The Global Carbon Cycles

There are two categories of flux depicted in Fig. 11.1 long-term fluxes, indicated by dashed arrows, and short-term fluxes, indicated by solid arrows. The long-term fluxes represent fluxes 

The global carbon cycle. Values in brackets are preanthropogenic reservoir sizes in Pg (10 g) values on the arrows are fluxes in Pgy . Dashed lines represent the long-term carbon cycle determined by weathering. Values are normalized to the flux of Die from rivers (see Chapter 2). Solid arrows are the shorter-term carbon fluxes associated with photosynthesis and respiration. 

The Wiggly vertical line indicates particulate C and DOC transport from the ocean euphotic zone to deep water. Symbols W, weathering of carbonates (CaCOj-hCOj + HjO  

2HCO + Ca ) and silicates (silicate + CO2 + H2O clay + HCOf - -cations) GE, gas exchange P, gross photosynthesis (CO2 + H2O CH2O (OM) + O2) R, respiration (CH2O (OM) 

Source The data are from the compilations of Pilson (1998), IPCC (2001), and Sabine et al. (2004). 

There are many review articles and books about the carbon cycle available with varying degrees of detail and points of emphasis (e.g. Bolin, 1970a,b Keeling, 1973 Woodwell and Pecan, 1973 Wood-well, 1978 Bolin et al., 1979 Revelle, 1982 Bolin and Cook, 1983 Degens et al., 1984). 

This chapter is an attempt to give an account of the fundamental aspects of the carbon cycle from a global perspective. An outline of the details we shall encounter is shown in Fig. 11-1. After a presentation of the main characteristics of carbon on Earth, four sections follow a section about the carbon reservoirs within the atmosphere, the hydrosphere, the biosphere, and the lithosphere a section covering the most important fluxes between the reservoirs a section giving brief accounts of selected models of the carbon cycle and a final section describing cultural influences on the carbon cycle today. 

The relevant time-scales vary over many orders of magnitude, from millions of years for processes 

There are more than 1 million known carbon compounds, thousands of which are vital to life processes. The carbon atom s unique and characteristic ability to form long stable chains makes life itself possible. Elemental carbon is found free in nature in three allotropic forms amorphous carbon, graphite, and diamond. Graphite is a very soft material, whereas diamond is well known for its hardness. Curiosities in nature, the amounts of elemental carbon on Earth are insignificant in a treatment of the carbon cycle. Carbon atoms have oxidation states ranging from 4- IV to — IV. The most common state is +IV in CO2 and the familiar carbonate forms. Carbonate exists in two reservoirs. 

Copyright 1992 Academic Pre s Limited All rights of reproduction in any form reserved 

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Figure 8.1 Diagrammatic model of the global carbon cycle. Questions marks indicate that no estimates are available. Figures are in units of lO tonnes of contained carbon but estimates from various sources sometimes differ by factors of 3 or more. The diagram is based on one by B. Bolin modified to include more recent data. ...

Schlamdinger, B., and Marland, G. (1996). The Role of Forest and Bioenegy Strategies in the Global Carbon Cycle. Biomass and Biocncrgy 10(5/6) 275-300. 

Up to this point, we have focused on aqueous equilibria involving proton transfer. Now we apply the same principles to the equilibrium that exists between a solid salt and its dissolved ions in a saturated solution. We can use the equilibrium constant for the dissolution of a substance to predict the solubility of a salt and to control precipitate formation. These methods are used in the laboratory to separate and analyze mixtures of salts. They also have important practical applications in municipal wastewater treatment, the extraction of minerals from seawater, the formation and loss of bones and teeth, and the global carbon cycle. 

POST ET AL. Climatic Feedbacks in the Global Carbon Cycle... 

The magnitude and fate of coastal-zone biological production is a major unknown in the global carbon cycle. Since river nutrient flux into these regions may be altered with C02-induced climate change, it is important that generation and fate of coastal-zone production be better understood. 

Vegetation, the Global Carbon Cycle, and Global Measures... 

The global carbon cycle is the continuous movement of carbon between the living and nonliving portions of the biosphere, driven in part by biological processes and resulting in a constant supply of carbon to life (Figure 1). The... 

Figure 1. The global carbon cycle. Estimates of reservoir size and annual fluxes are from Post et al. (4), Vegetation carbon reservoir was estimated from latest carbon density estimates. All values except the atmospheric reservoir are approximate only. All values are in gigatons. Fluxes are next to the arrows and are in gigatons ear.

Dead vegetation also afreets the global carbon cycle. Dead organic matter decomposes, releasing carbon dioxide to the atmosphere. Rates of decomposition vary with material, location, and climate. Non-woody organic matter decomposes rapidly woody organic matter slowly. Decomposition tends to occur faster at the soil surface than below. Decomposition is relatively fast in warm moist climates. In cold climates and in wetlands, decomposition is so slow that there is a net increase of stored carbon in the soil and organic soils called, "histosols, are formed. 

Another model, first introduced by Moore, et al. (2i), was used to examine the role of terrestrial vegetation and the global carbon cycle, but did not include an ocean component. This model depended on estimates of carbon pool size and rates of CO2 uptake and release. This model has been used to project the effect of forest clearing and land-use change on the global carbon cycle (22, 23, 24). 

Several studies, based on models, examined the effects of land-use change on the global carbon cycle and conclude that there is a net release of carbon due to land clearing. However, the results and conclusions of these studies are based on assumed sizes of vegetation carbon pools which are inputs to the models. For example, Melillo et al. 24) concluded that boreal and temperate deciduous forests of the northern hemisphere are net sources of atmospheric carbon. Their analysis used values for carbon density derived by Whittaker and Likens 19) from work by Rodin and Bazilevich (27). Rodin and Bazilevich extrapolated results of small, unrelated studies in Europe and the USSR to estimate total biomass of Eurasian boreal and temperate deciduous forests. Their estimates have since been extrapolated to forests worldwide and are used often today. 

Most estimates of global vegetation biomass densities are extrapolations from studies never intended to represent large areas (e.g. 79, 36) or they were derived from questionnaires sent to botanists (57). These estimates are still used commonly in the examination and modeling of the global carbon cycle. Some of the earliest estimates were made when almost no quantitative data were available and the data or the estimates were largely speculative. Other estimates are... 

Bjorkstrom, A. 1979. A model of CO2 interaction between atmosphere,oceans, and land biota. In The Global Carbon Cycle, Bolin, B. Degens, E. T. Kempe, S. Ketner, P., Eds. SCOPE 13 J Wiley Sons New York, NY, 1979 pp 403-457. 

Olson, J. S. Pfoderer, H. A. Chan, Y. H. Changes in the Global Carbon Cycle and the Biosphere ORNL/EIS-109, Oak Ridge National Laboratory Oak Ridge, TN, 1978. 

Baes, C. F., Bjdrkstrom, A. and MuIhoUand, P. J. (1985). Uptake of carbon dioxide by the oceans. In "Atmospheric Carbon Dioxide and the Global Carbon Cycle" (J. R. Trabalka, ed.). Report DOE/ ER-0239, US Department of Energy, Office of Energy Research, Washington, DC. 

Fig. 11-1 Major reservoirs and fluxes of the global carbon cycle, including time scales. Numbers given are Pg C (1 Pg C = lO g C) Pg C/yr, respectively. (After Sundquist, 1993.)...

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