Duration of Ice Buildup from Warm Conditions

After the polar regions the Tibet-Plateau, due to its high altitude, is the first to be widely glaciated. This raises the question of timing and magnitude, also with respect to a potential isotope signal this might produce.

Today in central Tibet the precipitation is 400 mm/yr (Lhasa 437 mm/yr: more in the east and less in the west). Assuming 50% evaporation, 200 mm water column remain to form ice. With a corresponding ice-column of 222 mm 5000 years are needed to form 1110 m ice.

With a potential weaker summer-monsoon (Kuhle 1982a, 1982c) and 167 mm/yr precipitation the observed 1000 to 2000 m thick inland ice could have formed within 10 000 years. This appears to be consistent with the steep flanks of the fluctuations of the Vostok ice-core. A cooling of 8°-10°C, with precipitation similar to current values, would provide the most simpel model of ice build-up. During warm times the precipitation was most likely higher. During cold times it might have been less. Thus it appears justified to use present-day values for the precipitation as first approximation.

On the other hand during glacial times (Flohn, pers. comm. November 1987) the aridity was globally 10-30% higher than at present. At a first glance an even more reduced precipitation might be expected.

While globally this mechanism is correct, increased cooling means a lowering of the snowline and increased precipitation at lower altitudes. In Tibet and in the Karakorum the situation is different. In the Karakorum and Hindukush, the most arid area under investigation (Fig.1, No.15, 16, 32-34, 36, 37, 55, 56), exponential increase in precipitation of 1000-2000 mm/yr presently occurs at altitudes above 4000 m on the southern side, and above 4700-5000 m on the northern side (Kuhle 2004c, p.96). On the other hand, on the valley floors between 1000 and 4000 m asl, precipitation fluctuates between 40 and 250 mm/yr (Kuhle 1994; Miehe et al. 2000). During glacials, the altitude of heavy precipitation is expected to have been lowered by 1200 m to 2800-3500 m asl to be in accordance with the observed ELA depression.

Thus during glacials a very substantial increase in precipitation in the supply area can be expected. Added to this is build-up of the ice cover, which led to an additional increase in the supply area surfaces, and thus to an improvement in the glacier nourishment. This autofeedback supports the concept of a rapid glacier builtup once it is triggered.

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