FLUXES AND A WORD ABOUT UNITS
In order to understand how carbon is cycled and how atmospheric CO2 will change in the future, scientists must carefully study the places in which carbon is stored (pools), how long it resides there, and processes that transfer it from one pool to another (fluxes). Collectively, all of the major pools and fluxes of carbon on Earth comprise what we refer to as the global carbon cycle.
As you might imagine, the actual global carbon cycle is immensely complex. It includes every plant, animal and microbe, every photosynthesizing leaf and fallen tree, every ocean, lake, pond and puddle, every soil, sediment and carbonate rock, every breath of fresh air, volcanic eruption and bubble rising to the surface of a swamp, among much, much else. Because we can't deal with that level of complexity, scientists often describe the carbon cycle by lumping similar objects or environments into simpler groups (forest, grassland, atmosphere, ocean) and focusing only on the processes that are most important at the global scale (see Global Carbon Cycle Diagram). As you might imagine, part of the trick is understanding just what those processes are.
The following section is a brief overview of some of the important pools and fluxes in the global carbon cycle (and note that, in our discussion, we will use the terms pool, stock and reservoir interchangeably). But first, it’s worth taking a moment to consider the numbers and units scientists often deal with. Because the quantities of carbon in the Earth’s major carbon pools can be quite large, it is inconvenient to use familiar units such as pounds or kilograms. Instead, we use other units that are better suited for expressing large numbers. For example, a Petagram of carbon (Pg), also known as a Gigaton (Gt), is equal to 10^15 grams or one billion tonnes. A tonne, also known as a metric ton, is equal to one thousand kilograms (1,000 kg). Because one kilogram is equal to 2.205 pounds, one metric tonne is the same as 2205 pounds. Taking this further, we can see that one Petagram is equal to just about 2,200,000,000,000 (or 2.2 trillion) pounds! Expressing this as 1 Pg is much simpler than working with that many zeros. Now we will consider carbon stored on Earth in four main reservoirs.
Depending on our goals, the Earth’s carbon pools can be grouped into any number of different categories. Here, we will consider four categories that have the greatest relevance to the overall carbon cycle. Keep in mind that any of these pools could be further divided into a number of subcategories, as we will occasionally discuss.
The Earth’s Crust: The largest amount of carbon on Earth is stored in sedimentary rocks within the planet’s crust. These are rocks produced either by the hardening of mud (containing organic matter) into shale over geological time, or by the collection of calcium carbonate particles, from the shells and skeletons of marine organisms, into limestone and other carbon-containing sedimentary rocks. Together all sedimentary rocks on Earth store 100,000,000 PgC. Recalling that 1 Pg is over two trillion pounds, this is clearly a large mass of carbon! Another 4,000 PgC is stored in the Earth’s crust as hydrocarbons formed over millions of years from ancient living organisms under intense temperature and pressure. These hydrocarbons are commonly known as fossil fuels.
Oceans: The Earth’s oceans contain 38,000 PgC, most of which is in the form of dissolved inorganic carbon stored at great depths where it resides for long periods of time. A much smaller amount of carbon, approximately 1,000 Pg, is located near the ocean surface. This carbon is exchanged rapidly with the atmosphere through both physical processes, such as CO2 gas dissolving into the water, and biological processes, such as the growth, death and decay of plankton. Although most of this surface carbon cycles rapidly, some of it can also be transferred by sinking to the deep ocean pool where it can be stored for a much longer time.
Atmosphere: The atmosphere contains approximately 750 PgC, most of which is in the form of CO2, with much smaller amounts of methane (CH4 and various other compounds). Although this is considerably less carbon than that contained in the oceans or crust, carbon in the atmosphere is of vital importance because of its influence on the greenhouse effect and climate. The relatively small size of the atmospheric C pool also makes it more sensitive to disruptions caused by and increase in sources or sinks of C from the Earth’s other pools. In fact, the present-day value of 750 PgC is substantially higher than that which occurred before the onset of fossil fuel combustion and deforestation. Before these activities began, the atmosphere contained approximately 560 PgC and this value is believed to be the normal upper limit for the Earth under natural conditions. In the context of global pools and fluxes, the increase that has occurred in the past several centuries is the result of C fluxes to the atmosphere from the crust (fossil fuels) and terrestrial ecosystems (via deforestation and other forms of land clearing).
