16 Mar 2022
16 Mar 2022
Status: this preprint is currently under review for the journal CP.

Threshold in orbital forcing for Saharan greening lowers with rising levels of greenhouse gases

Mateo Duque-Villegas1,2, Martin Claussen1,3, Victor Brovkin1, and Thomas Kleinen1 Mateo Duque-Villegas et al.
  • 1Max Planck Institute for Meteorology, Hamburg, Germany
  • 2International Max Planck Research School on Earth System Modelling, Hamburg, Germany
  • 3Center for Earth System Research and Sustainability (CEN), Universität Hamburg, Hamburg, Germany

Abstract. Numerous climate archives reveal alternating arid and humid conditions in North Africa during the last several million years. Most likely the dry phases resembled current hyper-arid landscapes, whereas the wet phases known as African Humid Periods (AHPs) sustained much more surface water and greater vegetated areas that "greened" a large part of the Sahara region. Previous analyses of sediment cores from the Mediterranean Sea showed the last five AHPs differed in strength, duration and rate of change. To understand the causes of such differences we perform transient simulations of the past 190,000 years with Earth system model of intermediate complexity CLIMBER-2. We analyse amplitude and rate of change of the modelled AHPs responses to changes in orbital parameters, greenhouse gases (GHGs) and ice sheets. In agreement with estimates from Mediterranean sapropels, we find the model predicts a threshold in orbital forcing for Sahara greening and occurrence of AHPs. Maximum rates of change in simulated vegetation extent at AHP onset and termination correlate well with the rate of change of the orbital forcing. As suggested by available data for the Holocene AHP, the onset of modelled AHPs happens usually faster than termination. A factor separation analysis confirms the dominant role of the orbital forcing in driving the amplitude of precipitation and vegetation extent for past AHPs. Forcing due to changes in GHGs and ice sheets is only of secondary importance, with a small contribution from synergies with the orbital forcing. Via the factor separation we detect that the threshold in orbital forcing for AHP onset varies with GHGs levels. To explore the implication of our finding from the palaeoclimate simulations for the AHPs that might occur in a greenhouse gas-induced warmer climate, we extend the palaeoclimate simulations into the future. For the next 100,000 years the variations in orbital forcing will be smaller than during the last hundred millennia, and the insolation threshold for the onset of late Quaternary AHPs will not be crossed. However, with higher GHGs concentrations the predicted threshold drops considerably. Thereby, the occurrence of AHPs in upcoming millennia appears to crucially depend on future concentrations of GHGs.

Mateo Duque-Villegas et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on cp-2022-26', Zhengyu Liu, 03 Apr 2022
    • AC1: 'Reply on CC1', Mateo Duque-Villegas, 10 May 2022
  • RC1: ' Review by Chris Brierley (UCL) of cp-2022-26', Chris Brierley, 05 Apr 2022
    • AC2: 'Reply on RC1', Mateo Duque-Villegas, 10 May 2022
  • RC2: 'Comment on cp-2022-26', Anonymous Referee #2, 18 Apr 2022
  • RC3: 'Comment on cp-2022-26', Anonymous Referee #3, 25 Apr 2022

Mateo Duque-Villegas et al.

Data sets

Threshold in orbital forcing for Saharan greening lowers with rising levels of greenhouse gases Publication Repository of the Max-Planck-Society (MPG.PuRe)

Clay mineralogy of three sediment cores from the Eastern Mediterranean Sea W. Erhmann, and G. Schmiedl

LR04 Benthic Stack L. E. Lisiecki, and M. E. Raymo

Mateo Duque-Villegas et al.


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Short summary
Using an Earth system model of intermediate complexity we quantify contributions of the Earth's orbit, greenhouse gases (GHGs) and ice sheets to the strength of Saharan greenings during late Quaternary African Humid Periods (AHPs). Orbital forcing is found as the dominant factor having a critical threshold and accounting for most of the changes in the vegetation response. However, results suggest GHGs may influence the orbital threshold and thus may play a pivotal role for future AHPs.