GEOLOGICAL AND HISTORICAL ASPECTS OF CLIMATIC CHANGE 
‘By C. E. P. BROOKS! 
Ferring, Sussex, England 
THE FACTS AS KNOWN AT PRESENT 
The Mild Climates of the Greater Part of Geological 
Time. The oldest known rocks have been dated by the 
uranium-lead ratio as having been formed about 1600 
million years ago, but the evidences of climate given by 
these very early deposits are scanty, and it is not until 
nearly the beginning of the Cambrian period, about 500 
million years ago, that a picture of world climate begins 
to emerge. (The succession of geological periods is shown 
at the bottom of Fig. 4.) All we can say of the pre-Cam- 
brian period is that at intervals of a few hundred million 
years glaciers or ice sheets covered various parts of the 
world; of the intervening periods we can say nothing. 
From the beginning of the Cambrian onwards our knowl- 
edge of the general level and zonal distribution of 
temperature becomes increasingly detailed, and it is 
quite clear that climate has alternated between mild 
and glacial, but that mildness has prevailed for nearly 
nine-tenths of the time. It seems appropriate, therefore, 
to begin this review of geological climates with a study 
of the warm periods. Two epochs may be selected as 
typical, the Jurassic and the Eocene. 
The Jurassic was the age of corals which extended 
into fairly high latitudes, indicating that in 50°-60°N 
the temperature of the water in shallow seas was about 
60F, or nearly 10F higher than the highest present- 
day ocean temperatures in those latitudes. Nearer the 
poles, however, corals were dwarfed or absent, and the 
general assemblage of animals differed from that in the 
tropics. This shows that climatic zones existed at that 
time, though they were less marked than at present. 
We know less about the climate of the land areas, but 
numerous salt beds in lake deposits point to a rather 
scanty rainfall which most probably fell in heavy show- 
ers of short duration instead of in prolonged cyclonic 
storms. There must have been sufficient vegetation to 
support the dinosaurs and other great land reptiles, but 
this was probably limited mainly to the river valleys. 
There is no sign of ice anywhere in the world; even the 
polar regions were too mild for ice sheets to form, and 
the absence of mountain ranges prevented the forma- 
tion of glaciers. 
In the Middle and Upper Eocene the climate was 
generally similar to that of the Jurassic, except that the 
rainfall of temperate regions was heavier. Land vege- 
tation was abundant as far north as northern Green- 
land, and Chaney [8] has given us a clear picture of the 
climatic zones. The most northerly flora known was 
near Cape Murchison in Grinnell Land (72°N) and 
included horsetail, yew, pine, spruce, poplar, birch, 
hazel, and grass—a cool-temperate flora but one which 
1. Retired from the Meteorological Office. London. 
was sufficiently remarkable when we think of the bar- 
renness of the region at present. Everywhere the north- 
ern boundary of the temperate flora appears to have 
been 15° to 20° nearer the pole than at present. At the 
same time the subtropical flora also advanced north- 
ward by about 10° of latitude, extending well into the 
United States and in places almost reaching the Arctic 
Circle. The subtropical as well as the polar regions 
appear to have been warmer than they are today, but 
the difference decreased from high to low latitudes. 
The subtropical flora differed completely from that of 
the Arctic, indicating well-marked climatic zones. 
The rainfall of temperate regions in the Kocene prob- 
ably exceeded the present rainfall. In Europe it oc- 
curred mainly in winter and early spring, giving place 
to a long, hot, and dry summer—the Mediterranean 
type of climate. The vegetation often shows damage 
by hail, even quite early in spring, pointing to msta- 
bility showers. In western United States, however, 
where the relief was more pronounced, rainfall was 
more evenly distributed through the year and may have 
averaged seventy inches. 
The picture we form of the climate of middle latitudes 
during the warm periods is one of mild winters and hot 
summers, and the absence of cyclonic depressions and 
gales. The “polar front” as we know it either did not 
exist or lay far to the north. The subtropical anticy- 
clones lay farther north than now. The arctic basin had 
prevailing west or southwest winds and mild humid 
weather with probably a good deal of rain, favouring 
a rich vegetation. Warm ocean currents extended into 
high latitudes, and zonal differences of temperature 
were reduced to a minimum. 
Ice Ages. The great “Ice Ages” stand out in sharp 
contrast to the long mild periods. The climate of large 
areas of the earth was very severe, but it was also very 
changeable from one millennium to another. The latest 
and best-known, the Quaternary Ice Age, began roughly 
a million years ago, and in this (geologically speaking) 
short period there have been four major advances and 
retreats of the ice, as well as a number of minor oscilla- 
tions. It is convenient to distinguish between the Ice 
Age as a whole, and the Glacial and Interglacial Periods 
into which it is divided. The details of the latter of these 
periods are described by R. F. Flint in this volume,’ 
but as they are important for the study of the causes 
of glaciation, they are briefly recapitulated below. In 
North America, northern Europe, and probably also 
parts of northern Asia, great sheets of inland ice, thou- 
sands of feet thick, spread from high ground over the 
plains and even crossed the shallow epicontinental seas; 
2. Consult ‘‘Climatic Implications of Glacier Research” by 
R. F. Flint, pp. 1019-1023. 
