Global monsoon in a geological perspective

Monsoon is now considered as a global system rather than regional phenomena only. For over 300 years, monsoon has been viewed as a gigantic land-sea breeze, but now satellite and conventional observations support an alternative hypothesis which considers monsoon as a manifestation of seasonal migration of the intertropical convergence zone (ITCZ) and, hence, a climate system of the global scale. As a low-latitude climate system, monsoon exists over all continents but Antarctica, and through all the geological history at least since the Phenorozoic. The time is ripe for systematical studies of monsoon variations in space and time. As evidenced by the geological records, the global monsoon is controlled by the Wilson cycle on the tectonic time scale (10⁶–10⁸ a). A “Mega-continent” produces “Mega-monsoon”, and its breakdown leads to weakening of the monsoon intensity. On the time scales of 10⁴-10⁵ a, the global monsoon displays the precessional cycles of ∼20 ka and eccentricity cycles of 100- and 400-ka, i.e. the orbital cycles. On the time scales of 10³ a and below, the global monsoon intensity is modulated by solar cycles and other factors. The cyclicity of global monsoon represents one of the fundamental factors responsible for variations in the Earth surface system as well as for the environmental changes of the human society. The 400-ka long eccentricity cycles of the global monsoon is likened to “heartbeat” of the Earth system, and the precession cycle of the global monsoon was responsible for the collapse of several Asian and African ancient cultures at ∼4000 years ago, whereas the Solar cycles led to the demise of the Maya civilization about a thousand years ago. Therefore, paleoclimatology should be focused not only on the high-latitude processes centered at ice cap variations, but also on the low-latitude processes such as monsoons, as the latter are much more common in the geological history compared to the glaciations. Supported by National Basic Research Program of China (Grant No. 2007CB815902)