Green Sahara tipping points in transient climate model simulations of the Holocene

Abstract

The ‘Greening’ and subsequent desertification of the Sahara during the early to mid-Holocene is a dramatic example of natural climate change. We analyse a suite of simulations with a newly palaeo-conditioned configuration of the HadCM3 coupled model that is able to capture an abrupt desertification of North Africa during this time. We find that this model crosses a threshold of moisture availability for vegetation at around 6000 years before present. The resultant rapid reduction in vegetation cover acts to reduce precipitation through moisture recycling and surface albedo feedbacks. Precursor drying events which are not directly forced also indicate that the model is close to a critical moisture level. Similar precursor-like events appear in a Holocene record from the East of the continent, hinting that the natural system may resemble some of the properties of this model simulation. The overall response is not fundamentally altered by the inclusion of solar irradiance variations or volcanic eruptions. The simulated timing of the abrupt transition is mostly controlled by orbital forcing and local positive feedbacks, but it is also modulated to some extent by the state of the atmosphere and ocean. Comparisons with proxy records across North Africa show good agreement with the model simulations, although the simulations remain overly dry in the East. Overall, a threshold response may present a useful model of the real transition, but more high-resolution palaeoclimate records would help to discriminate among the predictions of climate models.

Orbitally forced and internal changes in West African rainfall interannual-to-decadal variability for the last 6000 years

“…The strong and steady WAMR reduction simulated from mid- to late Holocene by the four transient simulations would also appear to be inconsistent with recent modelling studies suggesting abrupt rainfall changes in West Africa through changes in vegetation, and associated albedo feedbacks, or in the Atlantic Meridional Overturning Circulation (AMOC; Menviel et al 2021 ; Hopcroft and Valdes 2022 ; Li et al 2023 ). There are several possible reasons for this apparent inconsistency.…”

“…Secondly, our focus is on the trend in monsoon rainfall during the last 6 kyr. The abrupt shifts documented by Hopcroft and Valdes ( 2022 ) mostly occur between 8 and 6 kyr BP. Finally, the multivariate EOF analysis approach is designed to assess multiscale changes in WAMR variability rather than abrupt shifts, which are better identified using other methods such as the KDJ index (Li et al 2023 ) or information entropy (Falasca et al 2020 ).…”

“…Firstly, we are examining trends averaged over a large area (7–18°N by 15°W–20°E); it is possible that some parts of this domain exhibited more rapid shifts. In the HADCM3 transient simulations (Hopcroft and Valdes 2022 ), for example, abrupt changes in rainfall occur between 20°N and 30°N. Small but persistent changes in rainfall in this region can lead to significant changes in vegetation, which feedback on the water cycle.…”

Early warning signals of the termination of the African Humid Period(s)

“…Volcanic stratospheric aerosol optical depth is prescribed in the model based on the results from HolVol v1 bipolar reconstruction 57 and total solar irradiance has been reconstructed using cosmogenic isotopes 58 . In this study, we analyze the climate simulation forced with Holocene variations in orbit, ice sheets, greenhouse gases, solar irradiance and volcanic eruptions and using the paleo-conditioned configuration, HadCM3BB-M2.1d 15,59 .…”

West African Monsoon Dynamics and Its Control on the Stable Oxygen Isotopic Composition of Precipitation in the Late Cenozoic

“…The associated pressure gradient facilitated the moisture transport from the equatorial Atlantic into the continent. Overall, the changes in the orbital cycles and expansion of vegetation across the Sahel‐Sahara caused the strengthening of the WAM and its northward extent (Gaetani et al., 2017; Hopcroft & Valdes, 2022; Patricola & Cook, 2007). This WAM intensification and northward migration have been reflected in many proxy systems such as paleo‐lake levels (Hoelzmann et al., 1998; Prentice et al., 2000), leaf wax, and aeolian deposits in sedimentary cores from the Eastern Atlantic (deMenocal et al., 2000; Tierney et al., 2017) and archeological findings that indicate human habitation (Cremaschi & Di Lernia, 1999; Dunne et al., 2012; Gabriel, 1987; Hoelzmann et al., 2001; Manning & Timpson, 2014; Sereno et al., 2008).…”