The Radiation Balance

The preceding chapters presented a synopsis of data that describe warm and cold environments in Tibet and Central Asia. Important data are laterites. At the respective sites they either have been overridden by LGP glaciers or LGP glacial sediments overlie unconformably these laterites. Due to adiabatic cooling on the High Plateau north of the Himalayas no laterites have been found. Whether in older times Tibetan laterites could have formed in warm times before a glaciation commenced is not known. A drillsite at (36°48N/99°04'E, 3170 m asl) showed 6-15 Quaternary ice advances and retreats (Fig.2). The location near the NE fringe of the inland ice in the foreland of the Kukunor Shan permits tentatively a correlation with the Vostok ice-core. A drillsite in Central Tibet, probing through the Pleistocene into the Pliocene, studying also palynological samples, is needed to solve this question. Recent warming shrank some existing valley glaciers on the Himalayan south side (Kuhle 2004b, p. 18-20). It is therefore inferred that continued warming to Pleistocene greenhouse conditions, 2.5-3°C above preindustrial values, shrinks present valley glaciers further. Testing climate models for such times using a reduced mountain glaciation appears reasonable. Data about vegetation on the Tibet Plateau during Pliocene, Pleistocene and possible future warm times do not exist. Coupled GCMs such as the current version of CSM from NCAR might shed light onto this question. If the present vegetation and the present glaciers are modeled correctly the modeled vegetation of Pleistocene and Pliocene warm times can be tested against data from drillsites. This applies to the present long cores from Lake Baikal and to a necessary future drillsite on the Tibet Plateau. The ELA on the Himalayan South is expected to be at roughly 5300 m, on the Tibet Plateau at 5600 m asl. To provide milestones climate models have to meet hard data on the ice-coverage of the Tibet Plateau. The preceding sections provide them. This applies both to ice-thickness in valleys and to the extension of inland ice of considerable thickness on the Tibet Plateau.

These data are summarized (ice extension and thickness) as overview in Fig.3. This Figure, that is based on about 39 expeditions during the past decades shows that the preliminary maps of CLIMAP (1981), indicating a small mountain glaciation for the LGM, have now been improved by hard data that show a large inland ice for the LGP and LGM. The author suggests to use these up to date field data to drive or test climate models.

Today the Tibet Plateau has an altitude of 5200 m meters. The low latitude (about 30°N, corresponding angle of incidence) together with the high altitude implies that from 1360 W/m2 about 1000 to 1300 W/m2 reach the surface. To determine the actual radiation-balance between August and November in 1984 and 1986 climatic parameters were measured on Mt. Everest and Shisha Pangma in southern Tibet (28°N; Fig.1, No.14), as well as on K2 in NW Tibet (36°N; Fig.1, No.15).

Eight climatic stations were installed for that purpose at altitudes varying from 3800 to 6650 m asl. At the same time portable, handoperated instruments allowed comparative measurements in other places. Measurements of radiation, and radiation balance, on rock or scree, as well as on glaciers have been carried out. Approximately 25,000 data records on radiation, return radiation and albedo were obtained. The data are published in Kuhle & Jacobsen (1988) and Kuhle (1985, 1988b; 1989, p. 277-279; 1994; 1996c, p. 209-212; 2005a).

When weather conditions were such that radiation was not impeded by cloud cover, the values of incoming radiation were between 1000 and 1300 W/m2. The latter is approximately the solar constant at the upper limit of the atmosphere in relation to the corresponding position of the sun at that time. Theoretical incoming radiation on September 21st, as a mean value, is about 1180 W/m2 at the latitude of the area under investigation (30°N).

An unglaciated Tibet Plateau with its large fraction of dark matter absorbs most of this energy (albedo 15-20%), either directly heating rocks and disseminating the heat at night and by wind or indirectly through sublimination of snow, melting and evaporation. Lumping all processes together the Tibet Plateau is today one of the most effective radiative heating sources of the Earth. Thus a non-glaciated Tibet tends to stay in this situation. In other words: A non-glaciated Tibet-Plateau contributes to stabilizing the climate system in this state.

On glacier surfaces, especially on fresh surfaces in feeding areas, 85-90% of the shortwave radiation (0.3-3^m) is reflected. During the LGP about 97% of the highland area was covered by ice. Being glaciated, also at such high altitudes and due to geometric conditions (angle of incidence) the Tibet-Plateau reflected considerable amounts of energy. In addition at 6600-7000 m asl, the greenhouse-effect no longer applies since the glacial aridity reduces the moisture content of the atmosphere.

With evidence for a glaciated area of 2.4 Mio km2 and 1180 W/m2 as average value for September this is an energy input at the surface of about 2832 x 1012 W (1180 x 1 106 x 2,4 x

106) during non-glaciated times. This reduces by 85%, i.e. 24072 x 1011 W to only 4248 x 1011 W during glaciated times. This means that during glaciated times the Tibet-Plateau tends to keep the system in this state as well. Based on a rough approximation of the radiation energy-balance at the surface the Tibet-Plateau appears to have the potential to serve as one of several stabilizing factors of the climate system both during warm times and during glaciated times.

Also in the case of the scandinavian and northern hemisphere glaciers excluding the Tibet Plateau that means in both cases it must be noted that the preceding lines refer to energy input at the surface. In mid and mid-high latitudes the atmosphere itself absorbes also considerably. The processes discussed for high altitudes in Tibet apply also to high latitudes in Scandinavia, Eurasia and North America: Glacial aridity reduces also the moisture content of the atmosphere in high latitudes. Thus energy absorption by water vapor of the atmosphere might reduce. An optically more transparent atmosphere permits more shortwave radiation down to the surface. High albedo-values of the glaciers contribute to stabilization of the system in the glaciated state.

With an atmosphere approximately transparent to radiation, incoming radiation at Tibetan altitudes produces an energy input at the surface that is at least four times higher than that between 60°N and 70°N. This applies also to the Pleistocene North European inland ice centre (Bernhardt and Phillips 1958). Radiation loss through diffuse reflection of the atmosphere down to sea level (Lauscher 1956) is about 7%. Therefore the Northern Hemisphere glaciers excluding Tibet (32,5 Mio km2), scaled down to the approximated average radiation input (ca. 300 W/m2 instead of average ca. 1180 W/m2 in Tibet) contribute ca. 4 times to the stabliziation of the climate system either in a warm or in a cold state.

A quantitative test through long transient coupled GCM runs is a task for the future. Above approximatation might explain (data in the Vostok ice-core, Petit et al. 1999) why, after a glaciation was initiated (such as on the Tibet Plateau) the window of warm conditions appeared difficult to reach again.

A Tibetan inland ice surface of 2.4 x 106 km2 implies a cooling effect equal to at least 9.6 x 106 km2 (4 x 2.4 x 10 ) of Nordic inland ice. This latter value represents an ice sheet more than twice the size of the North European Weichsel ice.

A statistical test of the radiative impact of the Tibet inland ice can be found in Lautenschlager et al. (1987, pp. 8-40).

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