[34] At present the need in feedbacks for transformation of the weak insolation signals into global climate changes is almost a received fact. (Moreover, some authors assume the presence of not insolation-driven, but independent, self-oscillating global climate processes resulted from the influence of feedbacks in the course of interactions in the ocean-land-atmosphere-cryosphere system). A specific example shows the importance of the feedbacks effect.
[35] To verify the mechanism of Earth axis inclination angle variations climatic influence ( e decrease resulted in insolation decrease at high latitudes causes global cooling), Croll, and then Milankovitch, used the mechanism of positive feedback resulted from albedo change due to ice and snow cover area change mainly at high latitudes. However, e variations insolation changes of high and low latitudes being in antiphase, the solution of the problem of global climatic influence of angle e variations assessment would become quite difficult in the case of positive feedback for insolation variations at low latitudes, comparable to the discussed albedo relation at high latitudes.
[36] As exemplified above the account for feedback specific character will make it easier to resolve the problem relevant to the obliquity (and, in my opinion, with precession) zero average annual insolation variations for the entire Earth. It also can be a help in the global climatic value of variations for each of the three orbital elements estimation. The various Earth feedbacks are known to exist. They are not only positive (associated either with albedo change due to snow and ice volume and vegetation cover change or with greenhouse gases quantity change), but also negative as those related to the atmospheric circulation enhancement resulting in the lowering of latitudinal temperature gradients that increase during the time of glaciation.
[37] There is a good reason to assume that these types of feedbacks variously affect the orbital signals caused by variations of individual orbital elements. For example, the positive indirect relation caused by albedo change in mainly high latitudes of the Earth is likely to enhance strongly the insolation signal associated with variations of Earth axis inclination angle whose highest variations do also occur in high latitudes. Atmospheric circulation speed changes, caused by change of temperature gradients between pole and equator are most likely first of all to influence the same orbital signal. The feedback caused by the concentration of greenhouse gases in the atmosphere of the entire Earth oscillation is most likely to highly affect the direct (not precession modulations related) eccentricity signal changing the average annual insolation of the entire Earth. Not that clear mechanism of precession climatic influence, associated in particular with paleomonsoons [Barron et al., 1985; Clemens and Prell, 1991; Rossignol-Strick, 1983] is to a greater extent of regional rather than global importance, hence we can see a specific effect of the Earth feedbacks on the precession signal.
[38] Noteworthy that the above discussed was the specificity of various feedbacks forcing on certain orbital signals. However, there is still a certain possibility that all the signals are affected by the same feedbacks. An example is a crucial feedback caused by snow and ice cover albedo change [Budyko, 1977], that in Pleistocene forced the signals related to variations of all the three orbital elements. The albedo relation forces mostly those insolation signals that are related to eccentricity and obliquity variations, hence the ice sheets presence in the Quaternary period is the main factor defining rhythms of the Pleistocene global climatic cycles.
[39] Thus stated in Milankovitch theory identical linear forcing of insolation signals hasn't been properly verified. Such a statement confirms that Milankovitch followers had to accept a "non-linear forcing'' mechanism of eccentricity insolation signal to agree with empirical data [Berger, 1999; Hays et al., 1976; Imbrie et al., 1984; 1993]. Furthermore, the same set of empirical data suggests the absence of the linear amplification postulated by these authors for those insolation changes due to variations in the axial tilt and precession as well. So, Milankovitch insolation curves [Milankovitch, 1930] and especially Berger and Loutre [1991] for 65oN show the dominance of the precession contribution, whereas in the majority of records of indirect paleoclimatic parameters 41-kyr component caused by axial tilt dominates that of 23-kyr (see Figures 1 and 2). It resulted in the conclusion that the enhancing mechanisms are neither "linear'' nor "non-linear'', but are "individual'' for the variations of each orbital element [Bol'shakov, 2003c].
