The Carbon Cycle

We don’t always think about how elements impact our daily lives. But without carbon, we wouldn’t even have life.

In fact, carbon is the basis for all life on Earth, a crucial part of our bodies and all plants. We eat and breathe carbon, and use it in many other ways too. Carbon is also a major component of Earth’s atmosphere, oceans, and rocksl.

Carbon atoms connect these systems, moving through our world in a continuous cycle called the carbon cycle. As it moves, carbon takes on many forms and bonds with other elements in many ways.

Let’s follow the carbon cycle. Where and how is carbon stored and released? How does it move from one carbon sink, or reservoir, into another?

Start with the atmosphere

Thank goodness for carbon in the atmosphere! There, carbon bonds with oxygen molecules to form carbon dioxide, or CO2. Carbon dioxide traps some of the sun’s heat, warming Earth and making it livable. Without carbon dioxide, all the water on Earth would be ice.

Carbon dioxide gets into the atmosphere from a variety of sources including volcanic eruptions, exchange from the oceans, and burning of carbon locked up in rocks and soil.

Importantly, carbon also gets into the atmosphere from living things. Animals release carbon dioxide as they breathe. Plants release carbon dioxide as they decay.

Carbon dioxide is also be released into the atmosphere when living things are burned, such as forest fire. Humans have increased the amount of carbon dioxide by burning hydrocarbons which have been locked up underground as natural gas, oil and coal for millions of years. That increase in atmospheric carbon dioxide is shifting the balance of carbon dioxide, in the atmosphere, causing more of it to build up and warm Earth more.

How does carbon get from the atmosphere back to Earth? Largely through precipitation, which cycles carbon into oceans, soils and Earth’s biosphere.

Carbon Farm

On to the biosphere

The biosphere includes the parts of Earth that are occupied by living organisms. Carbon is the building block of life, including us, DNA, our food, and nearly everything we use and create.

As it cycles through the biosphere, carbon shape shifts, bonding easily with many other molecules. Plants take carbon out of the air and water, and use it for photosynthesis. Animals get carbon by eating plants or other animals. As long as they are alive, plants and animals store carbon. Plant or animals can also store carbon even after the organism has died.

Carbon in the oceans

The ocean is a huge part of the carbon cycle. In general, oceans store more carbon than they release. The oceans absorb carbon from the atmosphere as well as from rivers and erosion of rocks.

In the oceans, some carbon dioxide remains as a gas, dissolved in the water. Some is used by ocean plants for photosynthesis and absorbed into the bodies of marine animals. Too much carbon dioxide in the ocean makes the oceans too acidic. That can cause coral to die off and dissolve the shells of marine animals.

Oceans release some carbon back into the atmosphere. But much of the carbon in the ocean ends up stored in rocks. Those rocks form from organisms that have died and settled to the ocean floor.

Fossil Fuel

Moving into rocks

The thick layers of sedimentary rocks that have formed in Earth’s oceans and land hold more carbon than any other carbon sink on the planet. These carbon-rich rocks include limestone, which is made of calcium carbonate from carbon dissolved in sea water and the shells of marine organisms. Shale and coal are also formed from the carbon-rich bodies of animals and decayed plants.

After millions of years, carbon stored in sedimentary layers can become natural gas, coal, or oil. That’s why the oil used to make fuel such as gasoline is called a fossil fuel – because it’s literally made of fossilized creatures.

Soils are also part of the carbon cycle—just think of a forest floor, with its layers of decayed plants. In Arctic regions, soils are frozen into deep layers of permafrost. If permafrost melts, it can release large amounts of carbon.


Balancing Act

Through the carbon cycle, Earth’s atmosphere, biosphere, oceans, and geology are all connected. Nature has a remarkable ability to balance the amount of carbon in each of these parts of the cycle.

But like any cycle, a change in one part affects the whole cycle. By burning fossil fuels, humans have impacted the amount of carbon in the atmosphere. That change, in turn, causes ripple effects within the carbon that moves through Earth’s oceans and biosphere.

Written by Laura McCamy

Edited by Beth Geiger, MS Geology