Mid‐Holocene land‐surface conditions in northern Africa and the Arabian Peninsula: A data set for the analysis of biogeophysical feedbacks in the climate system

Hoelzmann, Philipp; Jolly, Dominique; Harrison, Sandy P.; Laarif, F.; Bonnefille, Raymonde; Pachur, Hans-Joachim

doi:10.1029/97gb02733

Cited by 268 publications

(263 citation statements)

References 71 publications

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“…According to reconstructions by Hoelzmann et al (1998) the location of the largest increase in lake abundance was south of 20 • N in the Sahara and along the East Coast of the Arabian Peninsula. The mid-Holocene lakes in eastern Sahara would have needed between 350-500 mm of precipitation a year, sometimes up to 700 mm yr −1 , to be sustained (Hoelzmann et al, 2000).…”

Section: Discussionmentioning

confidence: 99%

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The effect of a dynamic background albedo scheme on Sahel/Sahara precipitation during the mid-Holocene

2011

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Abstract.We have implemented a new albedo scheme that takes the dynamic behaviour of the surface below the canopy into account, into the land-surface scheme of the MPI-ESM. The standard (static) scheme calculates the seasonal canopy albedo as a function of leaf area index, whereas the background albedo is a gridbox constant derived from satellite measurements. The new (dynamic) scheme additionally models the background albedo as a slowly changing function of organic matter in the ground and of litter and standing dead biomass covering the ground. We use the two schemes to investigate the interactions between vegetation, albedo and precipitation in the Sahel/Sahara for two time-slices: preindustrial and mid-Holocene. The dynamic scheme represents the seasonal cycle of albedo and the correspondence between annual mean albedo and vegetation cover in a more consistent way than the static scheme. It thus gives a better estimate of albedo change between the two time periods. With the introduction of the dynamic scheme, precipitation is increased by 30 mm yr −1 for the pre-industrial simulation and by about 80 mm yr −1 for the mid-Holocene simulation. The present-day dry bias in the Sahel of standard ECHAM5 is thus reduced and the sensitivity of precipitation to mid-Holocene external forcing is increased by around one third. The locations of mid-Holocene lakes, as estimated from reconstructions, lie south of the modelled desert border in both mid-Holocene simulations. The magnitude of simulated rainfall in this area is too low to fully sustain lakes, however it is captured better with the dynamic scheme. The dynamic scheme leads to increased vegetation variability in the remaining desert region, indicating a higher frequency of green spells, thus reaching a better agreement with the vegetation distribution as derived from pollen records.

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Section: Discussionmentioning

confidence: 99%

“…The vegetation cover extended at least up to 23 • N (Jolly et al, 1998a), if not across the whole of North Africa (Hoelzmann et al, 1998), compared to about 12 • N today (de Noblet-Ducoudre et al, 2000). Lake abundance and lake levels also increased throughout North Africa and on the Arabian Peninsula (Hoelzmann et al, 1998).…”

Section: Introductionmentioning

confidence: 99%

The effect of a dynamic background albedo scheme on Sahel/Sahara precipitation during the mid-Holocene

2011

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“…Areal reconstructions (e.g. Petit-Maire and Guo, 1996;Hoelzmann et al, 1998;Anhuf et al, 1999), differ, so do model simulations (Claussen and Gayler, 1997;Texier et al, 1997;Broström et al, 1998;Doherty et al, 2000;Schurgers et al, 2006;Liu et al, 2007) (Fig. 4).…”

Section: North African Wet Phasementioning

confidence: 99%

“…The largest subtropical desert, the Sahara, was much greener and moister than today during the early and mid-Holocene according to palaeobotanic reconstructions Hoelzmann et al, 1998;Prentice et al, 2000), and lake level and water balance reconstructions (e.g. Yu and Harrison, 1996;Coe and Harrison, 2002).…”

Section: North African Wet Phasementioning

confidence: 99%

Late Quaternary vegetation-climate feedbacks

Claußen

2009

Clim. Past

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Abstract. Feedbacks between vegetation and other components of the climate system are discussed with respect to their influence on climate dynamics during the late Quaternary, i.e., the last glacial-interglacial cycles. When weighting current understanding based on interpretation of palaeobotanic and palaeoclimatic evidence by numerical climate system models, a number of arguments speak in favour of vegetation dynamics being an amplifier of orbital forcing. (a) The vegetation-snow albedo feedback in synergy with the sea-ice albedo feedback tends to amplify Northern Hemisphere and global mean temperature changes. (b) Variations in the extent of the largest desert on Earth, the Sahara, appear to be amplified by biogeophysical feedback. (c) Biogeochemical feedbacks in the climate system in relation to vegetation migration are supposed to be negative on time scales of glacial cycles. However, with respect to changes in global mean temperature, they are presumably weaker than the positive biogeophysical feedbacks.

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“…Possibly, the use of early Holocene (e.g., 9 ka) initial conditions is more appropriate because the climate of that time could have been closer to the mid-Holocene climate. This could for instance be the case in the Sahara/Sahel region that was in a 'green' state in the early-to-mid Holocene (from ∼11 to 5 ka) during the so-called African Humid Period (e.g., deMenocal et al, 2000), which includes the 6 ka time-slice (e.g., Hoelzmann et al, 1998;Prentice et al, 2000). Previous model studies have suggested that both the green and desert states could have been stable under 6 ka forcings (Renssen et al, 2003), implying that potentially simulations initialized with preindustrial conditions could produce a desert state in Northern Africa instead of the required green state.…”

Section: Introductionmentioning

confidence: 99%

On the importance of initial conditions for simulations of the Mid-Holocene climate

et al. 2006

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Abstract. Three simulations of the Mid-Holocene (6 ka) climate were performed with the ECBilt-CLIO-VECODE coupled atmosphere-ocean-vegetation model to study the impact of initial conditions. These experiments were forced with identical 6 ka forcings (orbital parameters and atmospheric greenhouse gas concentrations) and differed only in initial conditions. Two simulations were designed as equilibrium experiments, with one being initialized with preindustrial conditions as required by the protocol of the Paleoclimate Modelling Intercomparison Project (PMIP), while in a second experiment early Holocene (9 ka) initial conditions were used. These equilibrium simulations were run for 2100 years with 6 ka forcings. The third experiment was set up as a transient simulation, also starting from early Holocene conditions, but forced with annually changing orbital parameters and greenhouse gas levels. The results of the last 100 years are compared and reveal no statistically significant differences, showing that in this model the initial conditions have no discernible impact on the 6 ka climate. This suggests that the PMIP set-up for 6 ka simulations is valid, with the condition that spin-up phase should be long enough (at least 550 years) to allow the deep ocean to adjust to the change in forcings.

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Mid‐Holocene land‐surface conditions in northern Africa and the Arabian Peninsula: A data set for the analysis of biogeophysical feedbacks in the climate system

Cited by 268 publications

References 71 publications

The effect of a dynamic background albedo scheme on Sahel/Sahara precipitation during the mid-Holocene

The effect of a dynamic background albedo scheme on Sahel/Sahara precipitation during the mid-Holocene

Late Quaternary vegetation-climate feedbacks

On the importance of initial conditions for simulations of the Mid-Holocene climate

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