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Climate science

The carbon cycle

Published on
September 8, 2021

The carbon cycle is an important natural process which distributes carbon between soils, oceans, the atmosphere and living organisms over extremely long periods of time.

Photosynthesis captures carbon in carbon dioxide as plants use it to create energy for growth. This energy and carbon moves through the food chain into all living organisms.

The decomposition of organic matter like plants and animals releases carbon back into the atmosphere.

There is another alternative as to how plants and animals can decay. A combination of the heat from the earth’s crust and anaerobic decomposition turn these plants and animals into fossil fuels over millions of years.

This links to what is called the fast carbon cycle where the extraction of coal, oil and gas, made from decomposing plants releases carbon instantaneously which creates an imbalance of CO2 in the atmosphere

"when there is an excessive amount of cars on the road, or factories are burning coal 24/7, the carbon cycle is thrown off balance"


In essence, the carbon cycle is the process where carbon atoms travel from the earth and into the atmosphere and back. Carbon can be released through natural activities like volcanic eruptions, or when something decomposes.[1] Oceans release carbon as well. These types of carbon releases are not harmful, in of themselves, for the environment as they go through a cycle that allows the carbon to be used in different ways by different elements of the earth and can maintain a healthy pattern.[2] But, carbon is also released from manmade activities, mostly from the burning of fossil fuels through driving cars or industrial energy-use.[3]

Needless to say, these activities harm the environment. This is because fossil fuels release carbon at a much higher rate than natural carbon sinks can absorb. Thus, when there is an excessive amount of cars on the road, or factories are burning coal 24/7, the carbon cycle is thrown off balance.[4] To broaden our understanding, we will be going through the steps in the carbon cycle process and where exactly our man-made emissions come into play.

Step 1: Photosynthesis.

Photosynthesis is the process by which plants capture light energy from the sun and combine it with water and CO2 to produce energy and as a result, oxygen is produced as a byproduct.[5] This is why it is so important to preserve and develop our existing fauna - so that we can continue to combat the overflowing carbon dioxide in the atmosphere.[6]

Step 2: Decomposition.

While plants use a combination of sunlight, carbon dioxide and water to grow, their life cycle ends when they break down and decay. This process is called decomposition. Carbon dioxide is naturally released in this step as it involves bacteria breaking down organic matter such as glucose into carbon dioxide and methane.[7] This affects the afforestation and reforestation projects you back with carbon credits. Eventually, trees die and do in fact release carbon back into the atmosphere as they decay (or burn). Fortunately, not all carbon absorbed by the trees is subsequently released and the process takes a long time.

Step 3: Passing of carbon down the food chain / cellular respiration.

Now that the plants have gotten their fill of light energy and have turned this into carbon compounds, these compounds are passed through the food chain as animals eat them. The process continues up the food chain. Much carbon however, bonds with oxygen and is exhaled as carbon dioxide through the process of respiration.[8]

Step 4: Entry of Carbon into the atmosphere.

Plants & animals, after they have lived their full life-span, die and begin to decay. When they decay, they release carbon into the atmosphere. Sometimes, the breakdown of this matter is prevented.

Step 5: Compression of organic matter to become fossil fuels.

Instead, the pressure and heat from the earth’s crust, over millions of years, combined with anaerobic decomposition turn these plants and animals into fossil fuels, within rock formations, like oil and coal.[9]

Step 6: The fast carbon cycle.

We’ve covered the basics of the biological carbon cycle here. However, the extraction of fossil fuels like coal, oil and gas, which are made from the decomposition of plants, aquatic phytoplankton and zooplankton for combustion releases carbon instantaneously rather than over 10s or 100s of years. This creates the imbalance driving the increasing concentration of carbon dioxide in the atmosphere today to dangerous levels.[10]

Carbon D
Monthly average carbon dioxide in atmosphere.

The bright red line (source data) shows monthly average carbon dioxide at NOAA's Mauna Loa Observatory on Hawai'i in parts per million (ppm): the number of carbon dioxide molecules per million molecules of dry air.[11]

Sources for reference.

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