Introduction

Cyclic processes of exchange of carbon mass are of particular importance for the global biosphere, both in terrestrial and oceanic ecosystems especially owing to the close connections to the global climate changes.

This element is distributed in the atmosphere, water, and land as follows. According to existing data there are 6160 x 109t or 1.4 x 1016 mol of CO2 in the atmosphere (1680 x 109t of C). A major source of atmospheric carbon dioxide is respiration, combustion, and decay, compared with oxygen, whose main source is photosynthesis. In its turn, an important sink of CO2 is photosynthesis (about 66 x 109tyr-1 or 1.5 x 1015molyr-1). Since carbon dioxide is somewhat soluble in water (KH = 3.4 x 10~2 mol l-1 atm-1), exchange with the global ocean must also be considered. The approximate global balance of atmosphere-ocean water exchange is 7 x 1015molyr(308 x 109tyr-1) being taken up and 6 x 1015molyr-1 (264 x 109tyr-1) being released in different parts of the oceanic ecosystem. The residence time of CO2 in atmosphere is about 2 years, which makes the atmospheric air quite well mixed with respect to this gas.

However, a more recent analysis shows that the terrestrial ecosystems have much stronger sinks of carbon dioxide uptake.

In the global ocean, along with occurrence in living organisms, carbon is present in two major forms: as a constituent of organic matter (in solution and partly in suspension) and as a constituent of exchangeable inorganic ions HCO-, CO2~, and CO2:

The amount of CO2(aq) in the oceans is 60 times that of CO2(g) in the Earth's air, suggesting that the oceans might absorb most of the additional carbon dioxide being injected at present into the atmosphere. However, there are some drawbacks restricting this process. First of all, CO2 uptake into surface oceanic waters (0-100 m) is relatively slow (i1/2 = 1.3 years). Second, these surface waters mix with deeper waters very slowly (t1/2 = 35 years). Consequently, the surface oceanic waters have the capacity to remove only a fraction of any increase in the anthropogenic CO2 loading (Figure 1).

100

90

Amount injected into atmosphere (100 ppm)

80

\/

70

^^^^

60

Amount taken

50

into surface waters

40

Amount taken into

30

deep ocean

20

^^

10

0

-r-—r~

-1-1-1-1-1-1-1-1-

Time (years)

Figure 1 Calculated uptake of CO2 from atmosphere to the surface and deep oceanic waters.

The known analytical monitoring data obtained over many years at the Mauna Loa Observatory in Hawaii, a location far from any anthropogenic sources of carbon dioxide pollution, show a pronounced 1-year cycle of CO2 content (Figure 2).

One can see the peak about April and then through around October each year. These data indicate that the content of carbon dioxide in the Earth's atmosphere is not perfectly homogeneous. Some explanations would be of interest to understand this figure.

Hawaii is in the Northern Hemisphere where the photosynthetic activity of vegetation is maximal in summertime (May-September). In this period CO2 is removed from the air a little bit faster than it is added. The reverse situation occurs during winter. This is a reasonable explanation and accordingly the monitoring stations in the South Hemisphere show the highest concentration of CO2 in October, and the lowest in April (see http://www.mlo.noaa.gov).

A gradual increase in the partial pressure of carbon dioxide over the last decades is clearly pointed out in Figure 2. The value of p(CO2) was c. 315ppmv in 1958, it had reached 350ppmv in 1988, and >370ppmv in the beginning of twenty-first century. Accordingly, this trend can give a doubling of carbon dioxide content in the Earth's atmosphere sometime during the end of the twenty-first century and this seems a reasonable prediction.

Here we should refer to the opinion of some other authors who have argued that increased CO2 levels in the atmosphere may be a consequence of atmospheric warming, rather than the cause. The statistical analysis of various authors led to the conclusion that, although there is a correlation between p(CO2) and global temperatures, the changes in p(CO2) appear to lag behind the temperature change by c. 5 months. A possible explanation, if this trend is proved correct, would be that natural climatic variability like the solar activity alters the temperature of the global ocean, which contains about 90% of total CO2 mass. In turn, this leads to increase of CO2 flux from the warmer oceanic water to the atmosphere in accordance with Henry's law.

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