El control por parte de los consumidores

Because carbon plays such an important role in the mechanisms of living things and also has an impact on the earth’s climate through the greenhouse effect, the carbon cycle (Figure 2.19) is one of the important cycles associated with the water cycle.

Besides the fact that carbon is the basis of organic chemistry and that more than half a million different organic compounds have been identified to date, carbon also exists in the gaseous state in the atmosphere (as carbon monoxide CO and carbon dioxide

Water depth mm /year (Switzerland)

Water depth mm /year (Morocco)

Precipitation 1456 211

Runoff 978 32

Storage –6 6

Evaporation 484 173

External contributions 318 –

CO₂), in dissolved form in water as bicarbonate , and in the solid state in carbonates such as calcium carbonate (CaCO₃).

In addition to these compounds, carbon exists in pure forms such as graphite and diamonds, as well as the forms present in carbonaceous rocks.

Carbon also exists in seven isotopic forms (¹⁰C,¹¹C,¹²C,¹³C,¹⁴C, ¹⁵C,¹⁶C), two of which are stable (¹²C and ¹³C). The other isotopes are radioactive and have half-lives ranging from 0.74 seconds for carbon ¹⁶C to 5726 years for carbon ¹⁴C. The most abundant carbon isotope on earth is carbon ¹²C which accounts for approximately 99%

of the total quantity of carbon. The second most abundant form is carbon ¹³C.

Carbon is present in all the big terrestrial reservoirs but its cycle must be under-stood in the dimension of time. In essence, the carbon cycle is best perceived as several overlapping cycles with vastly different time scales. Figures 2.19 and 2.20 illustrate the carbon cycle with turnover times less than 1000 years (fast cycle) and in the order of 100 million years (slow cycle).

Schematically, it is important to show that the carbon cycle is not a single cycle but rather several closely linked cycles that occur on highly variable spatial (volume of the reservoirs) and temporal (turnover time) scales.

The Major Reservoirs for Carbon and its Processes of Transformation It is beyond the scope of this book to provide an exhaustive description of all the mechanisms linked to the carbon cycle. However, the following section discusses its main reserves and the flows that result.

Atmosphere (~747 Pg)

~93 Pg ~90 Pg ~120 Pg ~55 Pg deforestation

(~1-2 Pg)

Fig. 2.19 : Size of Reservoirs in Petagram (1 Pg = 10¹⁵g)) and fluxes [Pg/year] of the Carbon Cycle with a Renewal Time of less than 1000 years (Bolin, 1983).



HCO3

Atmosphere and the Greenhouse Effect

Carbon is present in the atmosphere in the form of carbon dioxide (CO₂), and the carbon molecule is also found in other gases such as carbon monoxide (CO) and methane (CH₄). It has been the subject of extensive scientific study because of the role carbon dioxide plays in the greenhouse effect and the problem of the increasing average temperature of the Earth’s surface. Despite its apparent “youth,” scientists have been studying the phenomenon known as the greenhouse effect for many years.

Although it was first described by J. B. Fourrier in 1827, it was Swedish chemist S. Arrhenius who proposed in 1895 that carbon dioxide emissions into the atmosphere could contribute to the increase in the planet’s average temperature by reinforcing the greenhouse effect.

Most of the solar radiation absorbed by the Earth is visible radiation, but some of this is reflected back into the atmosphere in the form of infrared. If this did not occur, the surface of our planet would continue to heat up. However, a portion of this infrared radiation is not re-emitted into space because it is absorbed by a number of gases called the greenhouse gases, and this allows the Earth to maintain an average surface temper-ature of 15°C. Without this greenhouse effect, the average tempertemper-ature would be - 18°C. The greenhouse effect was a critical factor in the appearance of life on Earth, and continues to make life possible. Among the gases that participate in the greenhouse

Sedimentation:

Fig. 2.20 : Principal reservoirs and fluxes [Pg/year] for the carbon cycle in the Earth’s Crust, with a characteristic turnover time in the order of 100 million years (Bolin, 1983). The left side of the figure concerns the oceans, while the right side shows the terrestrial carbon cycle.

