In this revision of his important and comprehensive study of the earth’s climatic history, first published in 1926, Dr. Brooks supplements his original material with an account of the fairly copious additions to knowledge of the subject made since then and also discusses critically many theories and hypotheses, new and old, put forward to explain the glaciations and warm epochs which fell to our planet’s lot in remote times. His own creed, which has found wide acceptance during the last 20 years, remains fundamentally unaltered. This is so well summarized in its essentials on pages 377–78 that full citation seems desirable:

An interesting theoretical consideration for glaciologists is Dr. Brooks’s demonstration, in Chapter I, that if, over an open sea, the mean winter temperature at the pole should fall from a trifle above the freezing point of sea water to only 0.6° F. (0.33° C.) below that value an ice field would form which would soon extend rapidly until it reached a latitude of about 78°, and then, in the absence of a land mass or other unfavourable conditions, more slowly to about latitude 65°. The ultimate lowering of winter temperature engendered by the initial decrease of less than 1° F. (0.56° C.) may, it is calculated, amount to some 45° F. (25° C.) in such circumstances.

Two of the more curious hypotheses mentioned (without approval) by Dr. Brooks as having been invoked quite recently to account for certain major climatic changes are (a) that in Palaeozoic times a small satellite of the earth, either original or captured, was disrupted by tidal and other forces to form a cincture of fragments round the equator, similar to Saturn’s rings; (b) that on several occasions, separated by intervals of 250 to 350 million years, the sun has suffered “nova” outbreaks causing short-lived thousandfold increases in radiation, followed by general cooling of terrestrial climates consequent on the resultant diminution in solar mass and radiative output. Perhaps the most plausible of the relevant modem astronomical hypotheses outlined in the book is that on its journey through space the sun from time to time encounters clouds of interstellar matter whose particles, on being absorbed, have their kinetic energy converted into heat. It may be recalled that long before the discovery of these cosmic clouds Newton envisaged the possibility that an accession of extraneous meteoric or cometary fuel to the sun might lead, sooner or later, to the extinction of all Life on the earth.