Which is the Largest Carbon Reservoir?

Which carbon reservoir is the biggest? To answer this, we explain how much carbon is stored in the Earth’s crust, the deep ocean, the atmosphere, and in plants and soils? We also examine how human activities have affected carbon sinks, and explain the main features of the fast and slow carbon cycles.
Deep Mangrove Tree Roots
Mangroves carbon reservoir. Photo: Matt Curnock /Coral Reef Image Bank

In this article, we answer the question: how much accumulated carbon is stored in each of the major carbon reservoirs? These carbon pools include: the oceans, plants and soil, the atmosphere, the Earth’s crust and mantle. We also look at how human activities have affected carbon sinks, and examine the latest estimates of volcanic outgassing.

The Importance of Carbon

Without carbon, life as we know it could not exist. It’s the basic building block of life and all living organisms contain carbon, because it’s able to create multiple, stable bonds with other molecules, providing the structure necessary for compounds like nucleotides, amino acids, sugars, lipids and Deoxyribonucleic acid (DNA).

As a result, Planet Earth has a system in place – known as the Carbon Cycle – to maintain adequate supplies of this important chemical. Like several other biogeochemical cycles, the carbon cycle has two different pathways: a fast one and a slow one.

The Phosphorus Cycle: The Slow Pathway
The Nitrogen Cycle: How Does It Work?
What is the Oxygen Cycle?

Carbon Cycles: Fast, Slow, Medium

In the fast cycle, carbon moves from the atmosphere to the biosphere via photosynthesis – either by plants or phytoplankton – and returns via respiration or decomposition. The fast carbon cycle takes place over days, weeks, months, or years and primarily concerns the supply of CO2 to living organisms.

In the slow cycle, carbon becomes buried in the ocean floor or the soil, before being lithified and absorbed into the Earth’s crust. It can remain locked up in the rock for tens of millions of years, or more, before being outgassed via volcanic or fumarolic activity. Types of slow-cycle carbon that have ended up in the lithosphere, include limestone sedimentary rock, or fossil fuels like coal, or natural gas or petroleum.

There is also a medium-speed carbon cycle, which lasts around two thousand years, in which atmospheric carbon is absorbed into the ocean, where it is downwelled into the depths by thermohaline circulation currents, before eventually returning to the surface and passing into the atmosphere.

Carbon Reservoirs

Here are the four basic carbon reservoirs or storage systems that have the most relevance in the overall carbon cycle. They are connected to one another by numerous pathways of exchange – including photosynthesis and respiration. (Note: The movement and amount of carbon passing along these pathways between reservoirs is called a flux.)

In both cycles, carbon is sometimes stored for varying periods of time. These storage areas (e.g. the soil) are commonly known as carbon reservoirs. Reservoirs that absorb CO2 are called carbon sinks; those that release CO2 are called carbon sources. The main reservoirs of carbon are found in the atmosphere, plants and trees, the soil, the ocean and the sub-surface rocks of the Earth’s crust.

For much of Earth’s history, the flow of carbon entering the Earth’s crust has been roughly balanced by the amount leaving, via volcanic outgassing. Scientists have identified only about four occasions over the past 500 million years, when CO2 levels went crazy.

One of these occasions was the Chicxulub asteroid strike about 66 million years ago, which is believed to have wiped out the dinosaurs. The impact of the asteroid vaporized carbon-rich rock, sending hundreds of billions of tons of carbon dioxide into the atmosphere. The initial effect (from the sky-darkening aerosols) was a cooling one, but after a few years the Earth experienced a sharp and prolonged period of warming.

Aside from these four anomalies, CO2 levels over the past 500 million years have been broadly stable. That is, until the Industrial Revolution in the late eighteenth century, when the sudden increase in the burning of coal for industrial power initiated global warming and our current climate crisis.

The Industrial Revolution arguably signals the start of the Anthropocene epoch, as well as the shift of carbon from the lithospheric reservoir to the atmosphere, and thereafter (mostly) to the oceanic carbon reservoir.

