Abstract
Abstract. We present how variations in plant functional diversity affect climate–vegetation interaction towards the end of the African Humid Period (AHP) in coupled land–atmosphere simulations using the Max Planck Institute Earth system model (MPI-ESM). In experiments with AHP boundary conditions, the extent of the “green” Sahara varies considerably with changes in plant functional diversity. Differences in vegetation cover extent and plant functional type (PFT) composition translate into significantly different land surface parameters, water cycling, and surface energy budgets. These changes have not only regional consequences but considerably alter large-scale atmospheric circulation patterns and the position of the tropical rain belt. Towards the end of the AHP, simulations with the standard PFT set in MPI-ESM depict a gradual decrease of precipitation and vegetation cover over time, while simulations with modified PFT composition show either a sharp decline of both variables or an even slower retreat. Thus, not the quantitative but the qualitative PFT composition determines climate–vegetation interaction and the climate–vegetation system response to external forcing. The sensitivity of simulated system states to changes in PFT composition raises the question how realistically Earth system models can actually represent climate–vegetation interaction, considering the poor representation of plant diversity in the current generation of land surface models.
Highlights
The African Humid Period (AHP) is an exceptionally interesting period to study drastic climate and vegetation changes in the past
We present how variations in plant functional diversity affect climate–vegetation interaction towards the end of the African Humid Period (AHP) in coupled land– atmosphere simulations using the Max Planck Institute Earth system model (MPI-ESM)
Differences in vegetation cover extent and plant functional type (PFT) composition translate into significantly different land surface parameters, water cycling, and surface energy budgets
Summary
The African Humid Period (AHP) is an exceptionally interesting period to study drastic climate and vegetation changes in the past. Vegetation cover marked a diverse savanna-like mosaic of xeric and tropical species whose ranges do not overlap today (Hély et al, 2014) The establishment of this so-called “green” Sahara (Ritchie and Haynes, 1987; Jolly et al, 1998) was presumably triggered by changes in the Earth’s orbit resulting in a stronger insolation and higher temperatures in the boreal summer than today, accompanied by an intensification and northward-shifted west African monsoon (Kutzbach, 1981; Kutzbach and Guetter, 1986). It has been shown that these externally triggered changes were amplified by internal feedback mechanisms involving ocean (Kutzbach and Liu, 1997; Braconnot et al, 1999; Liu et al, 2004), surface water coverage by lakes and wetlands (Coe and Bonan, 1997; Krinner et al, 2012), dust (Pausata et al, 2016; Egerer et al, 2016), soil albedo (Knorr and Schnitzler, 2006; Vamborg et al, 2011), and vegetation (Claussen and Gayler, 1997; Texier et al, 1997; Doherty et al, 2000; Rachmayani et al, 2015)
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