Follow the Carbon

Note: A gigaton of carbon (GtC) and a petagram of carbon (PgC) are the fundamental units of measurement of carbon at planetary cycling scales. One gigaton is equal to one billion metric tons of carbon (or one petagram, which is 10¹⁵ grams). They are interchangeable, and we will use GtC in this Snack.

1. Before you begin, ask participants if they can think of any of the earth’s carbon reservoirs. The five major carbon reservoirs are: Rock, Atmosphere, Oceans, Terrestrial Biosphere, and Fossil Fuels.
2. Each participant should count 100 grains of rice, and combine 5 of these to make 500 grains total. Find the mass of this amount. (In our sample, 500 grains of rice had a mass of 15 grams.) To create an accurate model of the relative abundance of carbon present in each of the five major carbon reservoirs, let each grain of rice represent 1 gigaton of carbon (GtC). If you have a large group, you will be able to use all those "extra" 100s of grains of rice for the next step.
3. Using the chart below (click to enlarge), calculate how many grams of rice represents the amount of carbon in each of the four reservoirs (listed in bold) except Rock.
   
4. Use a marker to label four of the gallon-sized bags with the name of each of the four major reservoirs except Rock. (Because rock contains a large amount of carbon, it cannot be physically represented by this model, so no bag will be needed for it.) Then, using the scale, measure the amount of rice that corresponds to the amount of carbon in each reservoir and place it into the correct bags. You may want to divide participants into four groups—one for each carbon reservoir—to do this step most efficiently.
5. If something is too large to be represented by a physical model, it can help to find a way to visualize it. To help imagine the amount of rice needed to represent the carbon in the Rock reservoir, determine how many grains will fit inside the 10 cm x 10 cm x 10 cm box. How many of those rice-filled boxes would you need to represent the amount of carbon in the Rock reservoir? How much space in the room would all those boxes take up? (A meter stick might be helpful to make these calculations.)
6. Place the appropriate carbon reservoir image with the bag of rice that represents that reservoir.

These investigations will help you model how carbon flows from one reservoir to another.

Note: Before you begin, notice the relative abundance of carbon in each of the five reservoirs. Rock contains far more carbon than the other four reservoirs combined. Since rock is part of the slow carbon cycle, it is not part of the exchanges you will model in this snack.

Pathway 1: Flow between the Atmosphere and Terrestrial Biosphere

The natural flux between the atmosphere and terrestrial biosphere is about 120 GtC per year in each direction. In the terrestrial biosphere, photosynthesis removes about 120 GtC from the atmosphere each year. Decomposition of biological material and respiration from plants and soil microbes returns 120 GtC to the atmosphere each year.

To model this interaction, remove 120 grains of rice from the Atmosphere bag and place it in a quart-sized bag. Then do the same with the Terrestrial Biosphere bag. Exchange these two equal-sized bags while discussing how the carbon flows from one reservoir to another. Model this yearly exchange several times while reviewing the ways in which carbon cycles from one reservoir to the other.

Pathway 2: Flow between the Ocean and the Atmosphere

Carbon cycles between the ocean and the atmosphere at a rate of 90 GtC per year in each direction. Most of this exchange occurs by diffusion at the surface of the ocean.

To model this interaction, remove 90 grains of rice from the Atmosphere bag and place it in a new quart-sized bag. Then do the same with the Ocean bag. Exchange these two equal-sized bags while discussing how the carbon flows from one reservoir to another. Model this yearly exchange several times, while reviewing the ways in which carbon cycles from one reservoir to the other.

Notice that, until now, the carbon cycle has remained in balance, and no reservoir has a net gain or loss.

Pathway 3: Flow from Fossil Fuels

Human use of fossil fuels (the burning of which releases carbon dioxide into the atmosphere) is changing the balance of carbon, adding an additional 9.4 (±0.5) GtC to the atmosphere each year. Land use changes, such as deforestation, remove part of the carbon sink (materials in the natural environment capable of absorbing excess carbon), thereby “contributing” that addition of 1.5 (±0.7) GtC excess carbon.  Human impacts are therefore contributing almost 11 GtC per year to the atmosphere. 

To model this interaction, count 11 grains of rice from the Fossil Fuels bag.

Not all of this carbon goes into the Atmosphere, as other reservoirs are absorbing some of this added carbon. Each year 4 GtC (represented by 4 grains of rice) from the Fossil Fuels reservoir are absorbed by the Terrestrial Biosphere, and 3 GtC (3 grains of rice) are absorbed by the Ocean reservoir. This results in a net gain in the Atmosphere reservoir of 5 GtC (5 grains of rice) per year with a budget imbalance of 0.5 GtC per year indicating overestimated emmisions and/or underestimated sinks (see the equation below).