Terrestrial Ecosystems: Terrestrial ecosystems contain carbon in the form of plants, animals, soils and microorganisms (bacteria and fungi). Of these, plants and soils are by far the largest and, when dealing with the entire globe, the smaller pools are often ignored. Unlike the Earth’s crust and oceans, most of the carbon in terrestrial ecosystems exists in organic forms. In this context, the term “organic” refers to compounds that were produced by living things, including leaves, wood, roots, dead plant material and the brown organic matter in soils (which is the decomposed remains of formerly living tissues).
Plants exchange carbon with the atmosphere relatively rapidly through photosynthesis, in which CO2 is absorbed and converted into new plant tissues, and respiration, where some fraction of the previously captured CO2 is released back to the atmosphere as a product of metabolism. Of the various kinds of tissues produced by plants, woody stems such as those produced by trees have the greatest ability to store large amounts of carbon. Wood is dense and trees can be large. Collectively, the Earth’s plants store approximately 560 PgC, with the wood in trees being the largest fraction.
The total amount of carbon in the world’s soils is estimated to be 1500 PgC. Measuring soil carbon can be challenging, but a few basic assumptions can make estimating it much easier. First, the most prevalent form of carbon in the soil is organic carbon derived from dead plant materials and microorganisms. Second, as soil depth increases the abundance of organic carbon decreases. Standard soil measurements are typically only taken to 1m in depth. In most case, this captures the dominant fraction of carbon in soils, although some environments have very deep soils where this rule doesn’t apply. Most of the carbon in soils enters in the form of dead plant matter that is broken down by microorganisms during decay. The decay process also released carbon back to the atmosphere because the metabolism of these microorganisms eventually breaks most of the organic matter all the way down to CO2.
The movement of any material from one place to another is called a flux and we typically think of a carbon flux as a transfer of carbon from one pool to another. Fluxes are usually expressed as a rate with units of an amount of some substance being transferred over a certain period of time (e.g. g cm-2 s-1 or kg km2 yr-1). For example, the flow of water in a river can be thought of as a flux that transfers water from the land to the sea and can be measured in gallons per minute or cubic kilometers per year.
A single carbon pool can often have several fluxes both adding and removing carbon simultaneously. For example, the atmosphere has inflows from decomposition (CO2 released by the breakdown of organic matter), forest fires and fossil fuel combustion and outflows from plant growth and uptake by the oceans. The size of various fluxes can vary widely. In the previous section, we briefly discussed a few of the fluxes into and out of various global C pools. Here, we will pay more careful attention to some of the more important C fluxes.
Photosynthesis: During photosynthesis, plants use energy from sunlight to combine CO2 from the atmosphere with water from the soil to create carbohydrates (notice that the two parts of the word, carbo- and –hydrate, signify carbon and water). In this way, CO2 is removed from the atmosphere and stored in the structure of plants. Virtually all of the organic matter on Earth was initially formed through this process. Because some plants can live to be tens, hundreds or sometimes even thousands of years old (in the case of the longest-living trees), carbon may be stored, or sequestered, for relatively long periods of time. When plants die, their tissues remain for a wide range of time periods. Tissues such as leaves, which have a high quality for decomposer organisms, tend to decay quickly, while more resistant structures, such as wood can persist much longer. Current estimates suggest photosynthesis removes 120 PgC/year from the atmosphere and about 610 PgC is stored in plants at any given time.
Plant Respiration: Plants also release CO2 back to the atmosphere through the process of respiration (the plant equivalent of exhaling). Respiration occurs as plant cells use carbohydrates, made during photosynthesis, for energy. Plant respiration represents approximately half (60 PgC/year) of the CO2 that is returned to the atmosphere in the terrestrial portion of the carbon cycle.
Litterfall: In addition to the death of whole plants, living plants also shed some portion of their leaves, roots and branches each year. Because all parts of the plant are made up of carbon, the loss of these parts to the ground is a transfer of carbon (a flux) from the plant to the soil. Dead plant material is often referred to as litter (leaf litter, branch litter, etc.) and once on the ground, all forms of litter will begin the process of decomposition.
Soil Respiration: The release of CO2 through respiration is not unique to plants, but is something all organisms do. When dead organic matter is broken down or decomposed (consumed by bacteria and fungi), CO2 is released into the atmosphere at an average rate of about 60 PgC/year globally. Because it can take years for a plant to decompose (or decades in the case of large trees), carbon is temporarily stored in the organic matter of soil.