How Much Carbon does Earth Contain?

According to recent figures from scientists at the Deep Carbon Observatory, as well as other experts 1 the best current estimate of total global carbon is: 1.845 billion gigatonnes (Gt). Please note: a single Gt (gigatonne) is the equivalent of 1 billion metric tonnes. So we can also express the total as: 1.845 billion billion metric tonnes.

How Much Carbon is Contained in Earth’s Main Reservoirs?

The main carbon reservoirs are categorized as being either below or above the Earth’s surface. Note that the total estimated amount of carbon which exists above Earth’s surface represents “2/1000ths of 1 percent” of Earth’s total carbon.

Carbon in Lower Mantle1.5 billion Gt
Carbon in Land/Sea Lithospheres0.315 billion Gt
Carbon in Upper Mantle0.03 billion Gt
Total Carbon Below Surface (99%)1.845 billion Gt
Deep Ocean (85.1%)37,000 Gt
Surface Ocean (2%)900 Gt
Marine Sediments (6.9%)3,000 Gt
Terrestrial Biosphere (4.6%)2,000 Gt
Atmosphere (1.4%)590 Gt
Total Carbon Above Surface (1%)43,500 Gt
Source: The Deep Carbon Observatory (DCO) Oct 1, 2019 2

NOTES

(1) “Marine Sediments” refers to all carbon in ocean sediments that is in the process of being compacted into rock (lithified). “Terrestrial biosphere” refers to all carbon in the soil, plants and trees. Fossil fuels would appear to be included under Land/Sea lithospheres.

(2) It is unclear which category permafrost carbon appears under. Typically estimated at about 1,400 Gt, one can only assume it, like fossil fuels, is included under Land/Sea lithospheres.

(3) The figure for atmospheric carbon appears to be at odds with previous estimates. The NOAA states that the amount of carbon in the oceans (around 37-38,000 Gt) is about 50 times larger than the amount in the atmosphere – in other words, 750 Gt. 3 4

Carbon Reservoir in the Earth’s Crust

The largest amount of carbon is stored in sedimentary rock formations within the lithosphere, the Earth’s crust. These sedimentary rocks are created either by the concretization of organic matter and mud into shale, over geological time, or by the formation of calcium carbonate particles (derived from the shells and skeletons of marine organisms) into limestone and other carbon-rich sedimentary rocks.

Sedimentary forms of carbon include a variety of hydrocarbon deposits which were compressed and heated over millions of years. Known as fossil fuels, these deposits derive from the remains of trees and vegetation (coal) that were aborbed into the soil, or from organic matter in the sediments of coastal marine basins and inland seas (petroleum and natural gas).

The Earth’s crust acts as a long-term carbon reservoir. When oceanic carbon dioxide is absorbed by marine organisms who die and form sediments on the sea bed, it takes millions of years for the carbon to become sedimentary rock, and millions more years for geological forces to melt it and expel it into the atmosphere through volcanic activity. In all, carbon takes between 100-200 million years to move between atmosphere, ocean, rocks and atmosphere in the slow carbon cycle.

Carbon Outgassing From the Lithosphere Reservoir

Carbon dioxide eventually exits the slow carbon cycle by being outgassed through volcanic eruption, fumarolic activity, volcanic vents or oceanic CO2 vents.

Earth’s total annual out-gassing of CO2 via volcanoes and through other geological processes has been recently assessed by scientists at the Deep Carbon Observatory (DCO) at roughly 300-400 million metric tonnes (0.3 to 0.4 Gt). Of this, roughly 280–360 million tonnes (0.28 to 0.36 Gt) of CO2 per year can be attributed to volcanoes. This encompasses CO2 outgassing from active volcanic vents, from the diffuse release of CO2 through soils, fissures and fractures, as well as volcanic lakes.

