Biospheric growth rate

Since the biospheric growth rate depends, among other factors, on the C02 supply, it is probable that the C02 increase induces, at least for part of the biosphere, an increased growth rate ("C02 fertilization"). A simple concept to take this into account is the introduction of a biota growth factor e if the atmospheric C02 pressure increases by p percent, the C02 flux to the biosphere increases by zp percent. Typically, values for e between 0 and 0.5 have been used in carbon cycles models [26,41]. 

Both rates of carbon exchange between the atmosphere and the biosphere and between the atmosphere and the ocean are sensitive to climate variations. It is therefore not surprising that over the past 20 years, the growth rate of atmospheric CO2 has varied by a factor of 2 even though fossil fuel source has increased steadily (Fig. 7). 

Table 11. Growth of liver and stomach cancer rates in the Samarkand nitrate biogeochemical province of the Desert region of biosphere, Uzbekistan, cases per 100,000 individuals.

For example, in the carbon cycle consider the balance between terrestrial photosynthesis and respiration-decay. If the respiration and decay flux to the atmosphere were doubled (perhaps by a temperature increase) from about 5200 x 1012 to 10,400 x 1012 moles y-l, and photosynthesis remained constant, the CO2 content of the atmosphere would be doubled in about 12 years. If the reverse occurred, and photosynthesis were doubled, while respiration and decay remained constant, the CO2 content of the atmosphere would be halved in about the same time. An effective and rapid feedback mechanism is necessary to prevent such excursions, although they have occurred in the geologic past. On a short time scale (hundreds of years or less), the feedbacks involve the ocean and terrestrial biota. As was shown in Chapter 4, an increase in atmospheric CO2 leads to an increase in the uptake of CO2 in the ocean. Also, an initial increase in atmospheric CO2 could lead to fertilization of those terrestrial plants which are not nutrient limited, provided there is sufficient water, removal of CO2, and growth of the terrestrial biosphere. Thus, both of the aforementioned processes are feedback mechanisms that can operate in a positive or negative sense. An increased rate of photosynthesis would deplete atmospheric CO2, which would in turn decrease photosynthesis and increase the oceanic evasion rate of CO2, leading to a rise in atmospheric CO2 content. More will be said later about feedback mechanisms in the carbon system. 

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