The Carbon Cycle
Every breath you exhale contains carbon dioxide. Every tree you pass on the way to school is pulling it out of the air. So where does all that carbon go, and why do scientists in Hawaii and Colorado track it so closely? In this page you will follow a single carbon atom on its trip around the planet, and you will see why burning gasoline and coal changes the trip.
Reservoirs and processes
Carbon is in everything alive, in the air, in the oceans, and in rocks. It never disappears. It just moves from place to place and changes form. This is the law of conservation of matter at planet scale.
A place where carbon is stored is called a reservoir (or sink when it takes in more carbon than it gives off). A process is what moves carbon from one reservoir to another.
Boxes are reservoirs. Arrows are processes. The main processes are:
| Process | What happens | Carbon moves from… to… |
|---|---|---|
| Photosynthesis | Plants use sunlight to turn $CO_2$ and water into sugar | Atmosphere → plants |
| Cellular respiration | Organisms break down sugar for energy and release $CO_2$ | Living things → atmosphere |
| Decomposition | Bacteria and fungi break down dead organisms | Dead matter → soil and atmosphere |
| Combustion | Burning wood or fossil fuels | Plants or fossil fuels → atmosphere |
| Ocean exchange | $CO_2$ dissolves in surface water and is released again | Atmosphere ⇄ ocean |
| Fossil fuel formation | Buried dead organisms are squeezed and heated for millions of years | Dead matter → fossil fuels |
Photosynthesis connects to chemistry: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$. This equation is balanced: each side has 6 C, 12 H, and 18 O atoms, so mass is conserved. The carbon atoms from $CO_2$ end up in the sugar. Respiration is the reverse trip.
Plants take carbon dioxide from the air and use it to make sugars. Name this process, and name the process that organisms use to release carbon dioxide back into the air…
Plants take carbon dioxide from the air and use it to make sugars. Name this process, and name the process that organisms use to release carbon dioxide back into the air when they break down sugars for energy.
Name two greenhouse gases that contain carbon or are involved in the carbon cycle. Then state one natural process and one human activity that release carbon dioxide into…
Name two greenhouse gases that contain carbon or are involved in the carbon cycle. Then state one natural process and one human activity that release carbon dioxide into the atmosphere.
Fossil fuels: carbon in long-term storage
Coal, oil, and natural gas are fossil fuels. They formed from the remains of plants and tiny sea organisms that died hundreds of millions of years ago, were buried, and were squeezed and heated underground. The carbon from their bodies got locked away.
When a power plant burns coal, or a car burns gasoline, combustion releases that carbon into the air as $CO_2$. For example, burning natural gas: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.
Compare the speeds. A wetland can store dead plant material for thousands of years. A fossil fuel took millions of years to form, but a power plant can burn it in seconds. That is the key: we move carbon out of long-term storage much faster than nature puts it back.
Coal, oil, and natural gas are called fossil fuels. Explain in one or two sentences what they are made from and what happens to the stored carbon when they are burned.
Coal, oil, and natural gas are called fossil fuels. Explain in one or two sentences what they are made from and what happens to the stored carbon when they are burned.
Aaliyah draws a diagram of the carbon cycle with these parts: atmosphere, ocean, forest, and fossil fuels. Describe the path of a carbon atom that starts as carbon…
Aaliyah draws a diagram of the carbon cycle with these parts: atmosphere, ocean, forest, and fossil fuels. Describe the path of a carbon atom that starts as carbon dioxide in the air, is taken in by a tree, and later is released again. Name each process and each reservoir it passes through.
Following a carbon atom
To describe a path, name each reservoir and each process in order.
A volcano erupts and releases carbon dioxide. At the same time, a large forest in the western United States is cut down and not replanted. Explain how each event affects…
A volcano erupts and releases carbon dioxide. At the same time, a large forest in the western United States is cut down and not replanted. Explain how each event affects the amount of carbon dioxide in the atmosphere, and classify each event as a natural or human-caused change in the carbon cycle.
Balancing the budget
Scientists measure how much carbon moves each year, in billions of metric tons of carbon. A reservoir stays stable when what goes in equals what comes out. If more goes in than out, it grows.