What’s more, DCO scientists estimate that about 400 of the 1500 volcanoes active over the past 11,000 years, are venting CO2 today. Another 770 may be producing diffuse emissions, notably in Yellowstone, USA, the East African Rift Valley, and the Technong volcanic province in China.

To put this into context, annual anthropogenic emissions of CO2 caused by the burning of fossil fuels and forests, are approximately 40 to 100 times greater than all volcanic emissions.

Himalayas

This long, slow process whereby atmospheric CO2 is removed from the air and sequestered in rock for huge periods of time, is the ultimate example of how Earth’s natural balance used to be maintained before humans started messing with it.

For instance, if levels of carbon dioxide in the atmosphere rose because of an increase in volcanic activity, then temperatures rose too, leading to more rainfall, which dissolved more rock, reabsorbing more CO2 that will eventually re-emerge through volcanic activity. A perfect carbon circle.

Some 60 million years ago, a huge tectonic plate (a piece of the Earth’s crust) known as the Indian Plate (or Gondwana), slammed into the massive Eurasian continental plate, roughly at the site of present-day India. This triggered a geological process that ended with a huge amount of rock in the middle of the two opposing plates being squeezed up into the air as both plates pressed against each other. Today, this protruding rock is known as the Himalayan mountain range.

The interesting part, is that geologists and paleoclimatologists now believe that this bare rock face absorbed so much CO2 in the form of chemical weathering, that the Earth’s temperature fell precipitously, terminating the hot spell known as the Cretaceous Thermal Maximum and ushering in a long-term period of climate change – only on that occasion it was global cooling. 5

Carbon Reservoir in the Deep Ocean

Earth’s oceans and seas, contain roughly 37,000 billion tonnes of carbon, the bulk of which is dissolved inorganic (mineral) carbon stored at great depths where it remains typically for thousands of years. The remainder, about 900 billion tons, stays near the surface of the ocean.

Carbon (CO2) near the surface is usually exchanged quite rapidly with the atmosphere through evaporation. Alternatively, it is used by phytoplankton for photosynthesis, or absorbed by crustaceans to make calcium carbonate – essential building material for shells and skeletons.

When these crustaceans die, they sink to the ocean floor adding to the huge mass of marine sediments. Over millions of years, a variety of physical and chemical processes convert these sediments into limestone rocks, such as the white cliffs of Dover.

The ocean is the largest above-surface carbon reservoir, containing over 87 percent of the world’s carbon outside of the Earth’s crust.

Carbon in the Atmosphere

The atmosphere contains approximately 590 billion tons of carbon, most of which is carbon dioxide, with a very small amount of methane. Although this is considerably less carbon than that contained in the Earth’s crust or oceans, the amount of atmospheric carbon is critical because of its influence on the greenhouse effect and global temperatures.

Carbon Reservoir in Plants And Soils

The pedosphere, Earth’s terrestrial soil-based carbon reservoir, includes plants, trees, shrubs, animals, soils , microorganisms and decomposers like fungi and bacteria. Unlike the carbon stored in the Earth’s crust and oceans, most of the carbon in the terrestrial ecosphere is organic – meaning it was produced by living things.

Living plants exchange carbon with the atmosphere quite rapidly through photosynthesis, and also respiration, in which about half of the photosynthesized CO2 is released back to the atmosphere. Decay is another major pathway, with leaves, plant litter, wood and roots, as well as the decomposed remains of small and medium-sized soil animals, all releasing CO2 into the air.

Of the various types of organic tissue produced by plants, the stems and trunks of trees store the most carbon. Altogether, Earth’s vegetation holds roughly 500 billion tons of carbon, with trees holding the most.

By contrast, the planet’s soil is estimated to contain 1,500 billion tons of carbon and this doesn’t include the northern circumpolar permafrost, in which it is believed up to 1,400 billion tons of carbon are stored.

