"The great cold of the glacial epoch, it is probable that the quantity of snow falling in the northern regions would be enormous. This would be particularly the case during summer, when the earth would be in the perihelion and the heat at the equator great. The equator would be the furnace where evaporation would take place, and the snow and ice of temperate regions would act as a condenser. Heat to produce evaporation is just as essential to the accumulation of snow and ice as cold to produce condensation... And, again, the formation of snow and ice facilitated the rate at which the earth lost its heat; and on the other hand, the more rapidly the earth parted with its heat, the more rapidly were the snow and ice formed.’' (Croll, 1875, pp. 74-75)
Feedback mechanisms, such as radiative effects of the ice fields, enhanced formation of cloud and fog, changes in sea level, and the mixing and redirection of warm and cold ocean currents would serve to enhance the climatic changes initiated by the orbital elements. According to Croll, 'The cause of secular changes of climate is the deflection of ocean currents, owing to the physical consequences of a high degree of eccentricity in the earth’s orbit.' That is, 'glacial cycles may not arise directly from cosmical causes, they may do so indirectly!’
Both Croll and Milankovitch were visionaries, both very extremely disciplined and principled; both had to face and overcome extreme challanges to advance their theories; both had the Herculean task of trying to unite heaven and earth; both provided missing link(s) between celestial mechanics and geology. If Croll’s theory that the earth’s climate ‘revolves’ around the sun started a Copernican revolution in climate dynamics, then Milankovitch’s served as the Newton of this field.''
James Rodger Fleming, 2006, James Croll in Context: The Encounter between Climate Dynamics and Geology in the Second Half of the Nineteenth Century ''Although Croll had no doubt that variations in the earth’s orbit were responsible for changes in climate, he was worried that the magnitude of the climatic changes demonstrated by the geologic record might be too great to be explained by the rather subtle changes in orbital geometry, even if these were amplified by the reflection of sunlight. Was is really possible that an increase in orbital eccentricity of only two or three percent would result in the developments of ice sheets massive enough to cover most of Europe and North America? Croll’s concern anticipated objections that were later raised by other researchers. He attacked the problem with characteristic ingenuity by hypothesizing that the orbital changes operated as a triggering mechanism capable of setting off a major response within the earth’s climate system. In the attempt to discover what this climatic response might be, Croll turned to the great warm currents of the Atlantic Ocean.
Today, the westward-flowing currents that moves across the equator is deflected northward by the coast of Brazil and joints the Gulf Stream. In this way heat is transported into the northern hemisphere from the southern hemisphere. But if some agency were to shift the Equatorial Current so that it stuck the coast of Brazil south of its easternmost point, that warm current would be deflected southward, heat would be carried to the opposite direction, and northern hemisphere would grow cooler.
What agency could be capable of shifting the Equatorial Current? To find an answer to this question, Croll developed an original (and essentially correct) theory that explained why the major oceans current flow in the direction they do. Croll pointed out that the westward-flowing currents, which flow near the Equator, and the poleward-flowing currents such as the Gulf Stream, both move in response to the trade winds, much as the water in a teacup is moved by blowing on it, and that the velocity of these trade winds depends, in turn, on the temperature in the polar regions. If the polar region in one hemisphere were to become colder, more heat transport should be required to balance the earth’s radiation budget in that hemisphere, and the trade winds there would blow harder.
In short, the colder the poles, the stronger the winds. Croll concluded from this analysis than when the precessional cycle causes the ice sheets to expand in one hemisphere, the resulting increase in the strength of the trade winds in that hemisphere forces the warm equatorial currents in all of the oceans to shift towards the other hemisphere so that even more heat is lost. This effect, Croll felt, would do especially pronounced in low-latitude portions of the Atlantic Ocean, were a bulge in the coastline of Brazil would deflect the Equatorial Current either northward or southward. Thus, the direct climatic effect of any astronomically induced change in radiation, already amplified once by the reflection-feedback effect, is amplified once again by the changing pattern of ocean currents.’'
John Imbrie and Catherine Palmer Imbrie, 1979, Ice Ages. Solving the Mystery, pp. 85-86 The future orbital configuration of the earth will lead to the inception of a new Ice Age. London, Daldy, Isbister & Co, 1st edition, 1875, 8vo xvi p., 577 p., col. front., illus., VII col. pl. (incl. maps, 2 fold.) : diagrs., index. Notes: ''List of papers which have appeared in Dr. A. Petermann's Geographische Mitteilungen relating to the Gulf-stream and thermal conditions of the Arctic regions'', p. 556-559, and ''List of papers by the author to which reference is made in this volume'', p. 560-562. Original blue cloth wrappers; recased; minor discolorations on spine; minor foxing on leaves, good condition. Items no 6315 and no 6272 are the same. Price: US$ 1,200,000.00