5. On the Global Climatic Forcing of the Total Annual Insolation Variations

[51]  Turning back to the title of paper by Loutre et al. [2004] it can be mentioned again that a possible climatic influence of annual insolation is not a problematic issue. The problem is how to more correctly determine the mechanism of full annual insolation variations climatic effect.

[52]  It should be noted that more than 150 years ago Herschel and von Humboldt stated that the account of full annual insolation of the entire Earth is required when climatic influence of insolation variations is discussed. Imbrie wrote on this subject: [Imbrie, 1982, p. 413]: "There has also been a tendency for investigators to believe they could model the response of the system from a radiation curve representing the input at a single latitude and season [e.g., Milankovitch, 1941; Kukla, 1968; Broeker and van Donk, 1970]. Since no one could be sure which insolation curve, if any, was the crucial one, investigators had great freedom to choose a curve that resembled a particular set of data. Understandably the resulting ambiguity did much undermine confidence in the validity of the time domain prediction. Starting in 1976, with the advent of numerical models that integrated the effect of insolation changes over all latitudes and seasons, this situation was much improved...'' However, those are just words and Imbrie himself used monthly (June) insolation at 65oN for modeling and paleoclimatic interpretations [Imbrie and Imbrie, 1980; Imbrie et al., 1993].

[53]  It is also the case when we refer to Berger, Loutre and Gallee paper. These authors wrote in 1998 [Berger et al., 1998, p. 616]: "Such time-depended climate models must therefore be forced only by the astronomical variations of insolation for each latitude and day...'', but on the next paper of the same article they write: "June insolation at 65oN is very often used as a guideline for the analysis of climatic changes and, in particular, for ice volume changes''.

[54]  Six years later we find a similar case in the paper by Loutre et al. [2004, p. 2]: "A more general version of the astronomical theory is now widely used, especially in climate modeling where changes in insolation at all latitudes and times of the year are taken into account. Nevertheless, it is often supposed that insolation at 65oN in June can be used for comparison with most proxy records''. The paper gives not a single reference to a "more general version''.

Figure 3
[55]  The use of semi-annual or monthly (diurnal) insolation for climatic modeling and paleoclimatic interpretations follows from the view of precession as a major orbital factor in control of global climate changes. For example the Loutre and Berger [2000] statement that the "cold orbit'' (giving rise to glaciations in the hemisphere discussed) shows high eccentricity value, low value of Earth axis inclination and the Earth in perihelion in the day of winter solstice arises from this view. However, as was mentioned above such concepts are not supported by paleoclimatic data: precession periods appear only slightly in paleoclimatic records of global changes (see Figures 1 and 2). Whereas Figure 3 shows that glaciations of the last million years are associated only with eccentricity curve minima, but not maxima. In this case these are not absolute eccentricity values that are important but the tendencies of its change: the value of some maxima associated with interglacials is lower than that of minima related to glaciations. (The later argues for the resonance mechanism for the explanation of 100-kyr glaciation cycles in the Pleistocene).

[56]  Thus the assumption about precession variations being the main factor in control of global changes in Pleistocene turns to be incorrect. (As one remembers this idea has been developed over more than 150 years by Adhémar, Croll, Milankovitch and his present devotees). Therefore a great attention to climatic influence of average annual insolation related to Earth axis inclination angle in publications [Elkibbi and Rial, 2001; Huybers and Wunsch, 2005; Loutre et al., 2004; Paillard, 2001; Raymo and Nisancioglu, 2003], seems quite obvious. It would be better if the scientists' attention in future wouldn't be focused only on the obliquity alike precession that was earlier regarded as a major tool controlling the global climatic changes. There are all the grounds that the understanding of a need to consider caused by all the three orbital elements annual insolation changes of the entire Earth and associated feedbacks will appear earlier than in the end of the 21st century.