A table shows carbon moving each year between reservoirs, in billions of metric tons of carbon. | Process | Carbon moved per year | |---|---| | Absorbed by land plants…
A table shows carbon moving each year between reservoirs, in billions of metric tons of carbon.
| Process | Carbon moved per year |
|---|---|
| Absorbed by land plants (photosynthesis) | 120 |
| Released by plants and soil (respiration and decay) | 119 |
| Absorbed by the ocean | 92 |
| Released by the ocean | 90 |
| Released by burning fossil fuels | 10 |
- a) Find the total carbon added to the atmosphere and the total removed in one year, using only the rows in the table.
- b) Does the atmosphere gain or lose carbon? By how much?
- c) Explain how this result relates to the greenhouse effect.
Natural and human changes
Many events change the carbon cycle. Sort them by cause.
| Event | Effect on atmospheric $CO_2$ | Cause |
|---|---|---|
| Volcanic eruption | Adds $CO_2$ | Natural |
| Wildfire started by lightning | Adds $CO_2$ | Natural |
| Burning gasoline and coal | Adds $CO_2$ | Human |
| Cutting a forest and not replanting | Adds $CO_2$ (trees burn or rot) and removes a sink | Human |
| Planting trees | Removes $CO_2$ | Human |
| Wind and solar power instead of gas or coal | Prevents $CO_2$ from being added | Human |
Deforestation does double harm: the cut trees release their stored carbon, and fewer trees remain to pull $CO_2$ out of the air.
Seasonal pattern. Measurements at stations like Mauna Loa in Hawaii show $CO_2$ dipping from April to September and rising from October to March. In the Northern Hemisphere, spring and summer bring leaves, so photosynthesis outpaces respiration and $CO_2$ falls. In fall and winter, plants lose leaves and decomposition continues, so $CO_2$ rises. Natural events can also change climate: a large volcanic eruption adds $CO_2$ and ash, and abrupt changes in ocean currents change how much $CO_2$ the ocean absorbs or releases. Over many years, though, the peaks keep climbing. This long-term rise means sources (mostly fossil fuel burning and deforestation) are bigger than the sinks.
Jayden measures the atmospheric carbon dioxide at a research station over one year. He finds that the concentration falls from April to September and rises from October…
Jayden measures the atmospheric carbon dioxide at a research station over one year. He finds that the concentration falls from April to September and rises from October to March.
- a) Use the carbon cycle to explain this seasonal pattern in the Northern Hemisphere.
- b) Over many years, the yearly peaks keep getting higher. Explain what this long-term trend suggests about the balance of carbon sources and sinks.
- c) Name two actions that could slow the increase, and explain how each affects the carbon cycle.
Compare two scenarios for the same region. In Scenario 1, a wetland stores dead plant material under water for thousands of years. In Scenario 2, a power plant burns…
Compare two scenarios for the same region. In Scenario 1, a wetland stores dead plant material under water for thousands of years. In Scenario 2, a power plant burns coal that formed from ancient plants.
- a) Describe the carbon pathway in each scenario, including whether carbon is stored or released.
- b) Compare how fast carbon moves in each case.
- c) Explain why burning fossil fuels can increase the greenhouse effect even though the carbon came from natural sources.
Back to the start: the carbon in your exhaled breath came from food, which got it from the air. Nature recycles that carbon in balance. Burning fossil fuels adds extra carbon from long-term storage, which is why the amount in the air is rising.
Summary
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Carbon is stored in reservoirs (atmosphere, ocean, living things, soil, fossil fuels) and moves between them through processes.
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Photosynthesis removes $CO_2$ from the air. Respiration, decomposition, and combustion return it.
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Fossil fuels are carbon from ancient organisms. Burning them releases carbon that was stored for millions of years.
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If carbon added to the atmosphere exceeds carbon removed, $CO_2$ rises and the greenhouse effect strengthens.
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Natural events (volcanoes, wildfires) and human activities (burning fossil fuels, deforestation) both change the cycle, but humans now add carbon faster than sinks can absorb it.