1008 
a widespread interruption in the growth of peat in 
Europe. It has been described as a “‘dry heat wave” 
lasting for perhaps 200 years. Lakes decreased in area 
and in a few places trees grew on their floors below 
the level of the outlet. From four such lakes in Ireland, 
Germany, and Austria it is estimated that the annual 
rainfall was only about half the present amount. The 
drought was not sufficiently mtense to mterrupt the 
steady development of forests, but it caused extensive 
migrations of peoples from drier to wetter sites. About 
1300 B.c. there was a period of heavy rainfall and 
floods which destroyed some of the Alpine lake villages. 
About 500 3.c. there was a great and rapid change 
to a colder and wetter climate. Over large areas the 
forests were killed by a rapid growth of peat. The levels 
of the Alpine lakes rose suddenly, flooding many of 
the lake dwellings, and most of the mountain area 
became uninhabitable, the few settlements being lim- 
ited to the warmest and driest valleys. Traffic across 
the Alpine passes, which had continued steadily since 
1800 B.c., came to an end. As many of the peat bogs 
formed during this period are now drying up, it seems 
that the rainfall of this sub-Atlantie period must have 
been greater than at present. This change of climate 
was by far the greatest and most abrupt since the end 
of the Ice Age, and its effect on the civilisation of 
Europe was catastrophic. It did not last long, however, 
for by the beginning of the Christian era conditions did 
not differ much from the present. 
Our knowledge of postglacial conditions in other 
parts of the world is less detailed, but the climatic 
changes appear to have run closely parallel. In Asia and 
northeast Africa the evidence consists mainly of the 
migration of whole peoples, and of the occupation and 
abandonment of marginal sites, where under average 
conditions there is only just enough water to support 
life. In North Africa, West and Central Asia, and China 
there was a wet period between 5000 and 4000 sB.c. In 
China the cultivation of rice extended about five de- 
grees north of its present boundary, pointing to a 
higher temperature. This warm wet period was followed 
by a period of gradually increasing drought which cul- 
minated about 2200 B.c. Conditions remained dry until 
1900 B.c., when there was a “complaint” that the 
marshes of the Nile delta had dried up and the river 
could be crossed on foot, presumably during the low- 
water stage. Then followed a period of fluctuating rain- 
fall, with another drought which the Chinese records 
place between 842 and 771 3.c. Migration and war 
almost ceased after 500 B.c., when caravans crossed 
deserts which are now almost impassable. A passage in 
Herodotus suggests that the level of the Caspian Sea 
stood much higher than now, and there is evidence of 
abundant water supply in Egypt and im the Saharan 
oases. On the whole, however, the case for a great in- 
crease of rainfall about 500 B.c. is not so strong in Asia 
and North Africa as it is in Europe. 
There is abundant evidence of postglacial climatic 
change in North America, but until the tree rings take 
up the story about 1000 8.c., the dating is uncertain. 
In eastern North America the rate of recession of the 
ice sheets at the close of the glaciation ran parallel 
CLIMATOLOGY 
with the recession rate in northwest Europe, and the 
peat bogs show similar alternations of wet and dry 
periods; there seems no reason to doubt that the 
changes were approximately synchronous. Hansen [12] 
gives the following succession: 
Eastern North America Northwest Europe 
Ice retreat (Hudsonian) Late-glacial 
Spruce, fir (cool, moist) Pre-boreal 
Pine (warmer but still cool) Boreal 
Oak and hemlock (warm, moist) Atlantic 
Oak and hickory (warm, dry) Sub-boreal 
Oak, chestnut, spruce (cooler, moister) Sub-Atlantic 
The history of the lakes of the Great Basin, described 
by Jones [32] and van Winkle [85], points to a long 
dry period which, according to the salt content of the 
lakes, ended some time between 2000 B.c. and a.p. 0. 
This period was brought to an end by a rainfall greater 
than that of the present and was followed by a gradual 
decrease. 
The evidence of the tree rings is unfortunately doubt- 
ful, as comparatively few trees date from before 500 
B.c. The curves drawn by Huntington and Antevs [32] 
both indicate a wet period which had definitely begun 
by 660 B.c. and reached an absolute maximum between 
480 and 250 B.c. All these facts are in sufficiently good 
agreement with the dating of the sub-Atlantic in Eu- 
rope at 500 B.c. 
The fluctuations of the lakes in equatorial Africa 
appear to run parallel with the alternations of wet 
and dry periods in Europe, though they cannot be 
dated precisely. On the other hand, in the dry subtrop- 
ical regions south of the main storm tracks the evidence 
for climatic change is much less definite. In the temper- 
ate parts of the Southern Hemisphere the postglacial 
variations of climate appear to have been generally 
similar to those of the Northern Hemisphere, but they 
have not yet been dated. In southern New Zealand 
the Climatic Optimum is represented by lowland rain 
forest, indicating increased rainfall and higher tempera- 
ture, but in the southern Argentine in the rain shadow 
of the Andes the increased warmth was accompanied 
by greater aridity. 
Climatic Changes during the Christian Era. After 
the beginning of the Christian era we have an increas- 
ingly detailed and accurately dated knowledge of cli- 
matic changes. The evidence includes fluctuaticns of 
lakes and rivers, growth of peat bogs, succession of 
floras, rate of growth of trees as shown by annual 
rings, advances and retreats of glaciers, locations of 
settlements and migrations of people (for which cli- 
matic reasons may be assigned with some show of 
probability), literary records and old weather journals, 
and finally instrumental records. The results of this 
‘great mass of detail, summarised in Table III, show that 
climatic oscillations are not entirely local events but tend 
to fit together into a world pattern. Asia and western 
North America tend to vary together; Europe partly 
varies with them but has changes of its own in addition. 
North Africa, represented principally by the Nile valley, 
varies partly with and partly agaist Asia. An opposi- 
tion between the temperate and subtropical regions 
is shown more clearly in Yucatan, as was first pointed 
