The carbon cycle

Carbon is a constituent of two important greenhouse gases, CO₂ and CH₄. It is therefore interesting to get an overview of the carbon paths between different reservoirs in nature. This overview does not differentiate between various forms of carbon. All quantities are expressed in petagram carbon. For a CO₂ molecule, it is only the weight of the C atom that counts. This makes sense since carbon comes in different forms; in the atmosphere mainly as CO₂ while living biomass contains an army of complex carbon compounds. By just weighing the C atoms, it becomes easier to set up "budgets" for how the carbon moves in the system. For those who want to compare the amounts of carbon presented here with similar amounts of greenhouse gases, which are often given in petagrams (= gigatonnes) CO₂ equivalents, then the following conversion factors can be used:

C -> CO₂: multiply by 3.664
CO₂ -> C: multiply by 0.273

The CO₂ concentration in the atmosphere is also given as "parts per million" (ppm). This unit indicates the number of CO₂ molecules in a parcel of dry air relative to all molecules in that air. If the atmosphere contains 400 ppm CO₂, this means that of 1 million air molecules (which mostly include N₂ and O₂) 400 molecules are CO₂. The IPCC operates with a conversion factor from ppm to PgC of 2.12; i.e. 400 ppm CO₂ corresponds to 2.12 * 400 = 848 PgC.

The natural cycle of carbon primarily involves two important processes: photosynthesis, where CO₂ is absorbed into plants and contributes to plant growth, and respiration, where CO₂ is released by both plants and animals when oxygen reacts with carbonaceous nutrients to provide energy.

A 2013 IPCC report has a chapter describing the carbon cycle [L78]. A figure on page 471 shows the main features of the carbon cycle:

I will try to summarise the most important points in this figure.

The figure shows the amount of carbon in different reservoirs (atmosphere, vegetation etc.), and the transport (flux) of carbon between the reservoirs. The amount of carbon shown for each reservoir is an average of the period 2000-2009, and is given as PgC, i.e. petagram carbon. One petagram is the same as one gigatonne, i.e. 1,000,000,000,000 tonnes. Transport between the reservoirs represents values that were valid when the report was written, i.e. in 2010 or a little later. The transport values are given in PgC/year.

The figure distinguishes between values that were valid in pre-industrial times (before 1750) and values that are due to human influence. Pre-industrial values are indicated in black, while numbers in red show human influence. The sum of these two values for a reservoir represents the total amount in the reservoir. Correspondingly, the sum of red and black numbers along a transport route is the total quantity transported over a year.

The figure contains a wealth of information, and for my part I find it difficult to get a good overview just by studying the figure thoroughly. I have therefore chosen to present this information a little more piecemeal, and will begin with the situation in pre-industrial times.

The numbers in the figure are intended to describe a "budget" in which the sum of transport to and from the reservoirs should end up as zero. This means that the numbers are given with an unnatural accuracy, and uncertainties are seldom taken into account. However, I have mostly chosen to use these numbers as they appear in the figure.

Wikipedia also has an overview of the carbon cycle [L77].

The carbon cycle in pre-industrial times

The most important carbon reservoirs include the ocean, the atmosphere, the soil and biomass on land. These reservoirs exchange carbon in different ways so that a state of equilibrium between the reservoirs is maintained over time. In addition, there are other reservoirs, such as sediments on the seabed, fossil carbon and carbon stored in permafrost, but in pre-industrial times these exchanged only insignificant amounts of carbon with the other reservoirs.

The ocean has by far the largest content of carbon in the form of inorganic carbon, i.e. CO₂ and compounds formed by CO₂. These are carbonic acid and salts formed by carbonic acid. A total of 38,000 PgC are stored in the ocean in the form of these compounds. The sea, however, is layered and there is a big difference in how the carbon behaves in the surface layer in relation to layers deeper down. The top layer (surface layer) is about 200 metres deep on average. The thickness of this layer varies both geographically and in time depending on seasons and latitude [L79]. What characterises this layer is that wind and waves lead to an upheaval of the water masses over a relatively short time period. Therefore, gases absorbed at the surface, such as CO₂, are mixed downwards throughout the layer. The surface layer is also warmer than the deep water, and since colder water is heavier than warmer water (for temperatures above 4 °C), this helps the deep water stay in the depths. The upheaval that occurs between the surface layer and the deeper layers takes 10-100 times as long as the upheavals within the surface layer.

The surface layer contains 900 PgC while the deep-water layer contains the remaining 37100 PgC. Each year, 90-100 PgC are exchanged between these two layers. Living organisms in the ocean are considered a separate sub-reservoir in the sea that also exchanges carbon with the surface layer and depth (approx. 50 PgC per year). Between the surface layer and the atmosphere, approximately 60 PgC per year are exchanged both ways.

Biomass on the surface of dry land (somewhat misleadingly called "vegetation" in the figure) includes live animals and plants and microorganisms in the soil. This reservoir contains 450-650 PgC. Soil contains 1500-2400 PgC in the form of dead organic material.

The atmosphere in pre-industrial times contained 589 PgC (uncertainty not given in the figure). The most important supply of carbon to the atmosphere derives from respiration from living biomass, as well as forest fires. These sources amounts to 107.2 PgC per year. In addition, the atmosphere receives smaller amounts of carbon from other sources. Net supply from seawater and fresh water totals 1.7 PgC per year. Photosynthesis transports 108.9 PgC per year from the atmosphere to living biomass.

The carbon cycle today

The carbon content in the atmosphere has increased by 230-250 PgC since pre-industrial times, and in 2010 it was approximately 829 PgC. This is due to the combustion of fossil carbon and cement production (approx. 7.8 PgC/year) as well as the net effect of changed use of land (approx. 1.1 PgC/year). Some of the carbon released in this way is absorbed by the ocean (approx. 2.3 PgC/year) and is taken up by the vegetation due to increased growth (approx. 2.6 PgC/year). The sum of emissions minus what is taken up in the sea and the vegetation constitutes an increase to the atmosphere of 4 PgC/year.

A more recent overview of the carbon cycle can be found in an article published under the auspices of The Global Carbon Project [L108]. This article [L107] shows the situation in 2018, and also has projections for 2019. In 2018, fossil emissions of carbon had increased to about 10 PgC/year, and the effect of changed use of land to about 1.5 PgC/year. The total man-made emissions of 11.5 PgC/year were partly absorbed in the sea (2.6 PgC/year) and partly taken up by vegetation on land (3.5 PgC/year). The growth of carbon in the atmosphere for 2018 amounted to 5.1 PgC. The sum of uptakes in the sea, on land and what was left in the atmosphere, was 11.2 PgC. With total emissions of 11.5 PgC, this gives an imbalance of 0.3 PgC (11.5-11.2) that has not been accounted for. All of these estimates are uncertain and based on independent data sources. Then it is no wonder that such an imbalance occurs, and the size of this imbalance is not really very big.

Latest update: 2021-07-10