PREDICTION OF C0 2 IN THE ATMOSPHERE 31 



atmospheric C0 2 concentration by the year 2000 of 394 ppM rather than 386. 

 The increased C0 2 concentration of 8 ppM according to Manabe and 

 Wetherald's calculation (Fig. 5 of my paper) will result in an additional warming 

 of less than 0.1 °C, which is well within the uncertainty of the climatic 

 prediction. 



Deevey: There is a third box between the atmosphere and the deep ocean, 

 i.e., humus, which has a size about the same as the biota and the mixed ocean 

 layer. Residence time in humus is of the order of hundreds of years. Would your 

 results be significantly influenced by the addition of such a box? 



Machta: Qualitatively, the additional removal of fossil-fuel C0 2 into the 

 humus would decrease the atmospheric growth rate in the model and thus 

 improve the fit between the 1958 to 1970 increment of 9 ppM observed at 

 Mauna Loa with that of the model estimate of 12 ppM. However, over 10% of 

 the total net primary production of the land biota would have to be transferred 

 to the humus to reduce significantly the 12 ppM increment in model 

 calculations. 



Shotkin: Would you care to speculate on what mechanisms in the 

 atmosphere might be responsible for the temperature decrease observed over the 

 last 25 years? 



Machta: Dr. Murray Mitchell is in the audience and is much better able to 

 answer the question than I. 



Mitchell: Of course I do not really know what is responsible for the cooling, 

 but I have the feeling that part of the answer lies in the accelerated pace of 

 explosive volcanic eruptions we have had in the tropics during the past quarter 

 century. These eruptions have pumped a lot of tephra into the middle 

 stratosphere after a hiatus of two or three decades. Dust at those altitudes tends 

 to backscatter a significant fraction of solar radiation to space and therefore 

 decreases the solar heating near the ground. Since the dust particles are too small 

 to affect appreciably the long-wave terrestrial radiation escaping from below, the 

 net effect of the dust is to cool climate. Other factors may enter the picture too. 

 For example, anomalous long-period thermal interactions between the oceans 

 and the atmosphere might account for part of the cooling, but I really cannot 

 say what these interactions look like or precisely how they operate. 



I should add something else. You have probably heard claims recently that 

 the increasing dust loading of the atmosphere from human activities may 

 account for the cooling. When dust enters the atmosphere from sources near the 

 surface, the situation is not the same as that of stratospheric dust injections from 

 volcanoes. Here it becomes crucial to know how efficiently the dust absorbs 

 solar radiation as well as how much radiation it backscatters. The fact is, we do 

 not know enough about the optical properties of anthropogenic dust to 

 determine whether it is tending to cool the earth or to warm it up. So to blame 

 the cooling trend on increasing atmospheric turbidity from man's activities is a 

 bit premature. 



