The Carbon Cycle

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Living systems are characterized by a continuous exchange ofmaterial and energy with their environment. The chemical energy responsible for maintaining life on the planet is produced by complex photochemical reactions involving the photochemical reduction of CO2 with water to organic forms of carbon and molecular oxygen. The process is referred to as photosynthesis. The photosynthetic conversion of CO2 and water to glucose and oxygen absorbs an energy equivalent of 2800 kJ mol-1:

The reverse reaction, respiration, oxidizes the organic carbon with atmospheric oxygen. It releases energy as heat and creates a flow of carbon dioxide back to the atmosphere. On a global scale the material turnover of this reaction is huge: 120 Gton of carbon is converted annually by the photosynthetic reaction of plants on land alone (IPPC Report). This quantity is generally referred to as the gross primary production (GPP). GPP corresponds to a conversion of 4.7 x 1021 J of energy per year, which exceeds the annual consumption of fossil fuels by a factor of circa 10. The total marine GPP is of the same order of magnitude and is estimated to amount to 102 Gton per year, exchanging CO2 between organisms and the water (dissolved inorganic carbon). The carbon exchange between the atmosphere and the hydrosphere (oceans) amounts to 90 Gton per year and is driven by photosynthesis and by transport processes between surface water and deep water. In a steady state all the fluxes from and to the atmosphere are well balanced, that is the photosynthetic production is balanced by an equal flux of carbon resulting from respiration processes. One differentiates between autotrophic respiration (i.e. the roughly 50 % of the GPP carbon flux which is released due to respiration processes of the photosynthetic organisms themselves) and heterotrophic respiration (respiration of organisms feeding on photosynthetic plants) and combustion ("natural" and anthropogenic combustion ofbiomass). The difference between GPP and autotrophic respiration is the so-called net photosynthetic production (NPP). Figure 3.3 summarizes the flows and reservoirs relevant to the global carbon cycle.

The carbon content of the atmosphere is small compared to the carbon content of the other reservoirs taking part in the exchange of carbon. Any deviation from a well balanced regime of carbon flows from and to the atmosphere will therefore lead to an accumulation or depletion ofthis reservoir within a short period oftime. Although the flow of carbon to the atmosphere due to fossil fuel burning is less than 5 % of the terrestrial GPP, this flow seriously disturbs the balance of flows and has led to the well-documented increase in the CO2 concentration in the atmosphere. The famous record of the CO2 concentration over the past nearly 50 years at the site of Mauna Loa in Hawaii reflects the sensitivity of the atmosphere to imbalances in carbon fluxes. The steady increase in the curve is due to the net excess of CO2 resulting from fossil fuels and deforestation, while the annual oscillations of the curve reflect a seasonal imbalance of the fluxes, due to the vegetation cycle and the fact that the land mass is not evenly distributed between the northern and the southern hemispheres. The observed fluctuations in the net increase in the CO2 content of the atmosphere (e.g. by 1.9 Gton in 1992 and 6.0 Gton in 1998) have been ascribed to variations in land and ocean uptake [27].

A very small part of the photosynthetic net production is buried in sediments, isolated from the oxygen of the atmosphere and thus sequestered from the rapid annual carbon turnover between biosphere and atmosphere. Under favorable

Geochemical Carbon Cycle
Fig. 3.3 The carbon flows between the important reservoirs in the geochemical cycle. From (Intergovernmental Panel on Climate Change (IPCC) (2001), Ref. [4]).

geological conditions, the accumulation and chemical transformation of part of this sequestered organic carbon in sediments has led to the formation of exploitable fossil fuel deposits over time. The rate of organic carbon sequestration in sediments is more or less balanced by oxidative weathering of old organic carbon deposits in sediment rocks. Estimates derived from isotopic analysis of carbon in sediment rocks reveal that the rate of deposition has varied over geological times. It ranges between 2 and 6 x 1018molMyr-1 [24], or, on average, about 5 x 1013gCyr-1, which corresponds to 0.1 % of the present annual net photosynthetic production (NPP). The rate of release of organic carbon from sediments by burning of fossil fuels is higher than the natural rate of weathering by a factor of 100. The "sudden" (on geological time scales) oxidation of sequestered organic carbon in the form of fossil fuels has led to the unprecedented surge in carbon dioxide concentration in the atmosphere, with its effect on global climate.

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