148 EHHALT 



annual production is quite large, 6.1 X 10 1 g/year. One can argue, however, 

 that the production by humid tropical areas is already included in Koyama's 1 l 

 original estimates, given in lines 6 and 7 of Table 2, because the tropical areas are 

 either forests, grass, brushland, or cultivated areas. Koyama's 1 ! values of annual 

 production by these sources, however, are much smaller than the value assumed 

 by Robinson and Robbins. 10 



If we, for the moment, accept all these figures and combine all the lower 

 estimates and then all the higher estimates for the various sources, we obtain 

 5.2 X 10 g/year as a lower limit and 15.1 X 10 1 g/year as an upper limit for 

 the "known" biogenic production of CH 4 . 



There are certainly more biogenic sources of atmospheric CH 4 which might 

 be of global importance, but their strength has not yet been estimated. One 

 example is the ocean, especially coastal waters. It has been shown by Swinnerton 

 and Linnenbom that ocean water contains dissolved CH 4 and is probably 

 slightly supersaturated with respect to the partial pressure of atmospheric CH 4 . 

 In addition, there are a few vertical profiles that show an increase in dissolved 

 CH 4 with depth in Gulf of Mexico water and also in the open ocean. 13 At 

 present there are not sufficient data to allow an estimate of oceanic production. 

 Considering the vast area, however, this source might be important. 



Of course there are other, nonbiologic sources, but geothermal areas, coal 

 fields, gas wells, industrial areas, and internal combustion engines constitute only 

 minor, although highly localized, sources. We may lump these sources of 

 1 C-free (dead) CH 4 together and assume, on the basis of the 14 C content of 

 atmospheric CH 4 , that their production is at most 25% of the biogenic 

 production. The total annual CH 4 production then lies between 6.5 X 10 14 and 

 19 X 10 14 g/year. 



It is interesting to compare the total biogenic production of CH 4 to the 

 figures listed in the last three lines of Table 2. From this comparison we find 

 that the release of biogenic CH 4 to the atmosphere equals or exceeds the annual 

 production of CH 4 from natural-gas wells. We further find that the released CH 4 

 is about 1% of the annual production of dry organic matter. Since the energy 

 content of CH 4 is about three times that of cellulose, about 1 to 3% of the solar 

 energy fixed by photosynthesis is lost to the atmosphere as CH 4 and escapes the 

 biologic food chain. Finally, by comparing the biogenic production rate of CH 4 

 to the total amount of CH 4 present in the atmosphere, we obtain a CH 4 

 turnover time of 2.6 to 8 years (see Table 4). 



SINKS OF ATMOSPHERIC CH 4 



Since the CH 4 cycle is in steady state, the production of CH 4 has to be 

 matched by its destruction. It was well known that CH 4 reacts with atomic 

 oxygen, especially with the excited 0*D atom, and with the OH radical. 

 However, it was assumed that the concentration of OH (and O) was too low in 



