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The Carbon Cycle

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The Carbon Cycle THE CARBON CYCLE Summary All life on Earth is based upon carbon. The carbon cycle describes a set of 1. The natural world processes governing the movement of carbon elements or compounds Principles of environmental systems through living organisms and the environment and how carbon is stored Ecological systems and released in various ways. The cycle can be described in terms of stores of carbon and the processes that govern exchanges between them. Water and the hydrologic cycle The carbon cycle underpins all li f e on earth and is important in The carbon cycle understanding the causes and effects of climate change. Climate and atmospheric systems Relevance to business Biodiversity The carbon cycle is vital to all life on earth and it is important to understand how it works and what happens when it is modified. Human activities impact upon the carbon cycle with a variety of far-reaching implications for human populations and economies. An understanding of the underlying principles of carbon management is increasingly required when considering action to combat climate change. Carbon Carbon is the fourth most abundant element on Ear t h and present in all living organisms. The arrangement of electrons on the carbon atom means that it easily forms covalent bonds (sharing electrons between neighbouring atoms) with other carbon atoms and those of other elements, particularly hydrogen, nitrogen, oxygen and phosphorus. Carbon can bond to itself, forming molecules in long chains, branches and rings. The complexity of these molecules helps form the basis of life. On average, living organisms include about 18 per cent carbon – around one hundred t im e s gr eat er than its concentration in the Earth. The total quantity of carbon on Earth is fixed and it is cycled and recycled through a series of geological and biological (collectively biogeochemical) exchanges known as the carbon cycle. This cycle is exceptionally complex and still only partially understood, but is becoming clear that human activities are impacting upon and altering the balance of the free carbon budget. Carbon stores Carbon resides in a number of ‘stores’ (Table 1) and exchanges or flows occur between them. They can be considered as sinks when they sequestrate carbon or sources when they provide it to another store. Carbonate rocks: carbon is present as CACO3 (e.g. limestone) formed by sedimentary geological processes from the shells of dead marine animals that fixed bicarbonate (from seawater) with calcium to produce calcium carbonate (CACO3). Kerogens and fossil stores: carbon is stored as geochemical carbonates and organic compounds including coal, oil, oil bearing shales (kerogens) and nat ur al gas formed by geological processes from organic deposits. Terrestrial biosphere: carbon is present in biomass (living plants and animals) and dead organic matter, including soil humus and peat. The Carbon Cycle Page 1 of 6 Atmosphere: carbon exists in the atmosphere as carbon dioxide (CO2). Oceans: carbon arises primarily from atmospheric CO2, entering water by -2 diffusion as dissolved CO2, or converted into carbonate (CO 3 ) or bicarbonate - (HCO3 ). These stores exchange carbon with each other through processes which are not yet fully understood and ar e difficult to measure. The timescales of these exchanges vary significantly. Carbon in rocks, kerogens and fossil fuels is fixed almost permanently (in human timescales) unless fossil fuels are burnt or carbonates processed. Carbon in the terrestrial biosphere, atmosphere and oceans is more freely available and cycles more actively. Table 1. Major stores of carbon Carbon store Size/gigatonnes carbon Rock carbonates <60,000,000 Kerogens 15,000,000 Ocean 38,400 Coal 3,510 Soil 1,500 Atmosphere 720 Living biomass 600 Peat 250 Oil 230 Natural gas 140 Source: Smithson et al (2002) Processes and flows All carbon stores are interlinked and ultimately interdependent. A number of processes and flows govern their exchange. Photosynthesis is the process by which plants convert solar energy into food to enable growth. Photosynthesis is conducted primarily by photoautotrophs like plants and algae; organisms capable of synthesizing organic molecules from carbon dioxide using sunlight as their energy source. Carbon dioxide and hydrogen from water are combined to form carbohydrates, mainly t he sugar glucose. This primary production forms the basis for food chains supporting most life on earth and is the route for bringing carbon into the biosphere from the atmosphere. In the oceans, marine biota use dissolved CO 2 for photosynthesis. As the atmospheric concentration of CO2 rises the biochemical ability of plants to fix carbon deceases thereby lowering their capacity to act as a carbon sink. As part of primary production, plants absorb CO2 from the atmosphere during photosynthesis and release CO2 back into the atmosphere during respiration. Respiration is the biological equivalent of combustion: it breaks hydrocarbons and carbohydrates into carbon dioxide and water thereby releasing energy held in the sugars for use in metabolism. The fluxes of carbon produced by photosynthesis and respiration are around 1,000 times greater than exchanges from the geological cycle. Combustion occurs through natural wildfires that oxidize organic matter, releasing CO2, and through anthropogenic activity, mainly t he burning of fossil fuels to release energy. Burning coal, oil and gas releases carbon previously stored in the geosphere into the atmosphere. The Carbon Cycle Page 2 of 6 Carbon is also returned to the atmosphere through decaying fungi and bacteria. These produce CO2 if oxygen is present and methane (CH4) if it is not. Both these gases are significant in the global radiation balance (see R 1. 