Page 7 - Perspective Paper
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20,000  years  (DeMenocal  &  Tierney,  2012;  Larrasoaña,  2021;  Tierney  et  al.,  2017).  Precessional  cycles
               (changes in the axial rotation of earth) have to a certain extent paced the hydroclimate of North Africa, but
               the precise timing and duration of dry/wet periods are not entirely consistent with the rhythm of orbital
               variation (see Figure 2.1).

                          Figure  2.1.  Top  image  shows  a  time  series  of  Earth’s  precessional  cycles  (red  line)  and  the
                          radiative forcing from changes in CO 2 . Bottom image shows a timeseries of Barium/Aluminium
                          ratio, which is a proxy for wet and dry periods (black line). The start and end of wet periods are
                          indicated by the green and blue vertical lines, respectively. Figure from Pausata et al. (2020) (CC
                          BY 4.0).

               The indication that there are additional factors that co-determine the onset of a regime shift makes that the
               orbital cycles alone cannot be used to predict the state of the climate. Research suggests that the tipping-
               point behaviour is coupled to the sensitivity of an area (Pausata et al., 2020), which further suggests that
               land cover change in the past might be more of a determining factor than has been previously understood.

               In place  of the  hyper-arid  landscape  the  Sahara  is  today,  the  region  has been  periodically  lined  with a
               considerable amount of lush green vegetation as well as rivers, lakes, forests, grasses and large mammals
               roaming  between  equatorial  Africa  through  northern  Africa  into  the  now-arid  regions  of  Central  Asia
               (DeMenocal et al., 2000; Larrasoaña et al., 2020; Larrasoaña, 2021). The latest African Humid Period (AHP)
               occurred during the early-to-mid Holocene beginning before 10,000 years ago and ending around 4000 years
               ago. Whether the termination of the last AHP occurred abruptly or stepwise is still under scientific debate
               though it is known the process was spatially and temporally variable (Lézine et al., 2011).

               The timing and geography of the termination of the AHP is a matter of debate, with some proxy evidence
               indicating an abrupt transition from wet to dry conditions (Bloszies et al., 2015; Bristow et al., 2018; Collins
               et al., 2017; DeMenocal et al., 2000; Salzmann & Hoelzmann, 2005) and some that indicate a slower, stepwise
               transition (Francus et al., 2013; Höpker et al., 2019; Kröpelin et al., 2008; A. M. Lézine et al., 2005; Ménot et
               al., 2020; Neumann, 1989; van der Lubbe et al., 2017).

               A strategic ‘living systems’ approach to climate stabilization                          7/26
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