GEOLOGICAL AND HISTORICAL ASPECTS OF CLIMATIC CHANGE 
sufficiently to prevent glaciers from forming; Wagner 
mentions a figure of 10F. Jeffreys [16], ener states 
that any appreciable rise of surface temperature from 
such a cause is not possible. 
Mountain Building and Climate. It is now generally 
agreed that the presence of mountains is a necessary 
condition for glaciation, the mountains forming a gath- 
ering ground and nucleus. It remains to be considered 
whether mountain building alone can cause world-wide 
cooling which is quantitatively swfficient to explain 
glaciation. Four factors are involved: 
1. The decrease of mean temperature with height. 
2. Reflection of solar radiation from the surface of 
clouds. 
3. Cooling power of surfaces of ice and snow. 
4. Loss of heat owing to increased evaporation. 
The average height of the land surface at present 
is about 2500 ft above sea level. At the maximum 
glaciation the average elevation may be taken as 3500 
ft. 
ICE SHEETS 
AND 
INTER- 
GLAGCIALS MILD CLIMATES 
SMALL ZONAL 
DIFFERENCES 
ELEVATION COOLING 
DENUDATION 
ELEVATION 
1013 
lowered the mean temperature over the land areas by 
about 1.5F, or 0.4F over the world as a whole. 
3. In calculating the cooling effect of increased areas 
of snow and ice, we must ignore for the moment the 
great Quaternary ice sheets, which were a consequence 
and not a cause of the initial coolmg. Hence we must 
limit the calculation to the area above the snow line 
and the surface of mountain glaciers. At present this 
is about 3 per cent of the land surface, more than half 
of which is between latitude 70° and the poles. When 
the continents were at their greatest height, this figure 
was probably rather over 6 per cent. Brooks [5] has 
calculated that if a land surface in high latitudes, 
formerly bare, has one per cent of its area covered by 
ice or snow, the mean annual temperature would be 
lowered by 0.3F. Hence the increased snow cover would 
have resulted in an average cooling of about 1F over 
the continents. Since the Quaternary elevation was 
greatest in high latitudes, the figure was most probably 
greater than this, perhaps double. Something must 
MILD CLIMATES 
SMALL ZONAL 
DIFFERENCES 
COOLING 
DENUDATION 
MOUNTAIN BUILDING AND ELEVATION 
SS SSS ZIV OO, OD. ENE 
Fic. 2.—Geographic cycle of mountain building, sea level, and climate. 
1. Owing to the cooling of ascending air by expansion 
the decrease of temperature with height is about 3F 
per 1000 ft. When the air descends again, it is warmed 
by compression, so that this effect is limited to the 
actual mountains. An elevation of 1000 ft is equivalent 
to an average cooling of 3F over the land areas, or 
0.8F over the world as a whole. 
2. Except in winter in high latitudes, clouds lower 
the mean temperature by reflecting from 60 to 75 
per cent of the solar radiation back to space. On the 
average, an increase of one-tenth in the mean cloudiness 
lowers the mean temperature by about 6F. The level 
of condensation varies according to the humidity of 
the air, but probably averages around 5000 ft over 
land areas. A mountain range increases the cloud 
amount to windward and over the crest and decreases 
it to leeward, but to a less amount. As a rough estimate 
we may assume an average increase of the cloud amount 
by three-tenths of the sky. At present 1214 per cent 
of the land is above 5000 ft, and an increase in the 
average elevation by 1000 ft is estimated to increase 
this area to 21 per cent. At the time of maximum 
elevation, therefore, increased cloudiness would have 
be added for the formation of sea ice in polar waters, 
and the average figure for the world as a whole may 
be taken as 1-2F. 
4. In the nonglaciated parts of the world the total 
precipitation, and therefore the total evaporation, dur- 
ing the great ice ages were considerably greater than 
they are now. The increase was due partly to the greater 
elevation of the land, and partly to stronger winds. 
It is difficult to form an estimate of the increase of 
rainfall, but considering all the evidence, a figure of 
20 per cent is probably a minimum. If we put the net 
fall of surface temperature (after allowmg for mereased 
back radiation from the atmosphere) as 6F at present, 
we find that increased evaporation at glacial maxima 
would have lowered the temperature by another 1.2F. 
Summing up, we find that the lowering of tempera- 
ture at the maximum elevation in the Quaternary can 
be estimated as follows: 
Decrease of temperature with height....... 0.8F 
Increased cloud amount.................- 0.4 
Increased area of snow and ice............ 15) 
Increased evaporation.................... 1.2 
The total is about 4F, averaged over the whole world. 
At the end of a long quiescent period the average 