[57]  Above the assumption made earlier in publications [Bol'shakov, 2003d; Hagelberg et al., 1991] of direct (not related to precession modulation) climatic effect of eccentricity variations through resonance mechanism has been justified. Nevertheless, such logic assumption is objectionable. For example, Maslin and Ridgwell [2005] believe that eccentricity cannot directly cause 100-kyr glacial-interglacial cycles. To prove the later they specify that the periods of eccentricity spectral peaks are 95, 125 and 400 kyr, while the only one 100-kyr period appears in paleoclimatic records. In fact, however, periods of approximately 95 kyr and 125 kyr occur in many oxygen-isotope records (see Figure 2 and [Bol'shakov, 2003b; Rial, 2003b]). The reason why the 100-kyr peak is often not divided in oxygen-isotope records was provided by Rial [1999]. He showed that in order to divide the broad 100-kyr peak into the components the long-term and well chronometrized records that are in fact just a few, are required.

[58]  The lack of 400-kyr periodicity in the Pleistocene paleoclimatic record (and 11th stage problem) are well explained with the given above mechanism of parametric resonance. In this case eccentricity insolation oscillations are like the mechanism triggering the main cycle of oscillation while a form of climatic response is as well defined by the forcing of other orbital signals and the state of climatic system during the time period discussed.

[59]  Thus the arguments of Maslin and Ridgwell [2005] against the direct forcing of eccentricity insolation on the climate can be easily removed. Nevertheless these authors conclude that the 100-kyr cycle is precession-controlled. Such conclusion was based on the fact that: " Raymo [1997] proposed that an episodic appearance of unusually low maximum of northern hemisphere summer insolation is a critical factor in control of the following glaciation''! Neither Raymo [1997] nor Maslin and Ridgwell [2005] give any physical justification of this assumption.

[60]  It should be added that to explain the 100-kyr cycle is commonly involved the so-called mechanism of "energy transfer from precession level to that of eccentricity'' due to minor eccentricity insolation change compared to that of precession or related to obliquity, calculated for one month and one latitude. Nevertheless, the mechanism of such transfer is not specified. It hasn't also been explained why the "high-energy'' precession signal is the weakest in Pleistocene paleoclimatic records. Therefore clear is the tendency to complicate the explanation of the paleoclimatic influence of eccentricity variations mechanism. Unfortunately, a similar tendency appears in Berger et al. [2005].

[61]  The authors study the origin of astronomical 100-kyr cycle. According to them eccentricity variations can't contribute that much into paleoclimatic records [Berger et al., 2005]: "As the 100-kyr variations in standing insolation due to eccentricity change are too small, they cannot be the direct cause of the ice ages''. Clearly, Berger et al. are true followers of Milankovitch as they focus only on quantitative changes in discrete insolation change accounting for monthly or diurnal insolation at a single latitude. They do not take into consideration thereby well-known (and above shown) qualitative differences in insolation variations related to certain orbital elements and actually affecting all the latitudes of the Earth all the year round.

[62]  The authors consider the "variations of eccentricity, of its first derivative, of the frequency modulation of obliquity, and of the inclination of the Earth's orbit on the invariable plane of reference'' to be the astronomical sources of 100-kyr signal forcing the Earth climate.

[63]  The science has got a reasonable principle, never use more complicated versions to explain any phenomenon until the simple ones haven't been settled. Hence the problem stated by Berger et al. [2005] doesn't seem quite based. Moreover, it's again a case of baselessly complicated version.

[64]  As a matter of fact it's hard to find any advantages in explanation of 100-kyr climate cycles assuming them caused not by eccentricity change, but by the change of its derivative. A definition of the mechanism of climatic influence (if it exists) due to the modulation of Earth axis inclination also should be further studied. As to the possibility of significant climatic influence of the ecliptic inclination changes, it clearly doesn't fit empirical data [Berger, 1999; Bol'shakov, 2003b]. First, the ecliptic inclination variations are characterized by a single period close to 100-kyr, whereas empirical data reveals two eccentricity periods of about 95 and 125 kyr (Figures 1, 2 and 3). Second, the considerable phase difference between ecliptic inclination oscillations and corresponding climatic component has been found [Berger, 1999].


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