Learn how carbon moves through Earth's ecosystems and how human activities are altering the carbon cycle.

Key points

  • Carbon is an essential element in the bodies of living organisms. It is also economically important to modern humans, in the form of fossil fuels.
  • Carbon dioxide—CO2\text {CO}_2—from the atmosphere is taken up by photosynthetic organisms and used to make organic molecules, which travel through food chains. In the end, the carbon atoms are released as CO2\text {CO}_2 in respiration.
  • Slow geological processes, including the formation of sedimentary rock and fossil fuels, contribute to the carbon cycle over long timescales.
  • Some human activities, such as burning of fossil fuels and deforestation, increase atmospheric CO2\text{CO}_2 and affect Earth's climate and oceans.

Carbon: building block and fuel source

About 18% of your body consists of carbon atoms, by mass, and those carbon atoms are pretty key to your existence!1^1 Without carbon, you wouldn't have the plasma membranes of your cells, the sugar molecules you use for fuel, or even the DNA\text{DNA} that carries instructions to build and run your body.
Carbon is part of our bodies, but it's also part of our modern-day industries. Carbon compounds from long-ago plants and algae make up the fossil fuels, such as coal and natural gas, that we use today as energy sources. When these fossil fuels are burned, carbon dioxide—CO2\text{CO}_2—is released into the air, leading to higher and higher levels of atmospheric CO2\text{CO}_2. This increase in CO2\text{CO}_2 levels affects Earth's climate and is a major environmental concern worldwide.
Let's take a look at the carbon cycle and see how atmospheric CO2\text{CO}_2 and carbon use by living organisms fit into the bigger picture of carbon cycling.

The carbon cycle

The carbon cycle is most easily studied as two interconnected subcycles:
  • One dealing with rapid carbon exchange among living organisms
  • One dealing with long-term cycling of carbon through geologic processes
Although we will look at them separately, it's important to realize these cycles are linked. For instance, the same pools of atmospheric and oceanic CO2\text{CO}_2 that are utilized by organisms are also fed and depleted by geological processes.
As a brief overview, carbon exists in the air largely as carbon dioxide—CO2\text{CO}_2—gas, which dissolves in water and reacts with water molecules to produce bicarbonate—HCO3\text{HCO}_3^-. Photosynthesis by land plants, bacteria, and algae converts carbon dioxide or bicarbonate into organic molecules. Organic molecules made by photosynthesizers are passed through food chains, and cellular respiration converts the organic carbon back into carbon dioxide gas.
Image credit: Biogeochemical cycles: Figure 3 by OpenStax College, Biology, CC BY 4.0; modification of work by John M. Evans and Howard Perlman, USGS
Longterm storage of organic carbon occurs when matter from living organisms is buried deep underground or sinks to the bottom of the ocean and forms sedimentary rock. Volcanic activity and, more recently, human burning of fossil fuels bring this stored carbon back into the carbon cycle. Although the formation of fossil fuels happens on a slow, geologic timescale, human release of the carbon they contain—as CO2\text{CO}_2—is on a very fast timescale.

The biological carbon cycle

Carbon enters all food webs, both terrestrial and aquatic, through autotrophs, or self-feeders. Almost all of these autotrophs are photosynthesizers, such as plants or algae.
Autotrophs capture carbon dioxide from the air or bicarbonate ions from the water and use them to make organic compounds such as glucose. Heterotrophs, or other-feeders, such as humans, consume the organic molecules, and the organic carbon is passed through food chains and webs.
How does carbon cycle back to the atmosphere or ocean? To release the energy stored in carbon-containing molecules, such as sugars, autotrophs and heterotrophs break these molecules down in a process called cellular respiration. In this process, the carbons of the molecule are released as carbon dioxide. Decomposers also release organic compounds and carbon dioxide when they break down dead organisms and waste products.
Carbon can cycle quickly through this biological pathway, especially in aquatic ecosystems. Overall, an estimated 1,000 to 100,000 million metric tons of carbon move through the biological pathway each year. For context, a metric ton is about the weight of an elephant or a small car!2,3,4^{2,3,4}

