Quaternary climate history

Past stable-isotope records from the ice core and the deep ocean sediment core revealed that the earth has experienced a repeated climatic rhythm of glacials and interglacials with a cycle of about 100 ka (thousand years) after 800 ka BP (before the present). According to a climate history since 3.5 Ma (million years) BP reconstructed by a Chinese scientist based on analysis of loess deposits in northeastern China, the climate has experienced alternate periods of desertification and inter-desertification (Geng 1986). In his interpretation, the typical profile of loess deposits consists of a series of alternate depositions of the eolian yellow loess layer transported from the inland desert region during the dry period (desertification), and the brown-reddish palaeo-soil layer formed in situ during the wet period (inter-desertification). Although the glacial and the interglacial do not necessarily correspond to the inter-desertification and the desertification, respectively, it may be said that the inland climate in China was drier during the interglacial, or warm, period than during the glacial, or cold, period. The present climate since 2 ka BP in northern China corresponds to a desertification.

Closed lake level records in the northern middle-latitude zone also suggest a drier climate during the hypsithermal interval (HI) at around 6 ka BP, the warmest period during the present interglacial, and a wetter climate during the last glacial maximum (LGM) at around 18 ka BP (Street-Perrott and Harrison 1985).

The saltiest lake water in the world, in the Dead Sea, is a result of rapid desiccation of the lake after the LGM to the HI, during which the lake level dropped by more than 200 m (Issar et al. 1989). However, in the Indus River basin on the same latitude of 30°N, the climate was drier during the LGM and wetter during the HI compared with the present climate. The Indus civilization flourished during the wetter climate following after the HI, and finally disappeared at around 3.3 ka BP (Khanna 1992) when a dramatic decrease in precipitation occurred during the course of global cooling after the HI (Lamb 1982). Climatic changes near the 30°N latitude were very variable and differed from region to region, as shown in table 1, because a small latitudinal shift of the position of the polar front could result in dramatic increases or decreases in local precipitation. It may be said that the climate in arid lands in the subtropical or middle-latitude zone is highly variable both in time and space, as evidenced by many past climatic records.

Table 1 Climatic Changes in the Zone of Latitude 30°N

a. Last glacial maximum.
b. Hypsithermal interval.