5 Climate and atmospheric systems). Figure 1. Carbon flows Source: NASA Earth Science Enterprise. GT = Gigagtonnes (1 billion tonnes) http://earthobservatory.nasa.gov/Library/CarbonCycle/carbon_cycle4.html Carbon can transfer from geological sinks through volcanic activity, normally in dramatic but sporadic events and through the weathering of carbon-rich rocks in slow but continuous processes. The biological fixation of carbon, where photosynthesis exceeds respiration, leads to the accumulation of organic matter and supplies the geological processes of coal and oil formation. Through these processes carbon is fixed in the geological store, resulting in a net removal of carbon from the atmosphere. However, these processes take millions of years and hence the current concern over the accelerated changes to the carbon budget resulting from human activities. The atmosphere stores about 750 gigatonnes of carbon, but this level is rising due to human activities. The annual receipt of carbon from fossil fuel combustion is thought to be about 5.5 gigatonnes and land use changes such as deforestation are thought to contribute 1.5 gigatonnes comprising an annual contribution of c. 7 gigatonnes. Around 2 gigatonnes of carbon diffuse into the oceans as dissolved CO2 or as carbonates. Measurements suggest that at least 3 gigatonnes remain in the atmosphere as a net contribution, but nearly 2 gigatonnes cannot be readily accounted for. It is unclear what is happening to the ‘missing’ 2 gigatonnes annually but is most likely being carried within the biosphere perhaps though increased plant and microbial activity. There is much uncertainty in this area, it is unknown whether this extra carbon can or will be absorbed in the future or how it is being stored. This lack of knowledge adds greatly to the uncertainty of planning for and attempting to mitigate climate change. Over the past century, the average carbon dioxide content of the atmosphere has risen from 290 to 340 ppm (parts per million) or higher, a rise of c.17 per cent. About 65 per cent of the additional carbon dioxide in the atmosphere The Carbon Cycle Page 3 of 6 comes from burning fossil fuel (and cement production); the other 35 per cent is derived from deforestation and the conversion of natural ecosystems into agricultural systems that results on the loss of organic content from soil. Natural ecosystems can store from 20-100 times more carbon than agricultural systems because of their more natural, complex and diverse structures. Thus, activities such as deforestation have both short and long-term impacts on carbon budgets. Forest clearance, particularly if burning is used, can result in a short term release of carbon dioxide that was fixed in the biomass. If the land is transferred to agricultural production the net capacity to fix carbon in the future is also diminished. In system terms this is an example of a positive feedback loop where change results in reinforcing the direction and strength of change (see R 1: 1 Principles of environmental systems). Increasing levels of carbon dioxide in the oceans are also of concern to scientists. This can lead to increased acidity and it is thought that may impact upon the productivity of phytoplankton and thus have ‘upst r eam ’ effects for carbon fixing and primary production in the oceans and thereby potentially serious implications for marine ecosystems. For example, increased acidity together with increased average temperatures are thought to be par t ly responsible for the recent dramatic worldwide decline in corals. Climate change The concentration of carbon dioxide in the atmosphere is recorded in a number of locations but the longest continuously recorded data come from the Mauna Loa station in Hawaii (Figure 2). The Mauna Loa record shows a 19.4 per cent increase in the mean annual concentration, from 316ppmv (parts per million by volume, dry air) in 1959 to 377 ppmv (parts per million per year) in 2004. Carbon dioxide is important in the global radiation balance because it acts as a greenhouse gas. Figure 2. C02 concentrations monitored at Mauna Loa observatory 1958 – 2004 380 370 360 350 340 330 CO2 concentration PPM concentration CO2 320 310 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 Year Source: Keeling & Whorf (2005) http://cdiac.ornl.gov/ftp/trends/co2/maunaloa.co2 There are other greenhouse gases both natural and human-made but carbon dioxide is produced in the largest quantities.
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