GEOLOGICAL AND HISTORICAL ASPECTS OF CLIMATIC CHANGE 
the elements of the earth’s orbit (p. 1011) are known to | 
have occurred, but are probably quantitatively in- 
sufficient. Variations of solar radiation could have pro- 
duced most of the observed effects, and it is not in- 
herently improbable that the sun is a variable star. 
The present trend of thought is therefore towards what 
Flint [11] terms the “solar-topographic hypothesis.” 
A fluctuation of solar radiation by 10-20 per cent on 
either side of the mean value combined with the geo- 
eraphie cycle to produce the variations of geological 
climate. This hypothesis seems to offer the best basis 
for future research, but it is still doubtful whether the 
solar or topographic partner predominates. One view 
is that topography governs the major swings of climate 
but the solar factor determines the incidence of glacia- 
tion; the other view is that solar variations decide the 
main climatic variation but that mountains are essential 
for glaciation. 
1017 
1006). The radioactive technique devised by Urry gives 
a prospect of dating the climatic changes revealed by 
these cores. Further, Ovey [26] foresees the possibility 
of tracing the sources of the marine glacial deposits 
and so tracking the paths of the icebergs, which will 
give us the ocean currents and winds of the glacial 
periods. 
At present, especially as regards the pre-Tertiary 
periods, geologists are too apt to characterise the whole 
of a geological period, lasting millions of years, as 
having a certain unchanging type of climate. This is 
improbable, and careful detailed studies of the fauna, 
flora, and lithology will result im a picture of the minor 
fluctuations superposed on the broad swings shown in 
Fig. 4. If a rough time-scale (not necessarily absolute 
dating) can be added, this will go far towards solving 
the problem of causes. 
The Factors of Climate. Parallel with the study of 
i Phas i ESTIMATED FROM FAUNA AND FLORA 
i | PSS ———- CALCULATED FROM GEOGRAPHIC 
i i Sal zs FACTORS 
4 tF 
= oe \ oN | 60 
° 1 
So) i \ i] a ‘ 
e 1 \ i : \ 
¢ 4 Nf Va } ESN IX > 
50 4 + aa : 50 
Luo 0 WY) t \ Wi + --- 
or 1 ! vt f C \ 
S) # ! Ya Vane 
iz / if \ i (] 
Be if \ y y ! Hl 1 a 
= am = : hee 
E \ 
\ ol, 
zZ a 1 
rm } ——_-—— |Paleozoic Mzsoz0ic {ere rhiar ri 
Sice 7 Cambrian | Ordovician [ilurian|Devonian|Carboniferous|Permian | trias [Jurassic [Crelaceous celine 20) 
| | | | N 
500 400 300 
200 100 O 
APPROXIMATE TIME-SCALE — MILLIONS OF YEARS 
Fig. 4.—Variations of temperature in geological time. 
The evidence from variations of rainfall is not clear. 
Increased solar radiation should increase total pre- 
cipitation, and so far as we can determine, precipita- 
tion in the early Tertiary was greater than at present, 
On the other hand the long warm periods of the Meso- 
zoic were characterised by desert deposits, pomting to 
a rainfall smaller than the present. This pomt needs 
further investigation. 
THE FUTURE STUDY OF CLIMATIC CHANGES 
The Palaeoclimatic Sequence. The greatest need at 
present is for a closer quantitative evaluation of cli- 
matic variations in geological time. We need better 
estimates of the distribution of mean annual tempera- 
ture and annual range, and the amount and seasonal 
distribution of rainfall. These data, plotted in conjunc- 
tion with the orography, may in time lead to an un- 
derstanding of the wind systems. In particular, we 
need to know more about conditions over the oceans. 
This gap is being filled for the Quaternary and later 
Tertiary by the cores raised from the ocean floor (p. 
the palaeoclimatic sequence must go a more exhaustive 
quantitative study of the effect of the various solar 
and topographic factors on climate, and particularly 
on temperature and on the circulation of the earth’s 
atmosphere. From a study of existing conditions we 
should be able to construct a more exact series of equa- 
tions of climate which can be applied to any other 
topography. The temperature relations are already 
known to some extent, and the urgent problem now is 
to study the effect of topography on the atmospheric 
circulation. 
When these parallel studies have reached a suffi- 
ciently advanced stage, we may be able to realise the 
dream of Kerner [18] that, from a large number of 
comparisons of quantitative estimates of local tempera- 
ture from geological data with calculations of the same 
values from topography (both local and general), we 
can reconstruct the variations of extraterrestrial factors 
and especially of solar radiation. A few such comparisons 
already exist, made mostly by Kerner himself. An inter- 
esting case is the middle Eocene of southeast England. 
