Preprints
https://doi.org/10.5194/cp-2022-27
https://doi.org/10.5194/cp-2022-27
 
09 Mar 2022
09 Mar 2022
Status: a revised version of this preprint is currently under review for the journal CP.

Comparison of the Green-to-desert Sahara transitions between the Holocene and the Last Interglacial

Huan Li1,2, Hans Renssen2,3, and Didier M. Roche2,4 Huan Li et al.
  • 1School of Geographic Science, Nantong University, Tongjingdadao 999, Nantong 226007, China
  • 2Vrije Universiteit Amsterdam, Faculty of Science, Cluster Earth and Climate, de Boelelaan 1085, Amsterdam, the Netherlands
  • 3Department of Natural Sciences and Environmental Health, University of South-Eastern Norway, Gullbringvegen 36, N-3800 Bø i Telemark, Norway
  • 4Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-INSU-UVSQ-CNRS, Université Paris-Saclay, Gif-sur-Yvette, France

Abstract. The desertification and vegetation feedbacks of the Green Sahara during the Last Interglacial (LIG) and the Holocene have been investigated by many studies. Yet the abruptness of climate and vegetation changes and their interactions are still under discussion. In this study, we apply an earth system model of intermediate complexity (iLOVECLIM) in combination with two dynamical vegetation models (VECODE and LPJ-GUESS) to simulate climate-vegetation changes during the Holocene and the LIG to compare the patterns of North African vegetation evolutions and mechanisms of their feedbacks during these two interglacials. Our results confirmed the existence of the 'Green Sahara' during the early LIG, which is as an analogue to the 'Green Sahara' during the Holocene. During both interglacials, an overall consistent transition from vegetated Sahara to desert is shown in our results, but the amplitudes of these transitions vary. These simulated Sahara vegetation transitions are nearly linearly related to the summer insolation declines at 20° N, resulting in faster declines of vegetation cover during the LIG than in the Holocene. The decline of vegetation cover peaks at 25 %/ka at around 122 ka BP, while during the Holocene the steepest vegetation cover decline is 10 %/ka at around 6 ka. Our results suggest net positive vegetation feedbacks to climates during the two interglacials. During the early LIG and Holocene, vegetation strengthens precipitation by a factor of 2 to 3 through the vegetation-albedo feedback when the vegetation cover is greater than 60 %. Vegetation cover decreases with declines of the incoming moisture transport by the atmosphere due to the reduced summer insolation at 20° N, weakening the summer monsoon during both interglacials. This desertification is accelerated when the positive vegetation-albedo feedback cannot offset the reduction of precipitation due to a weaker summer monsoon. The impacts of this positive vegetation feedback on precipitation decrease with decreased vegetation cover, during which the impacts of negative vegetation-evaporation feedbacks increase, accelerating the loss of soil moisture and vegetation cover. Overall, the net positive vegetation feedback is strong during the early phases of both interglacials, but the vegetation transition is more abrupt during the LIG than during the Holocene due to the more rapid changes in summer insolation during the LIG. The main difference between the two interglacials is the rate of precipitation change, which is relatively gradual during the Holocene, leading to a more gradual vegetation transition in comparison to the LIG.

Huan Li et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2022-27', Anonymous Referee #1, 08 Apr 2022
    • AC1: 'Reply on RC1', Hans Renssen, 19 May 2022
  • RC2: 'Comment on cp-2022-27', Qiong Zhang, 08 Apr 2022
    • AC2: 'Reply on RC2', Hans Renssen, 19 May 2022

Huan Li et al.

Huan Li et al.

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Short summary
In past warm periods, the Sahara region was covered by vegetation. In this paper we study transitions from this ‘green’ state to the desert state we find today. For this purpose, we have used a global climate model coupled to a vegetation model to perform transient simulations. We analyzed the model results to assess the effect of vegetation shifts on the abruptness of the transition. We find that the vegetation feedback was more efficient during the last interglacial than during the Holocene.