Ocean—Atmosphere exchange: Inorganic carbon is absorbed and released at the interface of the oceans’ surface and surrounding air, through the process of diffusion. It may not seem obvious that gasses can be dissolved into, or released from water, but this is what leads to the formation of bubbles that appear in a glass of water left to sit for a long enough period of time. The air contained in those bubbles includes CO2 and this same process is the first step in the uptake of carbon by oceans. Once in a dissolved form, CO2 goes on to react with water in what are known as the carbonate reactions. These are relatively simple chemical reactions in which H2O and CO2 join to form H2CO3 (also known as carbonic acid, the anion of which, CO3, is called carbonate). The formation of carbonate in seawater allows oceans to take up and store a much larger amount of carbon than would be possible if dissolved CO2 remained in that form. Carbonate is also important to a vast number of marine organisms that use this mineral form of carbon to build shells.
Carbon is also cycled through the ocean by the biological processes of photosynthesis, respiration, and decomposition of aquatic plants. In contrast with terrestrial vegetation is the speed at which marine organisms decompose. Because ocean plants don’t have large, woody trunks that take years to breakdown, the process happens much more quickly in oceans than on land—often in a matter of days. For this reason, very little carbon is stored in the ocean through biological processes. The total amount of carbon uptake (92 Pg C) and carbon loss (90 PgC) from the ocean is dependent on the balance of organic and inorganic processes.
Fossil fuel combustion and land cover change: The carbon fluxes discussed thus far involve natural processes that have helped regulate the carbon cycle and atmospheric CO2 levels for millions of years. However, the modern-day carbon cycle also includes several important fluxes that stem from human activities. The most important of these is combustion of fossil fuels: coal, oil and natural gas. These materials contain carbon that was captured by living organisms over periods of millions of years and has been stored in various places within the Earth's crust (see accompanying text box). However, since the onset of the industrial revolution, these fuels have been mined and combusted at increasing rates and have served as a primary source of the energy that drives modern industrial human civilization. Because the main byproduct of fossil fuel combustion is CO2, these activities can be viewed in geological terms as a new and relatively rapid flux to the atmosphere of large amounts of carbon. At present, fossil fuel combustion represents a flux to the atmosphere of approximately 6-8 PgC/year.
Another human activity that has caused a flux of carbon to the atmosphere is land cover change, largely in the form of deforestation. With the expansion of the human population and growth of human settlements, a considerable amount of the Earth's land surface has been converted from native ecosystems to farms and urban areas. Native forests in many areas have been cleared for timber or burned for conversion to farms and grasslands. Because forests and other native ecosystems generally contain more carbon (in both plant tissues and soils) than the cover types they have been replaced with, these changes have resulted in a net flux to the atmosphere of about 1.5 PgC/year. In some areas, regrowth of forests from past land clearing activities can represent a sink of carbon (as in the case of forest growth following farm abandonment in eastern North America), but the net effect of all human-induced land cover conversions globally represents a source to the atmosphere.
Geological Processes: Geological processes represent an important control on the Earth's carbon cycle over time scales of hundreds of millions of years. A thorough discussion of the geological carbon cycle is beyond the scope of this introduction, but the processes involved include the formation of sedimentary rocks and their recycling via plate tectonics, weathering and volcanic eruptions.
To take a slightly closer look, rocks on land are broken down by the atmosphere, rain, and groundwater into small particles and dissolved materials, a process known as weathering. These materials are combined with plant and soil particles that result from decomposition and surface erosion and are later carried to the ocean where the larger particles are deposited near shore. Slowly, these sediments accumulate, burying older sediments below. The layering of sediment causes pressure to build and eventually becomes so great that deeper sediments are turned into rock, such as shale. Within the ocean water itself, dissolved materials mix with seawater and are used by marine life to make calcium carbonate (CaCO3) skeletons and shells. When these organisms die, their skeletons and shells sink to the bottom of the ocean. In shallow waters (less than 4km) the carbonate collects and eventually forms another type of sedimentary rock called limestone.
Collectively, these processes convert carbon that was initially contained in living organisms into sedimentary rocks within the Earth's crust. Once there, these materials continue to be moved and transformed through the process of plate tectonics, uplift of rocks contained in the lighter plates and melting of rocks in the heavier plates as they are pushed deep under the surface. These melted materials can eventually result in emission of gaseous carbon back to the atmosphere through volcanic eruptions, thereby completing the cycle. Although the recycling of carbon through sedimentary rocks is vital to our planet's long-term ability to sustain life, the geological cycle moves so slowly that these fluxes are small on an annual basis and have little effect on a human time-scale.