The global carbon cycle is currently an important topic of discussion primarily because of its importance in the global climate system and the way human activities are altering it significantly. Carbon- a core constituent of all organic matter acts as a primary energy source providing us with food and fiber and Carbon dioxide (CO2) is the most influential agent for climatic change. The carbon cycle thus serves as the core to the “Life on Earth.” However, industrialization over the last two centuries has pushed this nature’s balance to a dangerous level. According to Strategic Plan for the Climate Change Science Program Final Report, July 2003, since 1750, CO2 concentrations in the atmosphere have increased by 30%, resulting in global warming and continuous increase in global atmospheric temperatures.
What is Carbon cycling?
According to Wikipedia, the encyclopedia, “The carbon cycle is the biogeochemical cycle by which carbon is continuously exchanged between the biosphere, geosphere, hydrosphere and atmosphere of the Earth. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments1 (includes fossil fuels).” The working processes in the Carbon Cycle are photosynthesis2, decomposition3, and respiration.
Biogeochemical cycle: Biogeochemical Cycling is the cycling or exchange of elements through the ecosystem-thus, the elements ["chemical”] cycle between biotic (living organisms, “bio-“) and abiotic (rock, air, water; “geo-“) reservoirs- thus involving a variety of biological, geological, and chemical processes. The four important biogeochemical Cycles are – water, carbon (and oxygen), nitrogen, and phosphorous. Interestingly, Carbon cycle interacts and overlaps with Water and Nitrogen cycles. “The changes in carbon (C) and nitrogen (N) fluxes are connected to changes in water cycle, land use patterns, agricultural practices, urban development, and vegetation, which can have adverse impacts on terrestrial and aquatic resources. Carbon dioxide levels in the atmosphere affect plants’ water use efficiency through stomatal response and total leaf area, thereby altering local and regional water cycles. In turn, Atmospheric nitrogen deposition can have a fertilizer effect, inducing changes in both water and carbon cycles.” Source:http://www.mnforsustain.org/water_climate_chap4_plan_for_science.htm )
Forms of carbon involved in the carbon cycle:
(Source: http://www.physicalgeography.net/fundamentals/9r.html )
Carbon is present as organic molecules in living and dead organisms found in the biosphere, as CO2 in the atmosphere, as organic matter in soils, in the lithosphere4 as fossil fuels5 and sedimentary rock deposits such as limestone6, dolomite and chalk, in the oceans as dissolved atmospheric CO2 and as calcium carbonate CaCO3 shells in marine organisms.
Major Sources of CO2 can be divided into two categories-
· Respiration By Producers, Consumers, and Decomposers
· Oxidation of Soil Organics
· Erosion of Carbonate Rocks
· Volcanic Emissions
· .Burning of Fossil Fuels (coal, oil)
· Burning of Forests and Fuel wood
· Agriculture (Plowing)
CARBON SINKS: Stores of carbon are thus known as carbon sinks/ reservoirs/ compartments. Sedimentary carbonates and kerogen are the largest carbon reservoirs; followed by marine dissolved inorganic carbon (DIC), soils, surface sediments, and the atmosphere. The quantities of carbon in various compartments is described in the table below
Amount in Billions of Metric Tons
Soil Organic Matter
1500 to 1600
38,000 to 40,000
Marine Sediments and Sedimentary Rocks
66,000,000 to 100,000,000
540 to 610
Fossil Fuel Deposits
Turnover time = pool size / flux rate, Turnover time is the length of time it would take to empty a compartment.
Turnover Time ( in years)
Peat and Soil Carbon
For instance, if a compartment contains 1000 tons of carbon, and 10 tons leave or enter the compartment per year, then the turnover time will be 1000/10 = 100 years
The Movement of carbon from one compartment to another: The atmosphere is a primary source of carbon dioxide to the Ecosystems. Using water and energy from solar radiation, the autotrophic organisms chemically convert the carbon dioxide to carbon-based sugar molecules with the means of photosynthesis. Through the metabolic addition of other elements, these molecules are then chemically
BOX MODEL OF THE GLOBAL CARBON CYCLE
The diagram shows estimates of the numbers of metric gigatons of
Carbon found in parts of the Earth system in the early 1970’s. The boxes represent amounts of
Carbon in various forms and the arrows represent the rates at which the carbon is moved from one part of the system to another. Source: http://mvhs1.mbhs.edu/mvhsproj/carbon/carbonstu.pdf
modified by these organisms to produce more complex compounds like proteins, cellulose, and amino acids. The organic matter produced in plants also moves down to the heterotrophic animals through consumption. Carbon dioxide then enters the sea water by diffusion7. In the sea, the carbon dioxide can remain as it is or can be converted into carbonate (CO3-2) or bicarbonate (HCO3-). Phytoplankton8 and other sea creatures convert bicarbonate with calcium (Ca+2) to produce calcium carbonate (CaCO3). This substance is used to produce shells and other water body parts by organisms like as coral, clams, oysters, some protozoa, and some algae. After these organisms die, these carbonate-rich deposits are physically and chemically altered into sedimentary rocks. Ocean deposits are by far the biggest sink of carbon on the planet. And from the ecosystems, Carbon is released as carbon dioxide gas by the process of respiration by both plants and animals involving the breakdown of carbon-based organic molecules into carbon dioxide gas. Finally, a number of organisms in the detritus9 food chain decompose organic matter into its abiotic components.
