Carbon Cycle 

Carbon is a fundamental part of the Earth system. It is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilizations—our economies, our homes, our means of transport—are built on carbon.  Forged in the heart of aging stars, carbon is the fourth most abundant element in the Universe. Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels.  Carbon moves from the atmosphere to the land, ocean, and life through biological, chemical, geological and physical processes in a cycle called the carbon cycle. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Carbon flows between each reservoir in slow and fast cycles.

Video: Keeping Up With Carbon

Video

Keeping Up With Carbon | https://www.youtube.com/watch?v=FgEZpX3n5mo | Source: NASA Goddard

This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon.

(Diagram adapted from U.S. DOE, Biological and Environmental Research Information System.)

Over the long term, the carbon cycle seems to maintain a balance that prevents all of Earth’s carbon from entering the atmosphere (as is the case on Venus) or from being stored entirely in rocks. This balance helps keep Earth’s temperature relatively stable, like a thermostat. This thermostat works over a few hundred thousand years, as part of the slow carbon cycle. This means that for shorter time periods—tens to a hundred thousand years—the temperature of Earth can vary. And, in fact, Earth swings between ice ages and warmer interglacial periods on these time scales. Parts of the carbon cycle may even amplify these short-term temperature changes.

The uplift of the Himalaya, beginning 50 million years ago, reset Earth’s thermostat by providing a large source of fresh rock to pull more carbon into the slow carbon cycle through chemical weathering. The resulting drop in temperatures and the formation of ice sheets changed the ratio between heavy and light oxygen in the deep ocean, as shown in this graph. (Graph based on data from Zachos at al., 2001.)

Rivers carry calcium ions—the result of chemical weathering of rocks—into the ocean, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone. (Photograph ©2009 Greg Carley.)

On very long time scales (millions to tens of millions of years), the movement of tectonic plates and changes in the rate at which carbon seeps from the Earth’s interior may change the temperature on Earth’s thermostat. Earth has undergone such a change over the last 50 million years, from the extremely warm climates of the Cretaceous (roughly 145 to 65 million years ago) to the glacial climates of the Pleistocene (roughly 1.8 million to 11,500 years ago). [See Divisions of Geologic Time—Major Chronostratigraphic and Geochronologic Units for more information about geological eras.]

​​​​​​Slow Cycle

Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 1013 to 1014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 1015 grams (1 Billion Metric Tons), whereas the fast carbon cycle moves 1016 to 1017 grams (10-100 billion Billion Metric Tons) of carbon per year.  

The movement of carbon from the Atmosphere to the Geosphere (rocks) begins with rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks—a process called chemical weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean. Rivers carry calcium ions—the result of chemical weathering of rocks—into the ocean, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone. In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on your faucet if you live in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives. Limestone, or its metamorphic cousin, marble, is rock made primarily of calcium carbonate. These rock types are often formed from the bodies of marine plants and animals, and their shells and skeletons can be preserved as fossils. Carbon locked up in limestone can be stored for millions—or even hundreds of millions—of years. Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale. This coal seam in Scotland was originally a layer of sediment, rich in organic carbon. The sedimentary layer was eventually buried deep underground, and the heat and pressure transformed it into coal. Coal and other fossil fuels are a convenient source of energy, but when they are burned, the stored carbon is released into the atmosphere. This alters the balance of the carbon cycle and is changing Earth’s climate.

Limestone, or its metamorphic cousin, marble, is rock made primarily of calcium carbonate. These rock types are often formed from the bodies of marine plants and animals, and their shells and skeletons can be preserved as fossils. Carbon locked up in limestone can be stored for millions—or even hundreds of millions—of years. (Photograph ©2008 Rookuzz (Hmm).)

The slow cycle returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide. When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.

Chemistry regulates this dance between ocean, land, and atmosphere. If carbon dioxide rises in the atmosphere because of an increase in volcanic activity, for example, temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering. Carbon stored in rocks is naturally returned to the atmosphere by volcanoes. In this photograph, Russia’s Kizimen Volcano vents ash and volcanic gases in January 2011. Kizimen is located on the Kamchatka Peninsula, where the Pacific Plate is subducting beneath Asia. However, the slow carbon cycle also contains a slightly faster component: the ocean. At the surface, where air meets water, carbon dioxide gas dissolves in and ventilates out of the ocean in a steady exchange with the atmosphere. Once in the ocean, carbon dioxide gas reacts with water molecules to release hydrogen ions, making the ocean more acidic. The hydrogen reacts with carbonate from rock weathering to produce bicarbonate ions.

Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85 percent of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean’s surface to its depths.In the meantime, winds, currents, and temperature control the rate at which the ocean takes carbon dioxide from the atmosphere. (See The Ocean’s Carbon Balance on the Earth Observatory.) It is likely that changes in ocean temperatures and currents helped remove carbon from and then restore carbon to the atmosphere over the few thousand years in which the ice ages began and ended. This means that for shorter time periods—tens to a hundred thousand years—the temperature of Earth can vary. And, in fact, Earth swings between ice ages and warmer interglacial periods on these time scales. Parts of the carbon cycle may even amplify these short-term temperature changes.The uplift of the Himalaya, beginning 50 million years ago, reset Earth’s thermostat by providing a large source of fresh rock to pull more carbon into the slow carbon cycle through chemical weathering. The resulting drop in temperatures and the formation of ice sheets changed the ratio between heavy and light oxygen in the deep ocean, as shown in this graph. (Graph based on data from Zachos at al., 2001.)

Carbon stored in rocks is naturally returned to the atmosphere by volcanoes. In this photograph, Russia’s Kizimen Volcano vents ash and volcanic gases in January 2011. Kizimen is located on the Kamchatka Peninsula, where the Pacific Plate is subducting beneath Asia. (Photograph ©2011 Artyom Bezotechestvo/Photo Kamchatka.)

Fast Cycle

The time it takes carbon to move through the fast carbon cycle is measured in a lifespan, which may not seem very quick. The fast carbon cycle is largely the movement of carbon through life forms on Earth or the Biosphere. Between 1015 and 1017 grams (1,000 to 100,000 million metric tons) of carbon move through the fast carbon cycle every year. On this time scale, the carbon cycle is most visible in life. Carbon plays an essential role in biology because of its ability to form many bonds—up to four per atom—in a seemingly endless variety of complex organic molecules. Plants and phytoplankton (microscopic organisms in the ocean) are the main components of the fast carbon cycle and convert carbon dioxide to biomass (like leaves and stems) through photosynthesis. In this process, they take carbon dioxide (CO2)  from the atmosphere by absorbing it into their cells with water to form sugar (CH2O) and oxygen. The chemical reaction looks like this:

CO2 + H2O + energy = CH2O + O2

During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars—for building plant structures. This process forms the foundation of the fast (biological) carbon cycle. (Illustration adapted from P.J. Sellers et al., 1992.)

During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars—for building plant structures. This process forms the foundation of the fast (biological) carbon cycle. (Illustration adapted from P.J. Sellers et al., 1992.)The bonds in the long carbon chains contain a lot of energy. When the chains break apart, the stored energy is released. This energy makes carbon molecules an excellent source of fuel for all living things. The carbon returns to the Atmosphere in the following processes but all involve the same chemical reaction:  

1. when plants and phytoplankton break down the sugar to get the energy they need to grow

2. when plants and phytoplankton die or decay (and are eaten by bacteria) at the end of the growing season

3. when plants and phytoplankton are eaten and digested by animals (including people) to get energy

4. when plants and phytoplankton burn in fires  

In each case, oxygen combines with sugar to release water, carbon dioxide, and energy. The basic chemical reaction looks like this:

CH2O + O2 = CO2 + H2O + energy

These four processes move carbon from a plant and put carbon gases into the Atmosphere. Changes that return carbon to the Atmosphere result in warmer temperatures on Earth.

The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing. Because plants and animals are an integral part of the carbon cycle, the carbon cycle is closely connected to ecosystems. As ecosystems change under a changing climate, the carbon cycle will also change. For example, plants may bloom earlier in the year and grow for more months (assuming sufficient water is present) as the growing season gets longer, altering the food supply for animals in the ecosystem. If more plants grow, they will take more carbon out of the atmosphere and cool temperatures. If, on the other hand, warming slows plant growth, habitats will shift and more carbon will go into the atmosphere where it can cause additional warming.