6.5.2.2 What Is the Understanding of Abrupt Changes in Monsoons?

In the tropics, precipitation-sensitive records and models indicate that summer monsoons in Africa, India and Southeast Asia were enhanced in the early to mid-Holocene due to orbital forcing, a resulting increase in land-sea temperature gradients and displacement of the ITCZ. All high-resolution precipitation-sensitive records reveal that the local transitions from wetter conditions in the early Holocene to drier modern conditions occurred in one or more steps (Guo et al., 2000; Fleitmann et al., 2003; Morrill et al., 2003; Y.J.Wang et al., 2005). In the early Holocene, large increases in monsoon-related northern African runoff and/or wetter conditions over the Mediterranean are associated with dramatic changes in Mediterranean Sea ventilation, as evidenced by sapropel layers (Ariztegui et al., 2000).

Transient simulations of the Holocene, although usually after the final disappearance of ice sheets, have been performed with EMICs and forced by orbital parameters (Box 6.1). These models have pointed to the operation of mechanisms that can generate rapid events in response to orbital forcing, such as changes in African monsoon intensity due to nonlinear interactions between vegetation and monsoon dynamics (Claussen et al., 1999; Renssen et al., 2003).