382 
rary freezing of shallow portions of the polar seas during 
the winter season. 
2. Complete disappearance of the anticyclonic polar- 
cap circulations with a corresponding poleward dis- 
placement of the climatic belts and zonal wind systems, 
notably with the contraction of the circumpolar storm 
tracks of either hemisphere into permanent cyclonic 
centers over the poles. 
3. Great warming of the polar regions, with a cor- 
respondingly weakened poleward temperature gradient, 
decreased circulation, and absence of storminess in 
middle latitudes. 
4. Expansion of the subtropical high-pressure belts 
with relatively storm-free, dry, and mild conditions 
prevailing through most of the middle latitudes. Pre- 
cipitation restricted to the local convective rather than 
cyclonic type, with a marked weakening of the inter- 
tropical convergence, hence with relatively dry condi- 
tions in the tropics as well as in the middle latitudes. 
This contrast between the typical glacial and inter- 
glacial climate is summarized here in its essential detail 
because it is quite characteristic of, though somewhat 
more extreme in degree than, the shorter-period cli- 
matic, secular, and anomalous fluctuations of the gen- 
eral circulation. 
Climatic Fluctuations. These include the consider- 
able fluctuations of climate during postglacial time. 
The last glacial maximum of the Pleistocene Epoch 
terminated officially some 8000 years ago, at approxi- 
mately 6500 3B.c. Geological evidence indicates that 
at that date the recession of both the Scandinavian 
and the North American ice sheets, and the correspond- 
ing amelioration of climate, had reached the point at 
which conditions stand today. However, during these 
8000 years there have occurred very substantial fluc- 
tuations of climate [4, 7, 12, 27], running in irregular 
cycles of from three to five thousand years. In ampli- 
tude these fluctuations probably have amounted to at 
least half of a glacial-interglacial cycle, and according 
to all evidence they have followed almost identically 
the glacial-interglacial pattern of climatic change. The 
evidence is quite clear that during the preceding twenty 
thousand years of the recession of the last ice sheet, 
and also during the preceding glacial maxima of the 
Pleistocene Epoch, a similar considerable secondary 
fluctuation of climate was superposed on the primary 
glacial-interglacial cycle, to such a degree that each 
glacial stage was marked by temporary peaks and re- 
cessions. 
During postglacial time, in the period of the Climatic 
Optimum, which lasted from approximately 4000 to 
2000 B. c., the world climate very closely approximated 
the interglacial type. Warm, dry, storm-free conditions 
prevailed in middle latitudes so that hardwood forests 
flourished in Scandinavia, in the British Isles, and in 
much of North America in regions where the climate 
is far too severe (cold and stormy) for such growth to- 
day. The polar regions were warm and stormy, the 
polar seas free of permanent ice, and the Greenland and 
antarctic icecaps probably several hundred feet lower 
than today. Lake levels in Asia, in northern and central 
COSMICAL METEOROLOGY 
Africa, and in the western United States reached their 
lowest levels of postglacial time, many of them drying 
up entirely, while most glaciers in middle latitudes 
completely disappeared. Most of these glaciers today 
represent new formations, not remnants preserved from 
the last glacial maximum. If this condition had _ per- 
sisted for fifty thousand instead of only two thousand 
years, it probably would have constituted a true inter- 
glacial period. 
At the start of the sub-Atlantic period, which was well 
established from about 1000 B.c. to a.p. 300, world 
climate took a strong turn towards the glacial type. 
Storminess and cold in Kurope became extreme, the 
extensive forests of the preceding Climatic Optimum 
were replaced by peatbogs, while glaciers in all parts of 
the world advanced well beyond preceding limits. The 
Caspian Sea, the nonoutlet lakes of the western United 
States, and the lakes of northern and equatorial Africa 
reached their highest postglacial levels, while rela- 
tively cool, moist conditions in the lower middle lati- 
tudes permitted the development of extensive civili- 
zations in the Mediterranean and on the steppes of 
central and western Asia in regions which subsequently 
became too arid for agricultural pursuits. During the 
period, severe arctic conditions were re-established in 
the polar regions. 
From approximately a.p. 400-1000 the world climate 
returned to a minor and abbreviated edition of the 
Climatic Optimum, mild extremes being reached in the 
higher latitudes during the eighth and tenth centuries. 
This period marked the peak of the Viking explorations 
and colonization in Iceland and Greenland. In their 
small boats the Vikings regularly traversed seas which 
today would be impassable for them by reason of ice 
and storms, and their colonies throve by agricultural 
pursuits in areas of Greenland which are now covered 
by glaciers. Dryness in the Mediterranean and central 
Asia led to mass migrations and to the crumbling of 
many civilizations. The degree of dryness was attested 
to by tree-ring records and by the lowest lake levels 
in Asia, Africa, and the western United States since 
the Climatic Optimum. Relative dryness in tropical 
regions which at present have heavy rainfall was evi- 
denced by the development of the Mayan civilization 
of Central America and the great city of Angkor in 
French Cambodia. The sites of both of these civiliza- 
tions were reclaimed by the jungle during subsequent 
centuries of wet climate. 
The eleventh and twelfth centuries saw a return of 
the glacial type of climate, which during the 13th 
and 14th centuries reached a peak of great storminess 
and climatic stress in Europe and other regions in higher 
middle latitudes. This period repeated all of the char- 
acteristics of the sub-Atlantic period to a moderate 
degree. Since the 14th century there has been some 
amelioration of conditions, particularly during the first 
half of the 17th and last half of the 18th centuries. This 
was followed by increased severity of climate during the 
early nineteenth century, and a turn for the better from 
1880 to the present. Although the climatic fluctuations 
of the past 500 years have been relatively slight, the 
