Climate Dynamics )2000) 16:643±659 Ó Springer-Verlag 2000

N. de Noblet-Ducoudre á M. Claussen á C. Prentice Mid-Holocene greening of the Sahara: ®rst results of the GAIM 6000 year BP Experiment with two asynchronously coupled atmosphere/biome models

Received: 20 April 1999 / Accepted: 20 January 2000

Abstract The mid-Holocene `green' Sahara represents than in LMD. This comparison illustrates the impor- the largest anomaly of the atmosphere-biosphere system tance of correct simulation of atmospheric circulation during the last 12 000 years. Although this anomaly features for the sensitivity of climate models to changes is attributed to precessional forcing leading to a strong in radiative forcing, particularly for regional climates enhancement of the African monsoon, no climate model where atmospheric changes are ampli®ed by biosphere- so far has been able to simulate the full extent of vege- atmosphere feedbacks. tation in the Sahara region 6000 years ago. Here two atmospheric general circulation models )LMD 5.3 and ECHAM 3) are asynchronously coupled to an equilib- 1 Introduction rium biogeography model to give steady-state simula- tions of climate and vegetation 6000 years ago, including The mid-Holocene climate, around 6 ka )6000 years biogeophysical feedback. The two model results are before present), was di€erent from the present-day in surprisingly di€erent, and neither is fully realistic. EC- several important respects. Paleoecological data, syn- HAM shows a large northward extension of vegetation thesized most recently by the BIOME 6000 project in the western part of the Sahara only. LMD shows a )Prentice and Webb 1998), indicate warmer growing much smaller and more zonal vegetation shift. These seasons in the Northern Hemisphere mid- to high lati- results are una€ected by the choice of `green' or modern tudes, and in some regions warmer winters as well, e.g. initial conditions. The inability of LMD to sustain a winter temperatures in north and northeast Europe were `green' Sahara 6000 years ago is linked to the simulated 1±3 K warmer than present )Cheddadi et al. 1997). The strength of the tropical summer circulation. During the largest di€erence between the mid-Holocene and present northern summer monsoon season, the meridional gra- climates however is in northern Africa where conditions dient of sea-level pressure and subsidence over the very much wetter than today are indicated by paleocli- western part of northern Africa are both much weaker in matological reconstructions using ancient lake sediments ECHAM than in LMD in the present as well as the mid- )Street and Grove 1976; Street-Perrott and Harrison Holocene. These features allow the surface moist air ¯ux 1985; Yu and Harrison 1996), o€shore marine pollen to penetrate further into northern Africa in ECHAM data )Dupont 1993), paleodune evidence )Sarnthein 1978), archaeological evidence )Petit-Maire 1989) and terrestrial pollen and plant macrofossil records )Street- Perrott and Perrott 1993; Jolly et al. 1998a, b). Ac- N. de Noblet-Ducoudre )&) Laboratoire des Sciences du Climat et de l'Environnement, cording to the BIOME 6000 reconstructions by Jolly DSM, Laboratoire mixte CEA-CNRS, et al. )1998b) grassland and shrubland vegetation Baà timent 709, Orme des Merisiers, 91191 were found at least as far north as 23°N, whereas today Gif-sur-Yvette Cedex, France the desert extends southwards to 12°N. E-mail: [email protected] It has been hypothesized )Kutzbach and Street- M. Claussen Perrott 1985; COHMAP members 1988) that di€erences Potsdam Institut fuÈ r Klimafolgenforschung, between modern and mid-Holocene climates were Postfach 601203, D-14412 Potsdam Germany Institut fuÈ r Meteorologie, Freie UniversitaÈ t Berlin, caused by changes in the Earth's orbit. Six thousand C.-H. Becker Weg 6-10, D-12165 Berlin, Germany years ago perihelion was close to the autumnal equinox, the tilt of the Earth's axis was about 0.7o greater than C. Prentice Max Planck Institute for Biogeochemistry, today, and the eccentricity of the Earth's orbit around Jena, P.O. Box 100164, D-07701, Germany the Sun was slightly larger )0.187 instead of 0.167 644 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara today). These changes led to increased solar radiation in used the same vegetation model. The only di€erence the Northern Hemisphere summer, which increased concerns the method of coupling )direct in ECHAM; land-sea contrast and ampli®ed the African and Indian anomaly based in LMD). summer monsoons, thereby increasing moisture trans- We ®rst eliminate the di€erent coupling methods as port into northern Africa. This hypothesis is now the cause of the di€erent model results, and then generally accepted, partly because experiments with investigate the role of di€erences in simulated features atmospheric general circulation models )AGCMs) re- of the large-scale circulation as possible causes of peatedly con®rmed its plausibility )e.g. Kutzbach and di€erential sensitivity of the coupled models. Guetter 1986; Mitchell et al. 1988; Phillips and Held 1994; Dong et al. 1996; Kutzbach et al. 1998). However, it has recently become clear that orbital forcing at 6 ka 2 Models and coupling strategies cannot produce a realistically large expansion of vege- tation in northern Africa when changes in land and 2.1 Biome model ocean surface conditions are not taken into account. This conclusion has been demonstrated with 18 AGCMs Biomes, i.e. macro-ecosystems in equilibrium with climate, are within the Paleoclimate Modelling Intercomparison computed using the BIOME model )version 1.0) developed by Project )PMIP) )Joussaume and Taylor 1995; Harrison Prentice et al. )1992). In the BIOME model, 14 plant functional types are assigned climate tolerances in terms of bioclimate variables of et al. 1998; Joussaume et al. 1999). AGCM sensitivity which the most important are coldest monthly mean temperature, studies for 6 ka, with prescribed but appropriately yearly