Page 7 - Perspective Paper

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).

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