Impact of Orbital Parameters in Low Latitudes

Data from the Vostok ice-core show for glacial times (Petit et al. 1999) temperatures of about 8,3-8.5°C below pre-industrial values. For LGP times an ELA depression of 1200 m was inferred (see above). Using an atmospheric temperature gradient of 0.007 C per meter this implies a temperature drop of 8.4°C. Thus the field data (ELA depression) and the isotope-data from the Vostok ice-core yield independently consistent results. Therefore, at a first glance, both for high-latitudes (Vostok ice-core) and the subtropics (Tibet-Plateau) a comparable drop of the temperature can be inferred. Using an atmospheric temperature gradient of 0.008°C/1 m (meaning extreme aridity) summer temperatures would drop 9.6°C.

An ELA depression is however a relative value documenting a balance of precipitation in the catchment area, altitude, temperature and ice-melting. In principle it can result from a temperature drop. It can also result from an increase in precipitation in the catchment area. This increase in precipitation may or may not be accompanied by a temperature drop. Conceptually the effect of an ELA depression by 1200 m can also be achieved by a respective uplift by the same value. Considering however the consistency with the Vostok ice-core data a temperature drop by 8.4°C is regarded as possible interpretation.

Progressive cooling, lowering of the equilibrium line, or uplift of the Tibetan plateau above the ELA must have led to an initially more extensive ice cover in some parts of the surrounding mountains, as well as on Tibet itself.

Due to geometric conditions the variation of orbital parameters is, regarding radiation differences, more important in the high latitudes than in the subtropics (Milankovic 1941; Schwarzbach 1974, pp.300, 303). Following the equations of the variation of orbital parameters contributes to temperature differences of about 3,5°C (Schwarzbach 1974). A global equilibrium line depression of about 500 m correlates with this 3,5°C drop in temperature. The ocurrence of an insolation maximum in connection with the earth being at perihelion is expected to have a higher effect than an insolation maximum in connection with aphelion. The same applies to a high insolation ocurring during NH summer versus SH summer (such as different albedos of the hemispheres). Apart from this, Milancovic radiation anomalies do not apply to the whole earth at the same time; they have an alternating cooling effect on the N and the S hemisphere.

Above it was shown that the Tibet Plateau receives radiative input that is close to the solar constant at this latitude. Thus radiative losses during glacial times are high. If an insolation maximum that results from a superposition of above-mentioned processes coincides with a glaciated Tibet-Plateau a stabilization of the glaciation by reflection is to be expected. If an insolation-maximum coincides with a non-glaciated Tibet-Plateau a warm time-interval is expected to be extended. If an insolation maximum coincides with a phase of either buildup or decay of glaciers, then depending on the degree of buildup/decay that already took place (amount of reflected radiation), decay or buildup will either be accelarated or stopped and reversed. Fluctuations of isotope values of the Vostok ice-core that tend to document a started but uncompleted deglaciation are consistent with this.

Thus, time-resolved, depending of the superposition of each of the involved factors the Tibet-Plateau might either have been a stabilizer of a warm age (scree effect), a pacemaker of a warm age, accelarating already ongoing decay of ice, a pacemaker of an ice-age, accelarating glaciation by backscattered radiation, a trigger of an ice-age or a stabilizer of an ice-age.

The isostatic effects mentioned above contributed also to this.

Even with an equilibrium line depression, or uplift of the plateau, of only 500 m, glaciers would have covered at least one third of Tibet. This is evidenced by the reconstruction of Late Glacial and Neoglacial ice margins and the corresponding equilibrium line depressions. The increase in the glacier surface area is a function of the depression of the equilibrium line (Figs.3b and 4c) and the average elevation of valley floors or the plateau. Owing to glacier tongues descending almost twice as far as what the ELA amounted to, an increase in surface area was achieved even before the equilibrium line has reached its lowest position. At that time large-scale foreland glaciations built up from the mountains towards the plateau, and in the shallow main valleys (Karakorum) this resulted in sudden gains in reflection surfaces as the pace at which they filled up with ice increased. These valley fillings and the build- up of glaciated mountain forelands extended the nourishment area of the glaciers. Such sudden increase in the glacier area could only originate from the high mountains above the plateau. The mountains acted, to a certain extent, as "crystallization centres" for the build-up of the ice. Glacier formation was promoted by mountains which towered high above the plateau. Initially, the volume to be filled was small due to the presence of shallow valleys. With a decreasing equilibrium line the proportion of supply areas increased. It follows that these shallow valleys were forced to fill up and that their glacier surfaces coalesced to form an ice cap. The glaciation of about one third of the Tibetan highlands, with subtropical radiation parameters, was more effective in triggering an autocyclic mechanism than all the remaining areas of the earth. This in turn led to the drop in temperature of about ca. 10°C, and thus to the High Ice Age proper.

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