temperature sum, and the ratio of actual to equilibrium modi®ed land-surface conditions )e.g. Street-Perrott evapotranspiration. The model predicts which plant functional type et al. 1990; Foley et al. 1994; TEMPO 1996; Kutzbach can occur in a given environment, i.e. for a given set of bioclimate variables. Competition between di€erent plant types is treated et al. 1998) have shown that positive feedbacks between indirectly by the application of a dominance hierarchy which climate and vegetation may have been involved, ampli- excludes certain types of plants as dominants based on the presence fying treeline shifts in boreal latitudes as well as vege- of other types rather than because directly of climate. Finally, the tation changes in the subtropics. Such sensitivity studies biomes are de®ned as combinations of dominant plant types. The BIOME model is based on physiological considerations )as formed the background of the GAIM 6000 year BP regards the choice of bioclimatic control variables and of their experiment )Anon 1994) which recommended the use of numerical prescribed thresholds for each plant type) rather than coupled atmosphere-biome models to quantify these relying on correlations between climate and biomes as they exist feedbacks. The GAIM 6000 year BP experiment in turns today. Therefore the BIOME model is suitable as a tool to assess stimulated the development of the BIOME 6000 project changes in natural vegetation in response to changes in climate. BIOME does not simulate the transient behaviour of vegetation, so )Prentice and Webb 1998), whose ®rst objective has been it is not suitable for full integration into an AGCM )see Foley et al. to provide a view of the biosphere that can be used as a 1998), but it can be interfaced with an AGCM through equilibrium benchmark for paleoclimate modelling. asynchronous coupling as used here. Feedbacks between vegetation and atmospheric cir- culation in the subtropics can partly be explained in 2.2 LMD atmospheric general circulation model terms of Charney's )1975) theory of a self-induction of deserts through albedo enhancement in the Sahel )Lof- The LMD AGCM )Sadourny and Laval 1984; Harzallah and Sa- gren 1995a, b; Claussen 1997; Claussen and Gayler 1997; dourny 1995) is a grid-point model that has 64 points regularly Texier et al. 1997; Zheng and Eltahir 1998). Interactively spaced in longitude )5.625°) and 50 points regularly spaced in the sine of latitude )2.5° in equatorial regions and 4° to 6° poleward of coupled atmosphere-vegetation models have been used 40°). Eleven vertical sigma levels are unevenly spaced, four being in to test the plausibility of this theory )Claussen 1997). the planetary boundary layer where the resolution is the ®nest, four However, recent applications of such models to the mid- in the free troposphere and 3 in the stratosphere. The model is used Holocene have yielded somewhat divergent results. with full seasonal cycle but no diurnal cycle for insolation. Calcu- Claussen and Gayler )1997) found a rather strong in- lations of radiative transfers are done using the scheme described in Fouquart and Bonnel )1980) for solar radiation and the scheme crease of vegetation cover in the Sahara, more or less developed by Morcrette )1991) for infrared radiation. Condensation agreeing with paleoecological reconstructions for the is produced using three schemes in a sequential mode: )1) a moist western part of northern Africa. Texier et al. )1997) also adiabatic adjustment )Manabe and Strickler 1964), )2) a Kuo-type found that biosphere feedbacks were important in this scheme )Kuo 1965) which allows for the formation of tropical convective clouds associated with large-scale moisture convergence, region, but were not able to produce such an extensive and )3) supersaturation for non-convective precipitation. greening. A recent sensitivity experiment )BrostroÈ m et al. The land-surface scheme is based on SECHIBA )Ducoudre et al. 1998), using a geographically realistic land-surface pat- 1993), with modi®cations to allow a more physiologically and tern for 6 ka based on Hoelzmann et al. )1998), ecologically realistic treatment of canopy conductance, seasonal variations of leaf area index )LAI) and their e€ects on land-at- was likewise not able to sustain the vegetation initially mosphere energy and water exchanges )de Noblet et al. 1996). Each prescribed. AGCM grid cell is assigned fractional covers of 17 di€erent land- These di€erences in results are certainly caused by surface types )biomes). The area of uncovered soil, surface albedo di€erences among the atmospheric models used )EC- and roughness length are computed every day, within the AGCM, HAM by Claussen and Gayler 1997; LMD by Texier depending on the foliage density )i.e. LAI). Values of albedo with and without fresh snow cover, of surface roughness and vegetation et al. 1997; CCM by BrostroÈ m et al. 1998). The two height are prescribed for each biome at full development. Other groups running coupled atmosphere-biosphere models prescribed parameters include )1) the maximum water storage of de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 645 the canopy per unit LAI )for rainfall and dew interception), )2) a minimum stomatal resistance, and )3) a maximum e€ective LAI. For all biomes, these prescribed values are updated within the AGCM, based on LAI. Parametrization of soil water allows for the moistening of an upper layer, of variable depth, during a rainfall event. The soil water capacity is set to 150 mm for a 1 m deep total soil all over the globe. A soil resistance limits the evaporation of soil water. It is proportional to the relative dryness of the upper soil layer; the coecient of proportionality was derived from mea- surements over a variety of soil types and is the same for all grid points. Latent heat ¯uxes are calculated separately for each biome and for the uncovered soil, and aggregated to provide lower boundary conditions for the AGCM.

