China droughts unpredictable

Tree ring research reveals four major droughts affecting China over the past 1,000 years, the latest, in the 1870s, causing "tens of millions of people to starve to death". Researchers do not fully understand what caused these droughts and are as yet unable to predict future occurrences:

Global climate models fail to accurately simulate the Asian monsoon, and these limitations have hampered our ability to plan for future, potentially rapid and heretofore unexpected shifts in a warming world.

How is it then that climate models are expected to accurately simulate global conditions?