Factors affecting carbon cycle: Fossil fuel burning used by industry, power plants, and automobiles for coal, oil, natural gas, and gasoline generation is the single largest source of carbon dioxide increase — producing 2.5 billion tons of CO2 in US alone every year. The difference between the rate at which carbon is replenished in new fossil reserves and the rate at which it is released from old reserves has created an imbalance in the carbon cycle. Secondly, land clearing or Deforestation activities for agriculture is another primary source causing imbalance in Carbon cycle-by clearing forests, we reduce the ability of photosynthesis to remove CO2 from the atmosphere.
Feedback Mechanisms: The carbon-climate feedback means the release of more CO2 into the atmosphere as a result of surface warming on Earth. Such a positive feedback implies that with global warming, we can expect more release of carbon into the atmosphere by some combination of terrestrial and marine processes. Much research needs to be done in this direction; else we will only be underestimating the magnitude of impending global warming. Today, we don’t have a full fledged mechanism causing this feedback, nor how strongly and rapidly it will operate though lot of simulation experiments has been performed using carbon cycle and climate models to compare historical atmospheric concentration values of carbon dioxide with simulated values. Sensitivity experiments are also being studied to quantify the extent the terrestrial feedback processes and oceanic fluxes that influence the global carbon balance in the model.
Carbon sequestration is the term describing processes that remove carbon from the atmosphere. There are two primary types of carbon sequestration- Short term and Long Term. The Long Term is where carbon dioxide is captured at its source (e.g., power plants, industrial processes) and then stored in non-atmospheric reservoirs (e.g., depleted oil and gas reservoirs, unmineable coal seams, deep saline formations, deep ocean). The short term focuses on enhancing natural processes to increase the removal of carbon from the atmosphere (e.g., forestation).
Terrestrial sequestration: The rate at which forests can sequester carbon is the fastest available till now though it is not enough to neutralize the balance. The terrestrial biosphere is estimated to sequester approximately 2 billion metric tons of carbon per year. Moreover, the restoration of desertified or damaged land will increase its carbon sink properties. The carbon sequestration potential of soils (by increasing soil organic matter) is also substantial; below ground organic carbon storage is more than twice above-ground storage. Mechanisms to enhance carbon sequestration in soil include conservation tilling, cover cropping, and crop rotation.
Ocean sequestration: One of the most promising ways to increase the carbon sequestration efficiency of oceans is to add micrometer-sized iron particles called hematite10 or iron sulfate to the water, which stimulates the growth of plankton. A recent study in Germany has concluded that any biomass carbon in the oceans would represent long term storage of carbon. This means that application of iron nutrients in select parts of the oceans, at appropriate scales, could have the combined effect of restoring parts of the oceans, at appropriate scales, could have the combined effect of restoring ocean productivity as well as reducing the emissions of carbon dioxide to the atmosphere.
Geological Sequestration: This occurs when Carbon dioxide is sequestrated in geologic formations such as: depleted oil and gas reservoirs, shale formations with high organic content, unmineable coal seams, and underground saline formations. Since 1996, approximately one million tons per year of recovered CO 2 is being stored into the Utsira Sand, a saline formation in the North Sea.
Past Research and Future Direction
The fact that CO2 increases the atmosphere’s ability to hold heat makes it a “greenhouse gas” and an important factor for climate change. Many scientists attribute the observed 0.6 degree C increase in global average temperature over the past century mainly to increases in atmospheric CO2. Scientists have developed techniques like analysis of gas bubbles trapped in ice, tree rings, and ocean and lake floor sediments for clues about past climates and atmospheres. It has been observed that over the past 20 million years, the Earth’s climate has oscillated between relatively warm and relatively cold conditions called interglacial11 and glacial periods. We are currently in an interglacial warm period, and human activities are pushing CO2 concentrations higher. On The International front, Intergovernmental Panel on Climate Change (IPCC), an international, interdisciplinary consortium comprising of thousands of climate experts has been formed solely for this purpose. Moreover, many nations have agreed to conditions specified by the Kyoto Protocol, a multilateral treaty aimed at averting the negative impacts associated with human-induced climate change. However, to understand the magnitude of Carbon Cycle and its Control Techniques, lot of needs to be done in terms of innovative research and development of improved process models, simulation techniques and new Carbon Sequestration methodologies.
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1 Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. Sedimentation is the deposition by settling of a suspended material.
2 Photosynthesis is an important biochemical process in which plants, algae, and some bacteria convert the energy of sunlight to chemical energy.
3 Decomposition refers to the reduction of the body of a formerly living organism into simpler forms of matter.
4 Lithosphere is the solid inorganic portion of the Earth (composed of rocks, minerals, and elements). It can be regarded as the outer surface and interior of the solid Earth.
5 Carbon based remains of organic matter that has been geologically transformed into coal, oil and natural gas. Combustion of these substances releases large amounts of energy. Currently, humans are using fossil fuels to supply much of their energy needs.
6 Sedimentary rock composed of carbonate minerals, especially CaCO3.
7 Diffusion is the movement of particles from higher chemical potential to lower chemical potential. It is readily observed for example when dried foodstuff like spaghetti is cooked; water molecules diffuse into the spaghetti strings, making them thicker and more flexible
8 Phytoplankton, like plants, obtain energy through a process called photosynthesis, and so must live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, or lake. Through photosynthesis, phytoplankton (and terrestrial plants) are responsible for much of the oxygen present in the Earth’s atmosphere.
9 The organic waste material from decomposing dead plants or animals
10 Hematite (AE) is the mineral form of Iron (III) oxide, (Fe2O3), one of several iron oxides
11 Glaciations are characterized by cool, wet climates and thick ice sheets extending from each pole. Sea levels drop due to the presence of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the earth has significant continental glaciation in the Arctic and Antarctic, we currently are in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an “interglacial”.