Biotic regulation: your questions
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Your questions on biotic regulation (September 2007)

How can we as humans reduce our net primary productivity consumption?

What are the spatial boundaries of the biota (in kilometers)?

What is the minimum spatial volume where one could see the greenhouse effect? E.g., would a cube of 1 m3 filled with chlorofluorocarbons suffice?

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Other biotic regulation questions and answers

 

How can we as humans reduce our net primary productivity consumption?

Asked by an anonymous visitor of our site on the 28th of September, 2007. Individual food consumption is fixed by the biological design of human beings. It cannot be diminished by any economic means. Therefore, the only way of reducing consumption of primary productivity by the humanity is to reduce the global population number of humans. This cannot be done at once. But due to the limited time span of human beings, it is possible, via regulation of birth rates, to reduce the global population number by an order of magnitude (i.e., approximately tenfold) in one century without compromising the social structure of the society and preserving progress in science and technology. For a quantitative analysis of the dynamics of such a reduction, see Appendix 1 in the text on demography (so far in Russian only).  

What are the spatial boundaries of the biota (in kilometers)?

Asked by Maksim, our site visitor, on the 28th of September, 2007. Biota is spread over entire surface of our planet, which is equal to 5.1 x 108 km2, or 510 million square kilometers. Considering the volume occupied by the biota, we should take into account depth of the oceans (\sim 4 km on average) and height of the atmosphere, where living organisms can still be found (up to 6 km). Taking oceanic depth 3.6 x 108 km2, we obtain that the volume occupied by the biota is equal to atmospheric volume 5.1 x 108 km2 by 6 km, plus oceanic volume 3.6 x 108 km2 by 4 km. This gives 4500 million cubic kilometers in total.  

What is the minimum spatial volume where one could see the greenhouse effect? E.g., would a cube of 1 m3 filled with chlorofluorocarbons suffice?

Asked by an anonymous visitor of our site on the 28th of September, 2007. The greenhouse effect depends on the so-called optical thickness t, which is defined as the ratio of the geometric thickness (height) z of the considered area to the free path length l of thermal photons, t = z/l. Free path length l = 1/ns, where n is concentration of greenhouse substances (number of molecules in one cubic meter), s is the absorption cross-section (in square meters) characterizing absorption of thermal photon by one molecule.
 
In the atmosphere the optical thickness of CO2 is of the order of unity, t ~ 1. Taking thickness of a uniformly dense atmosphere to be of the order of 10 km, we obtain that free path length is also 10 km (photons travel on average 10 km before they are absorbed by a CO2 molecule). Air density is 2 x 1025 molecules/m3, the relative amount of CO2 is 300 ppm = 3 x 10-4. This means that CO2 concentration is 6 x 1021 molecules/m3 and, hence, sCO2 ~ 2 x 10-26 m2. To achieve optical density t ~ 1 at z = 1 m, one has to increase CO2 density by 104 times, i.e. up to 6 x 1025 molecules/m3, three times air density, which is technically possible. Absorption cross section of chlorofluorocarbons (CFCs) is about four orders of magnitude larger than that of CO2. Therefore, in order to obtain t ~ 1 at z = 1 m for CFCs one needs to attain a CFC concentration of 6 x 1023 molecules/m3, i.e. a concentration equal to that of atmospheric CO2. This is technically achievable. Generally, the greater the concentration, the smaller volume one will need to see a preset value of the greenhouse effect.
 
Further reading: Gorshkov V.G., Makarieva A.M. (2002) Greenhouse effect dependence on atmospheric concentrations of greenhouse substances and the nature of climate stability on Earth. Atmospheric Chemistry and Physics Discussions, 2, 289-337. Abstract, PDF and discussion.
 

Main page in English Russian version of current document   2 October 2007.
© 2007 V.G. Gorshkov, A.M. Makarieva