TOTAL YIELD OF PHOTOSYNTHESIS OX EARTH 9 



reflected by ice and snow, so that not much more than 2 X 10'^ cal. can 

 be allocated to plant-covered land and plankton-filled sea. Of this 

 amount, at least lO^c are lost by reflection on water surface, and further 

 losses must occur through the absorption of visible light by water (c/. 

 Vol. II, Chapter 22) and dissolved ions. On land, too, 10 or 20% of all 

 radiation which falls on plant -covered areas is lost by diffuse reflection. 

 (Woods and forests are more efficient light traps than meadows and fields, 

 and therefore appear as dark spots on aerial maps.) 



Altogether, not more than 1.5 X 10-^ cal. are absorbed annually by 

 the plant pigments, and can thus be utilized in photosynthesis. It will 

 be shown in volume II, chapter 28 that, under natural conditions, the 

 energy conversion yield of green plants is of the order of 2%. This 

 brings us to the figure 3 X lO'-^ cal. for the probable annual energy 

 accumulation by photosynthesis, corresponding to the formation of 

 3 X 10" tons of organic carbon. (The heat of combustion of organic 

 matter is approximately 10^" cal. per ton of carbon contained in it.) This 

 agrees with the value derived above from crop estimates, and confirms 

 the utter impossibility of the much larger figures of Vernadsky. 



The reduction of carbon dioxide by green plants is the largest single 

 chemical process on earth. To make clearer what a yield of 10" tons per 

 year means, we may compare it with the total output of the chemical, 

 metallurgical, and mining industries on earth, which is of the order of 

 10^ tons annually. Ninety per cent of this output is coal and oil, i. e., 

 products due to photosynthesis in earlier ages. Similarly impressive is 

 the comparison of the energy stored annually by the plants, with the 

 energy available from other sources. The energy converted by photo- 

 synthesis is about one hundred times larger than the heat of combustion 

 of all the coal mined on earth in the same period, and ten thousand times 

 larger than the energy of falling water utilized in the whole world. On 

 the other hand, about three hundred times more solar energy is spent on 

 the evaporation of water from the oceans and continents than is utilized 

 in photosynthesis. Plants alone spend, according to the estimates of 

 Schroeder (1919-), l(i X 10-^ cal. annually on transpiration, or more than 

 ten times more than on photosynthesis. Since aciuatic plants have no 

 need for transpiration, all this energy is used up by land plants. The 

 ratio of transpiration energy to the energy of photosynthesis is, for land 

 plants, of the order of 50 or 100 to 1. 



The carbon dioxide cycle in, nature is of great importance for the 

 climate of the earth; even small changes in its photostationary state, 

 which probably have occurred in the history of the earth, must have had 

 far-reaching consequences. It is probable, for example, that in the pre- 

 glacial age, the carbon dioxide concentration in the air was higher than 

 now, and the climate warmer (because carbon dioxide prevents the escape 



