Variations of C02 level

One can see from Table 2 that the so-called equilibrium C02 level in the Earth's atmosphere would be higher than the present concentration. It can also be seen that Venus has a much greater C02 partial pressure than the terrestrial value. The difference between the two neighbouring planets can first of all be explained by the difference in their distance from the Sun which produced different initial temperatures. In the case of the Earth the temperature was less than 300 °C. Under these conditions gaseous C02 formed carbonate minerals by the following type of reaction (Urey, 1952):

which decreased the C02 level in the atmosphere to an important degree. Thus, the great majority of terrestrial C02 accumulated in the solid Earth as limestone and dolomite, while a huge amount of C02 on the Venus remained in the gaseous phase.

The second, but probably less important, factor in the decrease of C02 level of our atmosphere was the biospheric activity since plants used (and use) C02 to form organic carbon compounds (see reaction [2.4]). In any case this factor could only operate after the first had decreased the temperature below 100 °C. It seems likely that according to this process C02 and 02 levels varied inversely in the biospheric era (Rutten, 1966) of the Earth's history. Thus, the variations in Fig. 3 representing the increase of 02 levels during geological times also give an idea about the changes in atmospheric C02 concentration.

Finally, the C02 sources (respiration, decay of plants and humus) and sinks (photosynthesis of plants) became quasi-balanced. In establishing this natural equilibrium state the hydrosphere also played an important role. Before the industrial revolution of the 19th century, the COz production of the biosphere was around 480 x 109 t yr"1 (Butcher and Charlson, 1972), while the total amount of C02 burden was estimated to be 22S0 x 1091 (Bolin, 1977a). According to equation [1.1] these figures yield a rounded-off value of 5 years for the residence time.

The concentration of C02 in the air in the lowest layers of the troposphere shows expected diurnal and yearly variations (Junge, 1963). For example, photosynthesis can occur only in the presence of solar radiation. For this reason, in clean (non-

Variation of the C02 level in the atmosphere between 1880 and 1960 according to Junge (1963). (By courtesy of Academic Press and the author)

Variation of the C02 level in the atmosphere between 1880 and 1960 according to Junge (1963). (By courtesy of Academic Press and the author)

polluted) air the daylight concentrations are generally smaller than those measured during the night. The amplitude of this diurnal variations obviously decreases with increasing height. Furthermore, in accordance with the annual variation in plant metabolism, maximum C02 concentrations can be observed in the Northern Hemisphere at the beginning of the vegetation period (early spring). Correspondingly, the C02 concentration shows a yearly minimum during the autumn. These yearly variations become less pronounced with increasing altitude above ground level in the troposphere and they became negligible above the tropopause (Bischof and Bolin, 1966). In contrast, in the Southern Hemisphere, the amplitude of yearly variations is small even in the surface air. This results from the world-wide distribution of land biota. More exactly this means that forests are concentrated in this hemisphere in the tropics where the yearly changes in the intensity of solar radiation and in vegetation can practically be neglected.

The most important peculiarity of the variation of C02 level is its present gradual increase which apparently began at the end of the last century (Fig. 4). Thus, the concentration of carbon dioxide increased from the pre-industrial value of less than 300 ppm to 327 ppm in 1975 (Bolin, 1977a). Accordingly the present total atmospheric C02 mass is estimated to be 2,500 x 1091, about 11 % greater than its value before the industrial revolution. In other words this means that the delicate balance of C02 among different parts of the biosphere is already disturbed by the activity of men (see Chapter 6). All C02 sources, including human activity, at present yield around 500 x 109 t yr"1.

It should be mentioned that only about the half of anthropogenic C02 remained airborne in the past decades.4 However, this does not necessarily mean that the fraction of man-made COz stored in the atmosphere will always be the same in the future. For this reason it is essential to determine from past variations the factors governing the fate of anthropogenic carbon dioxide. It is also essential to include these factors in so-called reservoir or box models5 to calculate, on the one hand, the fraction absorbed by oceans and, on the other hand, the part of the emission used by the land biota. Since the uptake of carbon dioxide by ocean waters is governed by more or less known physical and chemical laws the response of land plants to the increase of C02 level, which is much more complicated, can be estimated by difference between total C02 input and oceanic absorption (e.g. Keeling, 1973).

We have to emphasize that the correct prediction of the future COz concentrations is one of most important tasks of atmospheric science at present. This is explained by the fact that the C02 content of our atmosphere regulates, among other things, the radiation balance of the Earth-atmosphere system by absorbing infrared radiation emitted by the surface. Thus, we cannot exclude the possibility that the increase of the carbon dioxide concentration may cause inadvertent climatic variations in the future (see Chapter 6).

* This can be estimated by comparing the value calculated from fossil fuel combustion to the increase of total atmospheric C02 burden.

s The reservoirs which must be taken into account are: the atmosphere, the oceans and the land biota. It is useful to divide both the ocean and land biota into two further reservoirs: surface ocean and deep ocean as well as short-lived and long-lived land biota (Keeling, 1973).

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