This activity models the fast carbon cycle, which involves the carbon reservoirs of the ocean, atmosphere, and biosphere. The fast carbon cycle takes place over months to years. The slow carbon cycle, which is the geochemical part of the carbon cycle, involves the cycling of carbon-containing rocks, takes thousands to millions of years, and is not modeled in this exercise.

The carbon cycling between the ocean, atmosphere, and biosphere was in balance until the Industrial Revolution, when fossil fuels were brought out of the rock (where they were part of the slow carbon cycle) and burned for energy, releasing a huge amount carbon (in the form of CO₂) into the atmosphere, and into the fast carbon cycle. As a result, the carbon cycle is no longer in balance.

Today, carbon from fossil fuels, normally part of the slow carbon cycle, is being added to the atmosphere, and the fast carbon cycle cannot absorb it at the same rate. This has increased the amount of carbon in the atmosphere by about 5 GtC per year (2018). While natural processes can use up this additional carbon, these processes are part of the slow carbon cycle, and take hundreds of thousands to millions of years.

The consequences of the additional atmospheric carbon are many, and include increased atmospheric and oceanic temperatures, sea level rise, and ocean acidification.

Each year, the oceans absorb and release about 90 GtC, largely via diffusion across the air-ocean interface. The physical processes that control the sinking of CO₂ into colder, deeper waters (where CO₂ is more soluble), and the mixing of ocean water at intermediate depths, are known collectively as the “solubility pump,” and are not part of this model. Phytoplankton photosynthesis converts CO₂ into organic carbon that is largely returned to ocean water as CO₂ via microbial respiration and decomposition. The small fraction of organic carbon that is encapsulated by certain plankton into degradation-resistant clumps that sink to the ocean floor is called the “biological pump.” Together, the solubility pump and biological pump control the amount of carbon transported to ocean depths and the exchange of CO₂ between ocean and atmosphere.

The carbon amounts used in this model are based on solid carbon, which is not the form that carbon takes in all of the earth’s carbon reservoirs. As you can see from this list, carbon can take many different forms.

Rock: 65,000,000 GtC

The carbon in rock is mostly solid carbonate, such as limestone (calcium carbonate, CaCO₃). Rocks are by far the largest reservoir of carbon on earth, but changes in the flow of carbon to and from this reservoir are extremely slow, and have no real impact on changes to the global carbon cycle at human timescales (tens to thousands of years). The carbon rock cycle is part of the slow carbon cycle, which takes hundreds of thousands to millions of years.

Atmosphere: 900 GtC (in 2018)

In the atmosphere, carbon is in a gaseous form. The most abundant carbon-containing gas is carbon dioxide (CO₂); others are methane (CH₄), and carbon monoxide (CO).

Ocean: 41,000 GtC

At the surface of the ocean, carbon shows up as dissolved carbon dioxide (CO₂), carbonic acid (H₂CO₃), bicarbonate ions (HCO₃^-), and carbonate ions (CO₃^-2). The relative abundance of these carbon compounds is controlled by the pH of the water. CO₂ dissolves in seawater, creating carbonic acid, which releases H+ ions. H+ ions combine with carbonate in seawater to form bicarbonate, which doesn’t easily escape the ocean.

Terrestrial Biosphere: 2,000 GtC

The carbon in the biosphere—mostly organic plant material and soils—takes the form of simple sugars like glucose or fructose, and more complex molecules like starch and cellulose.

Fossil Fuels: 4,000 GtC

The carbon in fossil fuels includes solid coal, liquid hydrocarbon petroleum, and gas hydrocarbon methane, which resulted from photosynthesis hundreds of millions of years ago and subsequent burial. Because fossil fuels are sequestered in rock, they are also part of the slow carbon cycle.

The amount of carbon moving between reservoirs is changing every year. The data use in this Snack were based on annual fluxes from 2005–2014. Here are some of the sources we used:

http://www.esd.ornl.gov/projects/qen/carbon2.html

http://www.ess.uci.edu/~reeburgh/fig1.html

Le Quéré, Corinne; et al. "Global carbon budget 2018." Earth System Science Data, 10, 2041 - 2194, 2018.  

IPCC Fifth Assessment Report, Climate Change 2013: The Physical Science Basis.  URL: https://www.ipcc.ch/report/ar5/wg1