[40] Hence, the proper account of different feedbacks is required to launch a correct paleoclimate orbital theory. It is precisely the account of the Earth feedbacks specificity in time and space that can make it possible to resolve the problem of zero average annual insolation changes of the entire Earth associated with the axial tilt and precession. Thus the real and reliable mechanism describing the climatic effect of different orbital elements variations, primarily precession, has to be developed.
[41] The conventional interpretation of precession climatic influence [Raymo and Nisancioglu, 2003 p. 1]: "...a glaciation can only develop if the summer high northern latitudes are cold enough to prevent the winter snow from melting, thereby allowing a positive annual balance of snow and ice'' can not be considered to be actually correct. Such an interpretation is incomplete for it considers just a half of the precession forcing, "cold summer'', whereas, as was discussed above one should account for a actually functioning full annual insolation cycle, i.e. long cool summer and short mild winter or long cold winter and short hot summer. It is the full annual insolation cycle for which the specific mechanism of precession climatic forcing should be found. One can not exclude that it is quite possible that Köppen's notion (long cool summer and short mild winter favouring the glaciation) might hold true to the northern hemisphere and the opposite one by Croll will be valid for southern hemisphere [Berger, 1980; Bol'shakov, 2003a, 2003c].
[42] It is also essential to remember that the colder summer results not only from the biggest distance from Sun, but also from the decrease of Earth inclination angle e. Angle e decrease causes average annual insolation decrease in high latitudes of both hemispheres. Caused by precession seasonal climatic contrasts are at the same time in antiphase in different hemispheres, while average annual precession insolation equals zero at any latitude. Obliquity variations result in single-phase change of average annual insolation at high latitudes in both hemispheres that is in turn forced by albedo feedback which may be considered the main reason of a stronger, as compared to precession, climatic influence of obliquity in Pleistocene. Thus as to the climatic forcing of summer temperatures mentioned above the obliquity rather than precession is to be considered as the most important. However, the orbital forcing of monthly or diurnal insolation automatically implies that it is precession that affects greatly the insolation changes (Figures 1 and 2). The above statement leads to incorrect notions of many scientists on the predominant precession influence on global climate change in Pleistocene time.
[43] The authors of papers [Loutre et al., 2004; Raymo and Nisancioglu, 2003] pay great attention to the hypothesis "...that a maximum in the insolation gradient enhances the poleward atmospheric transport of moisture from low latitudes... Moreover the high latitude cooling and the enhanced atmospheric transport favour the delivery of snow over the Northern Hemisphere. This mechanism thus suggests a possible direct link between annual mean insolation and glacial inception'' [Loutre et al., 2004, p. 9]. However it is important to remember that increased atmospheric circulation (at low angle e ), related to temperature gradients increase (and not insolation gradients) is the first of all negative feedback as it gives rise to temperature gradients decrease. Such a forcing is owing to the warm and humid air transfer from low to high latitudes and latent heat release during precipitation.
[44] Thus the feedback resulted from temperature gradient would actually try to weaken the positive albedo feedback. Of course, one should always remember that the main mechanism of the obliquity signal enhancement is the Croll's albedo feedback enhancing the effect of average annual insolation of high latitudes change. It is this mechanism that mainly increases the temperature gradients and thus atmosphere circulation. The fact that positive albedo feedback is greater than that of the negative caused by temperature gradient is confirmed by the presence of obliquity variations frequency in all the paleoclimatic records.
[45] The basis for full and simple explanation of climatic changes dynamics marked by orbital periodic patterns for the entire Phanerozoic is provided by global albedo changes. The Middle Pleistocene transition, for example, may be attributed to so-called parametric resonance mechanism. (About the possibility of resonance response on eccentricity forcing the reader is referred to Hagelberg et al. [1991]). According to this mechanism environmental conditions change results in system parameters and its resonance frequencies change. I understand the environmental changes as directed cooling in time interval from 2 till 1 million years ago that began as early as in the Eocene. It should have caused the volume of glaciations at high latitudes change and in turn changed their inertia. A possible mechanism of climatic oscillations cyclicity change at the turn of 1 m.y. can be shown as follows [Bol'shakov, 2001, 2003a].
