330 CARNEGIE INSTITUTION OF WASHINGTON. 



While it is quite certain that the average molecule of CO2 in the lower 

 part of the atmosphere is warmer than the average molecule of nitrogen, the 

 opposite contingency must be recognized. In the uppermost atmosphere 

 the CO2 may, indeed, habitually become the cooler, because it may there 

 radiate outward more than it absorbs from below. This radiated portion, 

 however, reenters the secondary series of absorptions and radiations sketched 

 above, but, by hypothesis, high in the atmosphere, with good chances of 

 early escape. In this case, the CO2 draws on the heat stored in the nitrogen 

 pockets or reservoirs and puts it again in the way of escape. Carbon dioxide 

 may thus serve as a cooling agency. In the large view, its function is regu- 

 lative, serving as a door of entrance into temporary storage for earth heat 

 and of exit into the mechanism of escape for any excess of heat in the body of 

 the atmosphere. But at the very low temperatures of the upper atmosphere 

 the radiating power of CO2 is low, since, according to Stefan's law, radiation is 

 proportional to the fourth power of the absolute temperature. 



Carbon dioxide is a "permanent" constituent of the atmosphere in the 

 sense that it remains a gas under ordinary conditions and is distributed 

 nearly uniformly in all latitudes and probably at all altitudes that much 

 affect the lower climatic zone. When, however, geological eras are con- 

 sidered, it is subject to important fluctuations. One type arises from its 

 effort to maintain equilibrium with many times its own amount in the ocean, 

 partly in a free and partly in a loosely combined state. The very massiveness 

 of this combination makes its fundamental changes slow. At the same 

 time, this gives range and capacity, and to that extent fits it to play a part 

 in climatic changes of the long-period type. A lowering of the temperature 

 of the ocean of 21° C. in the vicinity of the freezing-point about doubles the 

 amount of CO2 required for equilibrium, and here is another source of varia- 

 tion of the long-period order. The extent to which CO2 is consumed in the 

 solution of hmestone and other carbonates and in the formation of plant tissue 

 is still another that rises to the geologic order. Its great capacity for com- 

 bination is undoubtedly the reason why the quantity of CO2 now left in the 

 atmosphere is so small. The varying of the rate of combination as the land 

 is elevated and extended, on the one hand, or as it is base-leveled and sub- 

 merged on the other, must apparently lead to increases and diminutions of the 

 free and potentially free CO2 in the air and the ocean. These run hand in 

 hand with the more profound geologic movements and give to this atmospheric 

 factor a direct geologic relation. But it is itself dependent and hence an 

 analysis of the whole complex combination is necessaiy to its evaluation. 

 CLIMATIC FUNCTIONS OF THE VARIABLE CONSTITUENT, WATER- VAPOR. 



The third thermal factor in the atmosphere differs radically from the 

 others in its variability, the rapidity of its changes, and the contrasts of its 

 successive forms. The thermal effects of its entrance and exit are not less 

 important than its thermal service while in the air. The variation of the 

 amount of water-vapor in the air, from the low content in arid and frigid 

 regions to oversaturation in the torrid tracts, is some hundreds per cent of 

 its minimum. Water-vapor is perpetually being thrown into the air from the 

 earth's surface by evaporation and is soon sent back by precipitation as hquid 

 or solid. It runs rapidly through the gamut, solid-liquid-vapor-liquid-solid, 

 or some part of it, with extraordinary changes of thermal properties. It 

 therefore presents in itself a complex of thermal problems. 