The mass unit employed is the Petagram (Pg) or 10¹⁵ grams, unit of flux is Petagram / year.

effect, the main ones are water vapor and carbon dioxide, but the mix also includes methane (CH₄), nitrous oxide (N₂O), carbon monoxide (CO), ozone (O₃), chlorofluo-rocarbons (CFC) and their substitutes hydrochlorofluochlorofluo-rocarbons (HCFC), and a number of volatile organic compounds.

The greenhouse phenomenon is an essential regulating mechanism for life on Earth, but it has been demonstrated that increases in the greenhouse gases attributable to human activities (the burning of fossil fuels, agriculture, deforestation, etc) have raised the average temperature of the Earth. Without going into the probable conse-quences of this heating of the Earth, we should note that, for example, the rate of CO₂ increased from approximately 275 r20 ppmv (parts per million in volume) in about 1850 to 360 ppmv in 1992 (Rousseau and Apostol, 2000), thus raising the percentage of CO₂in greenhouse gases from 30% to 50%. Nonetheless, most of the greenhouse effect is still attributable to water vapor.

The following two figures (Figures 2.21 and 2.22) show the drastic increase in the carbon dioxide content of the atmosphere since the mid 19^th century in tons of carbon per capita, and since 1961 in parts per million of volume at the observatory of Mauna-leasing (Hawaii). The seasonal variations in CO₂ are due to photosynthesis. In spring, plants absorb a great quantity of CO₂ which they release in winter. An analysis of the CO₂ emissions and gross domestic products (GDP) of 162 countries for the years 1995, 1996 and 1997 shows a correlation of about 0.86 between these two measurements for the three years, which is considered statistically significant (Figure 2.23).

Despite the uncertainties involved in future projections concerning the conse-quences of increased greenhouse gases in the atmosphere, experts agree that global temperature will rise by 1.8^°C with an uncertainty of 0.8^°C by the year 2030 or even earlier (about 2010). It is important to recognize that this figure is an average; the amount of increase could vary drastically depending on geographic location and with

1955 1963 1971 1979 1987 1995

300

Fig. 2.21 : Monthly evolution of carbon dioxide content in the atmosphere from 1959 to 1989 measured at Mauna-loa (Hawaii).

the seasons. For example, the countries in southern Europe could see an increase in their average summer temperature of more than 2^°C. Other scenarios show a higher increase in winter temperatures compared to summer temperatures. This is particularly the case for the countries of central and northern European (Bader and Kunz, 1998;

Dessus, 1999). In addition to these increases in the surface temperature of the Earth, the entire water cycle and its energy exchanges will be altered, with the following probable consequences:

1850 1875 1900 1925 1950 1975 2000

year

tons of carbon per capita

Fig. 2.22 : Change in carbon dioxide content in the atmosphere since 1850 in tons of carbon per capita.

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

gross domestic product [$/capita]

Fig. 2.23 : Relationship of CO₂emissions in tons of carbon per capita to gross domestic product, 1999. Only a few countries are shown for the purpose of illustration. (Data from

World Bank, http://www.worldbank.org/data/).

• An average rise in sea level of approximately 50 cm, and all the consequences this implies for heavily populated coastal areas. Right now, 16 cities with more than 10 million inhabitants are located in coastal zones.

• Magnification of extreme climatic conditions, which is to say more storms, lengthening droughts, increased frequency of severe floods, etc.

•,ncreased precipitation in winter and decreases in summer.

Finally, beyond these physical aspects, it is essential to understand that world climate change means profound changes for all humanity, widening the gap between developed and less developed countries because it will worsen existing water shortages in an unprecedented fashion, and thus intensify food shortages.

In summary, we can draw the following conclusions. Increased quantities of greenhouse gases in the atmosphere due to the greenhouse effect increase the absorption of infrared radiation, and consequently raise the surface temperature of the Earth. This increase of the greenhouse effect causes changes throughout the entire hydrological cycle, and in particular, increases the process of evaporation, which adds to the increased greenhouse effect. It also causes modifications to circulation in both the world’s atmosphere and its oceans.

