[8] The first version of orbital theory (hypothesis) proposed by Adhémar [1842] assumes that glaciations in both hemispheres were related to precession-controlled long cold winters. This assumption was criticized by British astronomer J. Herschel and German naturalist A. von Humboldt [Croll, 1875; Imbrie and Imbrie, 1986]. They claimed the average annual insolation is not changed by precession variations at any latitude, as winter insolation decrease is compensated by summer insolation increase and vice versa, also at any latitude. Therefore a long cold winter is followed by a short hot summer in one hemisphere and simultaneously a long cool summer is followed by a short mild winter in another one. Hence, annual average heating (and temperature) do not change with precession variations.
[9] Thus, to explain the glaciations existence basing on orbital theory, it is essential to find a complementary factor that gives rise to appearance and retaining of ice sheets during millennia, vast spaces of high and middle latitudes to be covered despite inverse semi-annual precession insolation contrasts. J. Croll has found such a factor and, from my point of view, this discovery is an outstanding achievement of theoretical paleoclimatology.
[10] J. Croll realized that the direct influence of insolation variations on climate is slight, since the eccentricity-driven insolation changes are very small and average annual changes of Earth's total solar radiation having been connected with two other orbital elements are virtually null (are "compensated''). He wrote [Croll, 1875 p. 13]: "There is, however, one effect that was not regarded as compensated. The total amount of heat received by the earth is inversely proportional to the minor axis of its orbit; and it follows therefore, that the greater the eccentricity, the greater is the total amount of heat received by the earth. On this account it was concluded that an increase of eccentricity would tend to a certain extent to produce a warmer climate. All those conclusions to which I refer arrived at by astronomers, are perfectly legitimate so far as the direct effects of the eccentricity are concerned, and it was quite natural, and, in fact, proper to conclude that there was nothing in the mere increase of eccentricity that could produce a glacial epoch. How unnatural would it have been to have concluded that an increase in the quantity of heat received from the sun should lower the temperature, and cover the country with snow and ice! Neither would excessively cold winters, followed by excessively hot summers, produce a glacial epoch. To assert, therefore, that the purely astronomical causes to produce such an effect would be simply absurd... (emphasized by me - V. B.). The important fact, however, was overlooked that, although the glacial epoch could not result directly from an increase of eccentricity, it might nevertheless do so indirectly. Although an increase of eccentricity could have no direct tendency to lower the temperature and cover our country with ice, yet it might bring into operation physical agents which would produce this effect''.
[11] Croll was the first to introduce Earth positive feedbacks that enhance the insolation variations climatic influence as the physical agents. The first one is related to snow and ice cover albedo, the second one - to ocean current of the Atlantics. The glaciation mechanism was the following. During notably cold and long winters in one hemisphere, when the eccentricity was especially high and winter solstice was far from the Sun, next to the aphelion, the snow cover of this hemisphere grew significantly. The joint effect of cold winters, growing snow cover and positive albedo feedback resulted in the hemisphere annual temperature progressive decrease, despite the hot summers were followed by long cold winters. Temperature decrease in high latitudes increased the temperature gradient between pole and equator, enhanced the trade winds that in turn changed the water currents of the Atlantics. The latter should have resulted in further hemisphere cooling up to glacial conditions. Another hemisphere witnessed the onset of interglacial.
[12] Thus, according to Croll glaciations occur during the periods of the extremely high eccentricity values in one or another hemisphere, when precession-determined distance between Sun and Earth in winter of any hemisphere becomes very great. Using Le Verrier equations Croll calculated Earth orbit eccentricity changes for 3 million years to the past and 1 million years to the future. According to these calculations notably high eccentricity values correlate with time intervals of 980-720 kyr and 240-80 kyr. These were time intervals which Croll attributed to glaciations. The latest glaciation should have terminated 80 thousand years ago. Hence, it was Croll rather than Milankovitch (as sometimes several authors write) who was the first to calculate orbital elements to assess the time of paleoclimatic events.
[13] However, Croll's theory did not correlate with empirical data. It was evidenced that the last glaciation finished not 80, but approximately 10 thousand years ago. Besides, the coincidence of glaciations in northern and southern hemispheres has been shown. Therefore Croll's theory was rejected in the end of 19th century.
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