1. Introduction

[2]  Several papers, published during the last six years [Elkibbi and Rial, 2001; Huybers and Wunsch, 2005; Loutre et al., 2004; Paillard, 2001; Raymo and Nisancioglu, 2003], from my point of view, show the beginning of new stage in orbital (astronomical, Milankovitch) paleoclimate theory development. This new stage is related to M. Milankovitch theory reappraisal and could be characterized with D. Paillard words [Paillard, 2001, p. 343]: "The classical Milankovitch theory needs to be revised to account for the traditional peculiarities of the records, like the 100-kyr cyclicity and the stage 11 problem''. The impact of the Earth axial inclination on Pleistocene climate changes has received much attention from the authors of mentioned above papers. Effects connected with obliquity insolation variation and its gradient between high and low latitudes are considered to be the most important, that could manage climate during the period from 2.5 to 1 million years ago [Raymo and Nisancioglu, 2003] and might well accelerate Pleistocene glacial terminations [Huybers and Wunssch, 2005].

[3]  However, development of paleoclimate orbital theory (and Milankovitch theory revision) could be improved applying another approach. The approach implied is based on the history of theory development critical analysis. Based on this approach anyone can at once give an answer to the question stated in the Loutre et al. [2004]: "Does mean annual insolation have the potential to change the climate?''. In my opinion, the answer should be: "Yes, it does''.

[4]  Such an answer is quite obvious in case of solar constant change. In this case the increase of Earth total annual radiation will lead to global warming and its decrease - to global cooling. (I guess that Simpson hypothesis [Simpson, 1938], assuming that a minor solar radiation increase could result in glaciation, hasn't been properly based). For the orbitally-driven insolation variations the answer to the question stated should also be positive.

[5]  Eccentricity is known from the early 19th century to be the only orbital parameter changing the Earth's total average annual insolation. Assuming the Earth surface is a sphere the Earth's total insolation is not changed, but latitudinally redistributed by the axial inclination angle e variations. Angle e decrease results in average annual insolation decrease at high latitudes and insolation increase at low latitudes. e rise leads to opposite result. Hence, during inclination angle variations, average annual insolation of high and low latitudes changes in opposite phases (phase inversion occurs between 44o and 43o latitudes of both hemispheres). Besides, relative annual insolation change of high latitudes is higher than that of low latitudes.

[6]  J. Croll [1875] was the first to propose a proper mechanism of the Earth axis inclination variations climatic influence. He was also the first to account for the change of the total annual insolation along with the forcing of snow and ice albedo at high latitudes. Croll concluded that inclination angle e decrease would lead to global cooling, whereas e enhancement would result in warming, glacier melting at high latitudes in both hemispheres and ocean level rise. Milankovitch used the same mechanism in 1930 [Milankovitch, 1930]. Thus we may conclude that J. Croll got the answer to the question stated in paper [Loutre et al., 2004] more than 130 years ago.

[7]  Consequently, the important problem of average annual insolation climatic influence has got a long history which one should know and understand. Another reason is that the knowledge obtained leads to more fundamental understanding of orbital theory problems. Below is given the brief historical review of orbital theory development.


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