The Hydrosphere

Carbon is present in the liquid milieu in both organic and inorganic forms, dissolved or particulate. The oceans contain about 98% of the mobilizable carbon reserves on the Earth’s surface. The division of the carbon contained in the atmosphere and in water is strongly linked to pH and to the total quantity of inorganic carbon in the surface layer of the ocean, (that is, up to a depth of about 300 meters).

The ocean’s carbon dioxide content and the mechanisms of exchange are highly influenced by the metabolism of plants and the living environment. Carbon dioxide content is thus dependent on physical mechanisms such as the equilibrium with the atmosphere and the equilibrium between soluble and insoluble carbonates. The carbon dioxide content also depends on biological mechanisms such as photosynthesis and the respiration and decomposition of organic matter.

It needs to be stressed that although the carbon dioxide content of sea water is dependent on pH, the same does not hold true for fresh water, where CO₂content decreases as temperature rises.

Sea water maintains a pH balance of between 8 and 8.3, although this can rise to as high as 9 if photosynthetic activities are especially intense.

The CO₂ present in water is closely related to the CO₂ in the oceans by the means of a chain of dissociation of carbonates and bicarbonates that can be expressed as follows (Frontier and Pichod-Vidale, 1998):

This chain of dissociation has two distinct parts. The first corresponds to the

disso-ciation of carbon dioxide and the formation of carbonic acid (slow reaction). The second part of the reaction is relatively rapid. This chain shows that there is a mechanism of self-regulation for the quantity of CO₂ through the displacement towards the right of the equilibrium between CO₂ in its gaseous and dissolved forms.

The Lithosphere

Carbon in the solid phase is formed mainly of the carbonates (75% of the carbon in the Earth's crust), which include calcite CaCO₃, aragonite CaCO₃ (which has an orthorhombic crystal structure as opposed to the rhomboedric structure of calcite), siderite FeCO_s and magnesite MgCO₃. The origin of carbonaceous rocks is either sedimentary (calcite, aragonite, siderite, magnesite) or metamorphic rocks. Sedimen-tary rocks result from a transformation called diagenesis. There are two major categories of sedimentary rocks, one where the ratio of CaCO₃ is higher than 60%, and the second with a ratio of between 30% and 60%. The metamorphic rocks are the result of the transformation of rock as a result of changed physicochemical conditions.

The lithosphere constitutes the largest carbon reservoir, since it contains all the fossil fuels such as coal, oil and natural gas. Coal comes from the sedimentation of plant remains (forests of the Palaeozoic era, approximately 300 million years ago) at the bottom of the oceans or in natural depressions that were later covered by new soil.

Coal, which includes 70% to 90% of carbon-based substances, is a generic term that includes three fossil fuels rich in carbon: peat, lignite and hard coal (Rousseau and Apostol, 2000).

The second fossil fuel containing carbon is petroleum, which was formed by the decomposition of living organisms in the oceans approximately 500 million years ago.

Carbon accounts for approximately 80% to 90% of the mass of petroleum. During this ancient and slow period of decomposition of organisms and microorganisms, pockets of natural gas were sometimes formed. Natural gas is in fact a mixture of methane CH₄, propane C₃H₈, butane C₄H₁₀, carbon dioxide and other hydroxides that may also con-tain nitrogen or sulfur compounds.

The Role of Photosynthesis

Although we have already mentioned the role played by living matter in the carbon cycle, especially for the production of biomass, we need to add an essential process, formed by the cycle between photosynthesis and respiration which absorbs or produces carbon.

We have already discussed the significance of the interactions between all the cycles in which carbon plays a part with the other cycles of matter such as water and the closely connected energy cycle. The main processes for the production of carbon dioxide in the atmosphere are the biological or mineral precipitation of carbonates, the deterioration of basalts in the oceanic crust, breathing or fermentation of the biotic medium, metamorphism of carbonates, human activities, and volcanic degassing. The major processes of carbon dioxide consumption are the weathering of silicates, the dissolution of carbonates, photosynthesis, and the storage of carbon in fossil organic

matter. All these processes can be organized into three major cycles:

• The precipitation and dissolution cycle,

• The respiration and photosynthesis cycle,

• Degassing, precipitation and storage.