The geological carbon cycle

The geological pathway of the carbon cycle takes much longer than the biological pathway described above. In fact, it usually takes millions of years for carbon to cycle through the geological pathway. Carbon may be stored for long periods of time in the atmosphere, bodies of liquid water—mostly oceans— ocean sediment, soil, rocks, fossil fuels, and Earth’s interior.
The level of carbon dioxide in the atmosphere is influenced by the reservoir of carbon in the oceans and vice versa. Carbon dioxide from the atmosphere dissolves in water and reacts with water molecules in the following reactions:
CO2+H2OH2CO3HCO3+H+CO32+2H+\text{CO}_2 + \text H_2\text O \:\:\rightleftharpoons \:\: \text H_2\text{CO}_3 \:\:\rightleftharpoons \:\:\text{HCO}_3^- +\text H^+ \:\: \rightleftharpoons \:\:\text{CO}_3^{2-} + 2 \text H^+
The carbonate—CO32\text{CO}_3^{2-}—released in this process combines with Ca2+\text {Ca}^{2+} ions to make calcium carbonate—CaCO3\text{CaCO}_3—a key component of the shells of marine organisms.5^5 When the organisms die, their remains may sink and eventually become part of the sediment on the ocean floor. Over geologic time, the sediment turns into limestone, which is the largest carbon reservoir on Earth.
On land, carbon is stored in soil as organic carbon from the decomposition of living organisms or as inorganic carbon from weathering of terrestrial rock and minerals. Deeper under the ground are fossil fuels such as oil, coal, and natural gas, which are the remains of plants decomposed under anaerobic—oxygen-free—conditions. Fossil fuels take millions of years to form. When humans burn them, carbon is released into the atmosphere as carbon dioxide.
Another way for carbon to enter the atmosphere is by the eruption of volcanoes. Carbon-containing sediments in the ocean floor are taken deep within the Earth in a process called subduction, in which one tectonic plate moves under another. This process forms carbon dioxide, which can be released into the atmosphere by volcanic eruptions or hydrothermal vents.

Human impacts on the carbon cycle

Global demand for Earth’s limited fossil fuel reserves has risen since the beginning of the Industrial Revolution. Fossil fuels are considered a nonrenewable resource because they are being used up much faster than they can be produced by geological processes.
When fossil fuels are burned, carbon dioxide—CO2\text{CO}_2—is released into the air. Increasing use of fossil fuels has led to elevated levels of atmospheric CO2\text{CO}_2. Deforestation—the cutting-down of forests—is also a major contributor to increasing CO2\text{CO}_2 levels. Trees and other parts of a forest ecosystem sequester carbon, and much of the carbon is released as CO2\text{CO}_2 if the forest is cleared.6^6
Some of the extra CO2\text{CO}_2 produced by human activities is taken up by plants or absorbed by the ocean, but these processes don't fully counteract the increase. So, atmospheric CO2\text{CO}_2 levels have risen and continue to rise. CO2\text{CO}_2 levels naturally rise and fall in cycles over long periods of time, but they are higher now than they have been in the past 400,000 years, as shown in the graph below:
Image credit: "Threats to biodiversity: Figure 1" by OpenStax College, Biology, CC BY 4.0
That is a great question, one whose answer we still don't fully know.
The changes in CO2\text{CO}_2 levels move in step with changes in Earth's temperature. Both CO2\text{CO}_2 levels and temperature track with cyclical variations in Earth's orbit, known as Milankovitch cycles, but they can't be fully explained by these cycles alone. Scientists suspect that the periodic spikes in CO2\text{CO}_2 levels might be related to changes in carbon cycling in the ocean during the cold/low CO2\text{CO}_2 periods, but this is still an active area of investigation.7^{7}
For our purposes here, the key point is that the CO2\text{CO}_2 levels shown on the graph are higher in the present day than they have been for over 400,000 years, regardless of the cycles.
Why does it matter that there is lots of CO2\text{CO}_2 in the atmosphere? CO2\text{CO}_2 is a greenhouse gas. When in the atmosphere, it traps heat and keeps it from radiating into space. Based on extensive evidence, scientists think that elevated levels of CO2\text{CO}_2 and other greenhouse gases are causing pronounced changes in Earth's climate. Without decisive changes to reduce emissions, Earth's temperature is projected to increase by 1 to 5^{\circ}C by the year 2100.8^8
Also, while uptake of excess carbon dioxide by the oceans might seem good from a greenhouse gas perspective, it may not be good at all from the perspective of sea life. As we saw above, CO2\text{CO}_2 dissolved in seawater can react with water molecules to release H+\text H^+ ions. So, dissolving more CO2\text{CO}_2 in water causes the water to become more acidic. More acidic water can, in turn, reduce CO32\text{CO}_3^{2-} concentrations and make it harder for marine organisms to build and maintain their shells of CaCO3\text{CaCO}_3.9^{9} Both increasing temperatures and higher acidity can harm sea life and have been linked to coral bleaching.
A bleached coral appears in the front, while a healthy, unbleached, brown coral appears in the background. Image credit: Keppelbleaching by Acropora, CC BY 3.0
The debate about the future effects of increasing atmospheric carbon on climate change focuses on fossils fuels. However, scientists must take natural processes, such as volcanoes, plant growth, soil carbon levels, and respiration, into account as they model and predict the future impact of this increase.


This article is a modified derivative of the following articles:
The modified article is licensed under a CC BY-NC-SA 4.0 license.

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