Extended Abstract

Address of the author: Prof. Dr. Matthias Kuhle - Geography and High Mountain Geomorphology; Geographisches Institut der Universität Göttingen, Goldschmidtstr. 5, D-37077 Göttingen/Germany; E-mail: [email protected]

(2.4 Mio km2) and thickness (800-1000 m) of High Asian inland-ice. This alternations of inland-ice and non-glaciated situations followed upper pliocene subtropical conditions. Currently the Tibet-Plateau has an altitude of approx. 4400-5600m. Measurements showed that at its high altitude the incoming radiation approximates the solar constant. The low latitude contributes to a high energy input. During glacials about 75% of the incoming radiation is reflected. Thus low latitude and high altitude contribute to a substantial albedo-induced cooling. During non-glacial times much of the high energy input from low latitudes and high altitudes remains in the system. Therefore in either environmental situation, Tibet inland-ice or warm non-glaciated the Tibet-Plateau contributes to stabilizing the climate. The formation of glacials from non-glaciated conditions is as well delayed as the formation of non-glacial conditions from glacials.

Upper Pliocene suptropical sediments show that conditions to support the formation of an inland-ice have been established after about 2.75 Ma. This coincides with the formation of large glaciations on the northern hemisphere. The altitude and latitude induced energy loss by reflection from a glaciated Tibet-Plateau is regarded as one important co-factor to this. A small number of outcrops show laterite deposits overlain by glacial sediments. It is thus inferred that, also in Central Asia, considerable differences between glacials and warm times existed. A series of drillsites that documents Quaternary and Pliocene environments is thus needed.

For the future two scenarios are possible:

Currently the Tibet Plateau uplifts by 10 mm/a. To support a reglaciation the TibetPlateau needs to be elevated above the snow line. An anthropogenic warming by 1°C causes a prolongation of the current warm phase by ca. 15.35 ka as an uplift by additional ca. 160 (140-200) m is needed to support a reglaciation.

Currently for Central Asia pliocene subtropical environments are interpreted as being linked to a Tibet-Plateau that is lower than at present. Thus in Central Asia the reestablishment of pliocene conditions requires a massive change of boundary conditions to counteract the uplift of the Tibet-Plateau. Therefore for Central Asia the conditions of warm interglacials are currently regarded as more probable for the future than upper pliocene conditions. New drillsites are needed to address this issue.

The uplift of Tibet to high altitudes is synchronous with the onset of large glaciations on the Northern Hemipshere from ca. 2.75 Ma on. It is inferred that a causal link, namely the passing of a threshold value exists. Absolute datings with different methods assign MIS 4-2 to the observed glacigenic forms. Thus older glaciations are inferred to have comparable or reduced extensions. Using 13 climate stations radiation- and radiationbalance measurements have been carried out between 3800 and 6500 m asl in Tibet. They indicate that the incoming subtropical global radiation reaches its highest values on the High Plateau. The resulting radiation balance indicates that today Tibet is the most important heating surface on Earth.

In glacial times 70% of the incoming radiation was reflected by snow and firn of the glaciated 2.4 million km2. Assuming that 100% of the non-reflected incoming radiation is transformed to heat, including latent heat, 32% of the global energy deficit during glacial times is caused by the Tibet plateau. If the radiation deficit caused also a temperature-deficit, e.g. in warmer times in Tibet the incoming radiation was transformed to heat, in glacial times the Tibet Plateau would have been the most important cooling surface of the Earth. Interglacial periods are explained by glacio-isostatic lowering of Tibet by 650 m. The extended foreland glaciations are interpreted to have moved below the snow-line. At least rapid deglaciations are supported by this boundary conditions. A subsequent ice-age is supported if isostatic uplift, caused by the molten foreland glaciations, raised the Tibet-Plateau above the snow-line. The comparably small energy differences caused by the variation of orbital parameters are regarded as modifying component of climate changes. If a glaciated, and isostatically uplifted Tibet is subject to a warming of 4 C° a loss in glaciated surface would be observed but no deglaciation. The same impact on an isostatically depressed Tibet Plateau can contribute to a deglaciation. Thus the Tibet Plateau has a stabilizing effect for the global climate. Both warm intervals are stabilized once they are established (time-lag of isostasy) and glacials are retained after their establishment as well. Currently the Tibet Plateau uplifts by 10 mm/a. An anthropogenic warming by 1°C causes a prolongation of the current warm phase by ca. 15.35 ka. The reason is that the Tibet-Plateau has to be lifted isostatically to an altitude that exceeds those from the preceding glacial by ca. 160 (140-200) m. Based on the Vostok-Curve we should currently observe temperatures of 4.5 C° below preindustrial values if the conditions from the preceding deglaciation/glaciation are applied. The author interprets that the uplift since the LGP was insufficient to iniate a glaciation. Potential changes of geological boundary conditions are not included. Thus the stabilizing contribution of the Tibet-Plateau for the climate system, both for cold and warm time-intervals is one of the main results of this work.

Key Words: Quaternary and Neogene Climate, Tibetan Inland Ice, Stabilizing Climate

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