GEOLOGICAL AND HISTORICAL ASPECTS OF CLIMATIC CHANGE 
out by Huntington [15]. At present that country is 
covered by dense forests and the hot moist climate 
is enervating and inimical to a high culture. Buried 
Tasxp III. Ciimatic VARIATIONS DURING THE CHRISTIAN HRA 
Western North 
A.D Europe Asia aces Africa 
0 | As present Slightly As present | Good Nile 
rainier floods 
than now 
100 | Somewhat Rainy | Drier 
drier 
200 
Rainy 
300 Dry 
Dry Rainier 
400 Less dry 
Caspian Dry Rainy 
—15 ft 
500 | Drier 
Dry Rainy 
600 | Rather dry Slightly 
rainier 
Rainfall in- Rainy 
creasing 
700 Drier Drier 
Dry, warm | Rainy 
300 Dry _ pe- | Dry 
riod 
ended 
Rainier Rainy in 
China 
900 Caspian Rainier Rainier 
+ 29 ft 
Drier Slightly 
drier 
1000 Drier 
Colder Dry in China| Very rainy 
1100 | Heavy rain | Dry 
Caspian Dry Very dry 
—14 ft 
1200 |Rainy Dry 
Very stormy | Rainfall in- | Dry Rainy 
increasing 
1300 | Glacial ad- | Rainy, Cas- | Rainy Rainy 
vance, pian, etc. 
drier high 
1400 
Glacial min. | Dry in China| Dry Rainy 
1500 | Oceanic Rainy, Cas- Rainfall 
pian +16ft maxi- 
Continental mum 
1600 | Rapid ad- | Rainy Rainier Drier 
vance of | Caspian 
glaciers +15 ft 
1700 | Dry in west | Near present 
Glacial max. | Caspian 
rather 
high . 
1800 | Cold, rainier Rainy 
1900 | Rapid re- | Caspian fall- | Drier 
treat of ing 
glaciers 
1009 
in the forests are the ruins of magnificent Mayan 
cities, and it is inconceivable that these could have 
been built under present conditions. The first period 
of culture lasted from about 400 B.c. to a.p. 300. 
From a.p. 800 to 450 the forest encroached from the 
south, and from 450 to 900 it extended over the whole 
country; culture declined to a low level. There was a 
climatic recrudescence from 900 to 1100, deterioration 
from 1100 to 1300, and a slight improvement from 
1300 to 1450, after which conditions were continuously 
unfavourable. The dry periods in Yucatan coincide 
almost exactly with the rainy periods farther north. 
Special mention must be made of the advance of 
the glaciers in the Alps, Scandinavia, and Iceland, 
which began near the middle of the 16th century and 
was so striking that it has come to be known as the 
“Tittle Ice Age.” It may be divided into six periods: 
1. First advance, A.D. 1550-1650 (first maximum). 
About 1605, glaciers overran settlements which had 
been occupied since early days. 
2. Recession, 1650-1680, followed by fluctuations 
about a generally small extent until 1715. 
3. Rapid advance to a maximum about 1750-1760, 
which in many districts marks the greatest extension 
of glaciers since the Ice Age. 
4, Retreat until about 1790, but with fluctuations. 
5. Advance to a third maximum about 1850, with 
a minor maximum about 1815. 
6. General retreat, interrupted about 1890, then be- 
coming increasingly rapid. In Norway and Iceland the 
glaciers have not yet retreated to the positions they 
occupied in the 14th century. 
Generally speaking, the glacial advances were asso- 
ciated with cold winters and cool springs, and the 
retreats with strong westerly winds and a mild mari- 
time climate. The variations do not appear to be 
related to variations of total precipitation, but most 
probably indicate variations in the length of the period 
of ablation. 
Similar but less accurately dated fluctuations have 
occurred in glaciers in other parts of the world. In 
Spitsbergen they reached a maximum about the middle 
of the 19th century and have since retreated or become 
stagnant. The marginal glaciers of northeast Greenland 
have been receding since the late 18th or early 19th 
century. Alaskan glaciers for the most part reached 
maxima in the 18th or 19th century and have since 
receded, but some are still advancing, probably because 
their sources are at considerable heights. 
The broad pattern of climatic change since the end 
of the Ice Age is consistent with the hypothesis of an 
alternate weakening and strengthening of the planetary 
atmospheric circulation, associated with a poleward 
and equatorward shift of the wind zones. At times of 
minimum circulation the circumpolar vortex is con- 
tracted and anticyclones are frequent in middle lati- 
tudes. Winds are variable, rainfall is small, and climate 
continental, with cold winters and hot summers. This 
was the general situation during the long dry period 
from about a.p. 400 to 1000. When the circulation 
is stronger, westerly winds predominate, rainfall is 
