Occurrence rhythms and mechanisms of Green Sahara periods over the Quaternary

As Earth's largest desert, the Sahara experienced recurrent greening phases in the geological past, which has nurtured the earliest human civilizations.  Understanding the occurrence of these Green Sahara periods (GSPs) and their driving mechanisms is crucial for reconstructing climatic history and predicting future climate trends.  Traditionally, orbital forcing, particularly precession, has been considered the main control.  However, geological records indicate that the occurrence of GSPs does not strictly follow orbital pacing, but instead exhibits prolonged absences (the skipped GSPs) during specific intervals.  To investigate their cause, we conducted a series of climate sensitivity simulations quantifying different forcings in contributing GSPs.  Results reveal that atmospheric CO2 and ice sheet extent also play a modifying role.  Using quantitative estimation, we developed a simple but powerful theoretical model to estimate the pace of GSPs.  In agreement with geological records, our theoretical reconstruction suggests that the skipped GSPs represent a persistent phenomenon throughout the Quaternary.  Furthermore, we predict that the Sahara will probably not regreen within the next precession cycle unless the cumulative atmospheric CO2 emissions reach ~3,840 Petagrams of Carbon (Pg C).  These findings provide a scientific basis for reconstructing Saharan paleoclimate and offer valuable insights for predicting its future climate change.