[46] Earlier than one million years ago ice volume wasn't sufficiently high and Earth surface temperature was sufficiently low to provide the extent of ice comparable to those of the Pleistocene ice sheets. The change of glaciations mainly grouped at high latitudes volume in this situation was caused by rather short-period forcing of axial tilt according to empirically found 41-kyr periodicity of these changes. Thus it may be concluded that the corresponding time constant of glacial oscillations had the same periodicity at that time. Long-period forcing of eccentricity variations and associated with them global temperature oscillations were too weak to cause ice sheets extension. It is due to these reasons the variations of corresponding 100-kyr period are not reflected in climatic records of that time interval. (Eccentricity periods found by Clemens and Tiedemann [1997] in oxygen-isotope record of Pliocene-early Pleistocene might well be an artefact because the amplitude corresponding climatic variations is very small). Glaciers extended with time as global cooling increased.
[47] Probably it happened about one million years ago that Earth surface temperature and glacier mass at high latitudes reached critical values with the respect to the effect of insolation changes resulted from eccentricity variations. In this case even the "eccentricity'' fall of temperature was sufficient to prevent melting of glaciers extending from higher latitudes towards lower latitudes. On the other hand with glaciations mass and area growth made greater positive feedbacks due to albedo and presence of greenhouse gases in the atmosphere that in turn forced the extension of glaciation. Obviously it led to increase of time constant of glaciers extension and breakup. In my opinion it was the joint effect of these three factors that determined new rhythm pattern of glaciations during the last million years.
[48] Thus for the last million years the development of global glaciations dynamics is mainly determined by the simultaneous forcing of eccentricity and obliquity variations enhanced by positive feedbacks effect against the background of the global cooling. (Similar mechanisms of "the Middle Pleistocene transition'' with ice volume increase regarded as one of the main factors has been proposed by other researchers as well [Berger, 1999; Clark et al., 1999; Hagelberg et al., 1991; Imbrie et al., 1993]. However, these mechanisms do not that clearly state the leading part of eccentricity and related to Earth axis inclination variations insolation signals).
[49] According to the above proposed simple pattern the further increase of glacier volume would result both in increase of area of their distribution and extend the glacial oscillations period. Thus the growth of recent glaciation might well witness the climatic event of a longer 400-kyr eccentricity cycle which didn't show itself during the last 2 million years [Berger, 1999; Imbrie et al., 1993]. It is this statement that has been confirmed by the published data [Heckel, 1986; Veevers and Powell, 1987] showing the 400-kyr cyclicity that expressed in ocean level oscillations took place during the maximum phase of Permian-Carboniferous glaciation! (The Permian-Carboniferous Gondwana land glaciation is known to have greater as compared to those of Pleistocene, extents, i.e. its boundary reached 30oS [Veevers and Powell, 1987]). It means that the above data argue for the proposed mechanism establishing the relation between the insolation variations resulted from Earth axis inclination angle and eccentricity variations, changes in ice sheets volume and rhythm of their increase and decrease.
[50] The above emphasizes the key role of the ice sheet volume as affecting the orbital periods of climatic cycles. Hence the absence of ice would result in the appearance of different mechanisms of climatic oscillations. Thus one can guess, contrary to other scientists, that there would be no climatic cycles directly related to eccentricity and obliquity variations during thermal epochs. This assumption is generally confirmed by empirical data [Herbert and Fisher, 1986; Kent et al., 1995; Olsen, 1986, and others]. Thus in Mesozoic, Eocene and Miocene 23-kyr (precessional) oscillations modulated by eccentricity variations took place. Independent evidence for other orbital cycles hasn't been reported. The author has shown that the statement about direct appearance of 100-kyr eccentricity variations in records of Cretaceous by Herbert and Fisher [1986] is most likely to be a mistake of interpretation [Bol'shakov, 2003b].
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