Soil carbon is stored in the remains of plants and the decomposed remains of worms, termites, ants, fly larvae, beetles, centipedes, millipedes, slugs, snails and other organic residues. 6

It’s not clear how “blue carbon” stored in coastal and estuarine ecosystems is classified. Absorbed from the atmosphere by mangrove forests (mangals), seagrasses, and other marine vegetation, this blue carbon is stored underwater in the saline soil. But does it belong to the Earth’s surface (pedosphere) or the sea (hydrosphere)? Coastal ecosystems are renowned for their carbon capture and storage abilities, as they absorb and store CO2 at ten times the rate of mature rainforests.

Have Human Activities Affected Earth’s Carbon Reservoirs?

Yes. Since the Industrial Revolution we have extracted increasing amounts of fossil fuels for use in furnaces, power plants and vehicle engines. As a result, since about 1800, levels of carbon dioxide in the atmosphere have increased by over 45 percent, mostly due to the use of coal and petroleum and – to a lesser extent, natural gas – for industrial and transport purposes. (Wood burning also emits CO2 but wood is not classified as a fossil fuel.) All this has raised global temperatures by roughly 1 degree Celsius and the rate of increase is rising rapidly.

Humans have also interfered with the terrestrial carbon reservoir. As part of our drive for greater global prosperity, large areas of forest have been cut down and cleared, especially in the Amazon Rainforest.

Trouble is, deforestation is a double whammy. Trees photosynthesize large amounts of atmospheric CO2, so the forest acts as a huge carbon sink. But when it is cut down and starts decaying, it stops removing CO2 from the air and starts emitting it. And if the logs are burned, they emit even more CO2. 7

Is the forest replaced with something else? Yes. But it’s usually replaced with crops or pasture which store less carbon. In addition, deforestation exposes more soil which vents CO2 from decayed plant matter into the atmosphere.

At present, inappropriate land use (including deforestation) is responsible for the release of 4.8 billion tons of carbon dioxide into the atmosphere. 8 See also: 7 Effects of Climate Change on Plants and Trees.

The effects of global warming on oceans have also been severe. When CO2 emissions from the burning of fossil fuels enter the troposphere, roughly half remains there, one quarter is absorbed by land plants and trees, and another quarter is absorbed into colder areas of the ocean. 9

In terms of heat, something like 93 percent of all the stored heat of global warming from 1971 to 2010 has been absorbed by the ocean. Of this, about 63 percent has gone into the upper ocean and about 30 percent into the deep ocean. 10

In a nutshell, anthropogenic activities are having significant effects on every above surface carbon reservoir, including the atmosphere, the soil and the oceans.

If you need advice about reducing your personal carbon emissions, please read our article: How to Reduce Your Carbon Footprint.

References

  1. “A Framework for Understanding Whole-Earth Carbon Cycling.” Cin-Ty A. Lee, et al. October 2019. []
  2. “Scientists Quantify Global Volcanic CO2 Venting; Estimate Total Carbon on Earth.” []
  3. Carbon cycle.” NOAA. []
  4. Global Carbon Cycle.” University of North Dakota Energy & Environmental Research Center. []
  5. Neogene cooling driven by land surface reactivity rather than increased weathering fluxes. Nature 571, 99–102 (2019). Caves Rugenstein, J.K., Ibarra, D.E. & von Blanckenburg, F. []
  6. For a new database, named SoilHealthDB, that focuses on four main conservation management methods: cover crops, no-tillage, agro-forestry systems, and organic farming, see: “A database for global soil health assessment.” Jian, J., Du, X. & Stewart, R.D. Sci Data 7, 16 (2020). []
  7. For the impact of man-made land use changes on the contribution of biomes like the taiga to the terrestrial carbon sink, see: “Recent divergence in the contributions of tropical and boreal forests to the terrestrial carbon sink.” []
  8. Deforestation and Climate Change.” Climate Council. Annika Dean. 21 August 2019. []
  9. “Ocean-Atmosphere CO2 Exchange.” NOAA. []
  10. “Climate Change: Ocean Heat Content.” Climate Gov []
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