and explain why a small percentage change in the largest reservoir could still matter for the atmosphere.
Biology changes the cycle over time
Life has reshaped the carbon cycle more than once.
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Evolution of photosynthesis (over 2.4 billion years ago). Cyanobacteria began taking CO₂ out of the air and releasing oxygen. Over time, CO₂ fell and O₂ rose, changing the atmosphere and the oceans (the Great Oxidation Event).
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Land plants (about 450 million years ago onward). Forests buried huge amounts of carbon. In the Carboniferous, this produced the coal beds of the eastern US and lowered atmospheric CO₂.
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Shell-building organisms (over 500 million years ago). Marine life with CaCO₃ shells created a new long-term reservoir: limestone on the seafloor.
Each of these natural changes took millions of years. Burning fossil fuels returns carbon that took hundreds of millions of years to store, and it does so in about two centuries. Rates matter: ecosystems, ocean chemistry, and climate can adjust to slow change, but a fast flux outruns natural sinks, so CO₂ builds up in the atmosphere.
Back to the beginning: the carbon in your breath was recently in a plant, which took it from the air. The carbon in coal was in the air 300 million years ago, was stored by swamp plants, and is being returned now. The speed of that return is why it matters.
Emma studies a graph from a long-term record of atmospheric carbon dioxide measured on Mauna Loa, Hawaii. The curve rises over many years but also shows a regular…
Emma studies a graph from a long-term record of atmospheric carbon dioxide measured on Mauna Loa, Hawaii. The curve rises over many years but also shows a regular zigzag, with a peak each spring and a low each late summer or early fall in the Northern Hemisphere. Explain both features. Identify which biological process causes the yearly zigzag, why the low occurs when it does, and which human activity best explains the long-term rise. Use the terms reservoir, flux, and photosynthesis in your answer.
Analyze how biological processes changed the carbon cycle over Earth's history. In your answer, (a) describe how the evolution of photosynthesis changed the atmosphere's…
Analyze how biological processes changed the carbon cycle over Earth's history. In your answer, (a) describe how the evolution of photosynthesis changed the atmosphere's carbon dioxide and oxygen, (b) explain how the evolution of shell-building marine organisms created a new long-term carbon reservoir, and (c) compare the timescale of these natural changes with the timescale on which burning fossil fuels is now moving carbon. Conclude with one sentence stating why the speed of the change matters for Earth's systems.