Environmental variability within the Chinese desert-loess transition zone over the last 20000 years

The desert/loess transition zone in northern China is sensitive to climate variability, which is controlled mainly by the relative strengths of the East Asian summer and winter monsoons. Sandy loess layers found in the Loess Plateau and palaeosol sequences found in the sandy desert demonstrate latitudinal shifts of the southern desert margin over the last 20000 years. Stratigraphic investigations together with radiocarbon and some thermoluminescence dating, show that during the last glacial maximum the desert margin was at its most southerly position (38°N). During the early Holocene, it moved northward about three degrees in latitude (41°N). At present, the desert margin is again close to its most southerly position (38°21 N), yet the northern boundary position of modern summer monsoon activity is placed at 41°45 N, which is close to the estimated Holocene Optimum desert margin (41°N). This situation cannot be explained from natural climate models. Hence, an external driving mechanism needs to be considered, and the most obvious one is that caused by human activity on the natural environment over the last 3000 years. Historical evidence tends to reinforce this consideration.