GEOLOGICAL AND HISTORICAL ASPECTS OF CLIMATIC CHANGE 
theory raises many difficulties, and it breaks down 
completely over the Late Palaeozoic glaciation. It is 
true that the variations in the rainfall of the north 
temperate zone since A.p. 1000 have run fairly parallel 
with the variations of sunspots, but the geological 
variations of climate were on a scale many times greater 
and would require enormous and prolonged sunspot 
outbursts which seem quite improbable. 
Changes of solar radiation could account for some, 
but not all, of the postglacial changes of climate. 
It is difficult to find any other explanation for the 
warmth and heavy rainfall of the Climatic Optimum, 
but the rainfall maximum about 500 B.c. was accom- 
panied by a fall of temperature. The alternating halts 
and retreats of the late glacial period could have been 
due to solar changes, but other explanations have been 
put forward. Lewis [21] considered that a glacial period 
could be initiated by a comparatively small merease 
of precipitation, which might be due to the temporary 
diversion of an ocean current, and that once the ice 
sheets reached a size sufficient for the development of 
a glacial anticyclone, they grew by a “runaway” process 
until the increasing loss of solar energy by reflection 
from the ice and snow lowered world temperature, and 
consequently the precipitation level, below the sub- 
sistence level of the ice sheets. In this connection the 
recent recession of the glaciers of the “Little Ice Age” 
is especially interesting. By comparing the decrease in 
the volume of ice since 1850 with the rise of sea level, 
Ahlmann [1] found that the recession had been confined 
to the marginal glaciers, the central ice sheets of Green- 
land and the Antarctic being almost untouched. There 
has been no appreciable decrease of snowfall, so that 
the recession must be due to increased ablation. In 
the marginal areas ablation is due mainly to heat con- 
duction from warm air, that is, to a stronger atmos- 
pheric circulation. On the main surfaces of the ice sheets 
ablation is due to solar radiation, and Ahlmann infers 
that the warming of the Arctic is associated with an 
“increased atmospheric circulation without any appre- 
ciable change of solar radiation. 
Changes in the Elements of the Earth’s Orbit. With 
constant solar radiation, the heat reaching the outer 
limit of the earth’s atmosphere in a year remains 
practically constant in any latitude, but the seasonal 
distribution changes. There are three variables: 
1. The obliquity of the ecliptic, or the angle which 
the plane through the equator makes with the plane 
of the earth’s orbit round the sun. The greater the 
obliquity, the greater the contrast between the heat 
received in summer and winter. Milankovitch [22] cal- 
culated the variation as 214° in a period of 40,400 years; 
the last maximum was about 8000 B.c. 
2. The eccentricity of the earth’s orbit, which varies 
from 0.0 to about 0.07 in a period of 100,000 years. 
The hemisphere with winter in aphelion has a short 
hot summer and a long cold winter, that with winter 
in perihelion has a short mild winter and a long cool 
summer. At present the Northern Hemisphere is in 
perihelion in winter, but the excess of land quite out- 
weighs the solar effect. 
1011 
3. The precession of the equinoxes, by which the 
season in which perihelion falls advances through the 
year in a period of 21,000 years. About 8500 B.c. the 
Northern Hemisphere had winter in aphelion and a 
more extreme solar climate than now. 
Many attempts have been made to account for the 
succession of glaciations by these astronomical changes. 
The early theories, such as that of Croll, require alter- 
nate glaciation in the two hemispheres; they were un- 
sound meteorologically and the astronomical dating 
did not agree with the geological time scale. These 
defects in the theory have been overcome to a large 
extent, and the latest exposition, by Zeuner [36], is qual- 
itatively in good agreement with the facts. Zeuner shows 
that the variation of the present snow lie with latitude 
follows closely the variation of radiation im the summer 
half-year. A rise of winter temperature increases the 
snowtall and the corresponding decrease of summer 
temperature enables the snow cover to persist through 
the year. Hence small obliquity with high eccentricity 
and summer in aphelion lead to glaciation. Zeuner’s 
dating of the various glacial stages is shown on the 
right of Table I, and is seen to agree reasonably well 
with the estimates by Penck and Brickner. 
Undoubtedly these changes in the seasonal distribu- 
tion of radiation must have some effect on climate, 
but they were almost certainly quantitatively msuffi- 
cient to aécount for the enormous range between glacia- 
tion and deglaciation, even when secondary effects are 
exploited to the full. The variations of radiation are 
small and complex near the equator, and cannot pos- 
sibly account directly for the alternation of pluvial and 
interpluvial periods. Moreover, as Zeuner recognises, 
such factors cannot account for the Ice Age as a whole. 
They have presumably been continuously in operation 
throughout geological time, but traces of them are 
rare. According to Bradley [3] the Eocene of Colorado, 
Utah, and Wyoming, covering several million years, 
shows a periodicity of 21,000 years. The Cretaceous 
in the United States also shows a cycle estimated as 
of this length. It is possible that the coal seams in the 
Upper Carboniferous (Pennsylvanian) represent periods 
of great eccentricity with northern winter in perihelion 
and a small obliquity of the ecliptic, giving little annual 
range of temperature; this would account for the ab- 
sence of annual growth rings. The intercalated sand- 
stones would represent the opposite condition of sum- 
mer in perihelion. 
The continental climate of the pre-boreal, about 
8000 to 7000 8.c., fits in with the large obliquity and 
winter in aphelion, but could equally well have been 
due to the remains of the ice sheets and, in Europe, to 
the elevation of Scandinavia which converted the Baltic 
into the fresh-water Ancylus Lake. Astronomical 
changes cannot possibly account for the Climatie Opti- 
mum, the subsequent deterioration in the sub-Atlantic, 
and the ‘Little Ice Age” of the 17th to 19th centuries 
A.D. The trend of modern thought is against the as- 
tronomical theory. 
Tidal Variations. We must mention here a suggestion 
by Pettersson [28] that changes of climate during 
