Multi-proxy records from ice caps, lakes and pollen records from the Tibetan Plateau are used to reconstruct the palaeoclimate during the period 30-40 ka B.P. Lake geomorphological and lacustrine sedimentological studies show that numerous high lake level stands and/or freshwater lakes occurred in large areas of western China at this time, almost covering the entire Tibetan Plateau. Today the area contains mostly shallow water and saline-hypersaline water lakes under extremely dry conditions. Pollen-based reconstruction of Tibetan vegetation shows systematic differences from the present; the northern limit of alpine steppe-forests was shifted ca. 400 km and the alpine conifer forest extended ca. 400-800 km beyond their present western limits. Variations in d 18O curves from ice cores from the plateau suggest that the climate was exceptionally warm and wet with temperature 2-4? higher and precipitation 40? to over 100? higher than today. All of the evidence consistently suggests the existence of an exceedingly strong summer monsoon climate over the plateau.

The occurrence of such warm and wet conditions can be attributed to the stronger summer low pressure over the Plateau, which strengthened the influence of the summer monsoon from low latitude oceans. The vigorous evaporation of the tropical ocean surface would also play an important role in promoting the penetration of moisture-rich southwest monsoon over the Tibetan Plateau. These climate patterns with strong summer monsoon circulation during the period 30-40 ka B.P. inflect the 20 ka earth orbital precessional cycle, when the Tibetan Plateau received extraordinary strong solar radiation which thus enlarged the thermodynamical contrast between the plateau and the mid-south part of the Indian Ocean.