The Global Carbon Cycle

carbon source versus sink

What is the world’s largest natural carbon sink? Wetlands. But what is the world’s largest carbon stock? The ocean. My adoration of wetland ecosystems is not hindered by the fact that oceans are a larger carbon stock, though. Wetlands still win at sequestering the largest amount of carbon per year in peat. We can detect this because of something called a “carbon sink”. Carbon sinks have net negative carbon emissions. On the opposite end, carbon sources have positive emissions.

carbon source versus sink

Gases are leaving and entering systems like wetlands and forests all the time, but for a water body as big as the ocean you aren’t going to see massive changes in its carbon content (at least not in a short time period). Wetlands have the ability to form peat over thousands of years. Plants can perform photosynthesis (converting CO2 + H20 -> sugar + O2) on even shorter timescales- as little as a second. For example, this 1985 UK study measured canopy photosynthesis of Scots pines in eastern England to be 1.2 milligrams of CO2 per square meter per second!

carbon cycle schematic

Although leaves form fast and frequently, microbes that feed on decaying leaves will release CO2 back into the atmosphere over time. Peatlands can also release thousands of years of decayed organic plant matter in the form of CO2 or other gases when wildfires burn underground for days or even weeks. It is the longevity and abundance of plants that maintains a carbon sink. Just think of the Douglas fir, which can live to be twelve HUNDRED years old. Or the American Beech, which can live to be 400 (if someone doesn’t mow it down). How many hundreds of leaves would it live to have? How many seedlings would take root nearby and grow up to have hundreds of leaves too? Think about the thickness of the trunk and the height of the tree. Visualize the depth of the ugly bog that holds 12,000 years of organic matter, protected by its own anaerobic conditions that limit decomposition.



Respiration from humans and other animals, volcanoes…these are other natural sources of carbon dioxide. Finally, the far right of the carbon cycle diagram shows a not-so-natural part of the global carbon cycle.

low angle photo of maple leaves
Photo by Skitterphoto on

These diagrams can get quite complex when you add in all the different fluxes and stocks that exist. Keeping everything in terms of carbon rather than CO2 can simplify things. It means less mass conversions and 1 less periodic element to deal with. However, to convert units into CO2 all you would need to do is use some simple stoichiometry. Click on that link if you want a lesson in molar conversions by Khan Academy- otherwise, press on.

If you were to add up all the global carbon sources, sinks, and stocks you would find that Earth naturally rids of 4 Gigatons of carbon each year. The current imbalance is because (1) anthropogenic emissions are overloading the system and (2) natural sinks like forests and wetlands have been & are being destroyed.

animal lake outdoors raccoon
Photo by Photo Collections on

According to the National Wetland Condition Assessment performed by the EPA and released in 2011, wetlands can look healthy but actually not be functioning at full capacity. Scientists surveyed a variety of wetlands throughout the coterminous US. 48% percent of wetland area was found to be in good condition, 20% in fair condition, and 32% in poor condition. Wetland condition was determined by vegetation, soil, hydrology, water chemistry, algae, and buffer characteristics. The largest hindrances to wetland health were:

  1. Surface hardening from roads
  2. Vegetation removal/loss due to grazing, mowing, forest clearing (for timber or construction, I assume)
  3. Ditches being built through the wetland – this will alter the flow
  4. Invasive plant species

Ensuring that the wetlands we have are healthy will enable them to continue being a strong carbon sink in the global carbon cycle. Let people know why swamps are GOOD to have around. And let ME know if you have any other fun facts about the global carbon cycle in the comments!

First graphic was made by yours truly. Have a great night 🙂

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