青藏高原于 2 5～ 1 7Ma前第二期强烈隆升即相当于喜马拉雅运动的二期 ,所达到高度与宽度 ,足以改变环流形势 ,它和同时期热带太平洋的变暖 ,南极冰盖出现后越赤道气流增强 ,亚洲东缘、东南缘边缘海盆的扩大、亚洲大陆的向西伸展、副特提斯海的萎缩等因素结合 ,共同加强了大陆与大洋的热力差别和动力作用 ,孕育了以夏季风为主的亚洲季风系统 ,替代东亚地面老第三纪的行星风系 ,导致东亚干旱草原带大收缩与湿润森林带大发展等的重大环境变化 .其具体出现时间 ,在高原东北边缘的临夏剖面为 2 1 8MaBP .植被从疏林草原转变为森林 . The intense uplift of the second stage of the Qinghai-Tibet Plateau from 25 to 17 Ma is equivalent to the second stage of the Himalayan movement. The height and width reached by the Qinghai-Tibet Plateau are sufficient to change the circulation situation. It is consistent with the warming of the tropical Pacific over the same period and the emergence of the Antarctic ice sheet The increase of the cross-equatorial airflow, the expansion of the marginal basin in the eastern and southeastern Asian margins, the westward expansion of the Asian continent and the shrinkage of the Tethys Sea will jointly strengthen the thermal differences and impetuses of the mainland and the oceans, The monsoon-based monsoon system replaces the old Tertiary continental wind system in East Asia, resulting in major environmental changes such as the great contraction of the arid steppe zone in East Asia and the great development of moist forest belt. The specific appearance time of the Asian monsoon system on the northeastern edge of the plateau The Linxia section is 218 Ma BP. The vegetation is transformed from a forestland to a forest.