The consequences are even more far-reaching. Under present conditions, summer warming of the Tibetan plateau leads to areas of low surface pressure and hence to a marked atmospheric pressure gradient in relation to the relatively cold adjacent oceans. Strong winds are the result: the East Asian and the Indian summer monsoons (Findlater 1974). The pattern is reversed in winter. Then the high albedo of the winter snow cover of the Tibet Plateau leads to cold-induced high surface pressure and thus to a compensatory flow of air to the low surface pressure areas. This occurs over the now relatively warm oceans (i.e. the winter monsoon circulation, Flohn 1981; Ding et al.1995; Xiao et al. 1995). Inevitably, the existence of a perennial Tibetan ice sheet must have modified this seasonally alternating large-scale pattern of monsoonal circulation. Whereas the summer monsoon would have been either weaker or non-existent (Sirocko et al. 1993), the winter monsoon must have been much stronger. Data from deep-sea cores from the Arabian Sea are consistent with this. They allow the reconstruction of changes in the upwelling system off Arabia due to variations in strength of the SW Indian monsoon circulation. For the last 500,000 years the data show that the summer monsoon was substantially weaker during the glacial phases, but the winter monsoon was stronger (Anderson and Prell 1993; Emeis et al. 1995). From 2.5 MaB.P. onwards loess accumulated on the loess plateau of China. It is interpreted as record of the onset of winter monsoon circulation (Kukla and An 1989; An et al. 1990; Ding et al. 1992).
High-resolution loess-paleosol sequences from China spanning the last 2.5 Ma permit a reconstruction of the intensity fluctuations of the East Asian summer monsoon. They supply further confirmation that the summer monsoon had been dramatically weaker during glacial times (Rutter and Ding 1993). For the East Asian winter monsoon, however, the same sequences record a marked increase in intensity during glacial phases (Ding et al. 1995; Xiao et al. 1995). Both marine and terrestrial sediment records document a highly significant correlation between variations in global ice volumes and corresponding counterfluctuations of monsoon circulation throughout the entire ice age era.
Thus two interpretations for neogene and future climates are possible:
The first is that warm tropical oceans in connection with an ice-covered Arctic Ocean and the polar night cause pronounced latitudinal temperature gradients. These contribute to high wind speeds during northern hemisphere winter. This includes also, as GCM runs that had been driven by reconstructed sea surface temperatures from the Pliocene showed a strong winter monsoon. Whether this contributes to the Tibetan inland-ice needs to be studied.
The second is that the Tibetan inland-ice itself generates respective regional temperature gradients.
If both mechanisms apply both strong latitudinal temperature gradients, particularly warm tropical oceans and the Tibetan inland-ice can stabilize winter conditions that support an ice-age.
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