2.3 ECHAM atmospheric general circulation model

The ECHAM AGCM is a spectral model developed at the Max- Planck Institute for Meteorology )Hamburg). The physics as well Fig. 1 Schematic view of the asynchronous coupling between an as its validation is described in detail by Roeckner et al. )1992). atmospheric general circulation model and a biome model

Fig. 2a, b Regional partition of di€erent biome types for pre- sent-day and 6 ka climates, ex- pressed as percentages of the total continental surface in the region considered; a the high northern latitudes )45°N±90°N/ 180°W±180°E); b northern Af- rica )0±30°N/20°W±50°E). All distributions have been simu- lated by BIOME )Prentice et al. 1992) using )1) the climatology of Leemans and Cramer )1991): Data; )2) outputs from the LMD model: LMD; )3) the climate simulated in the 1st iteration of LMD model using the anomaly approach: HOL2; )4) the climate simulated in the 1st iteration of LMD model using the absolute approach: EXP2 646 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara ab present-day present-day + Green Sahara

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Fig. 3a±d Prescribed biome distribution in Africa in the LMD model simulated by the LMD model when starting with c present-day for a present-day, b present and an anomalously green-Sahara. Biome vegetation, d agreen-Sahara distribution simulated by BIOME using the 6 ka equilibrium climate

In this study, ECHAM )level 3) is run with 19 vertical layers 2.4 Coupling strategies )hybrid sigma-p coordinates) and a triangular truncation at zonal wave number 42 )i.e. a 2.8° Gaussian grid). The radiative transfer Both AGCMs are coupled to BIOME as shown in Fig. 1. Asyn- equation is solved according to Hense et al. )1982) and Rockel et al. chronous coupling is used because the biome distribution responds )1991). The vertical turbulent transfer of momentum, heat, water slowly to climatic forcing and is modelled in BIOME as a function vapour, and cloud water is based on the Monin-Obukhov theory of the average climate over a number of years, whereas the climate )Louis 1979) modi®ed for liquid water loading )Brinkop and responds to vegetation changes 102±103 times faster through Roeckner 1995). The cumulus convection scheme comprises the mechanisms internal to the general circulation of the atmosphere. e€ect of deep, shallow and mid-level convection on the budget of This strategy is implemented in di€erent ways in the two coupled heat, water vapour and momentum )Tiedtke 1989). The precipita- models. The di€erences concern the frequency of iterations and the tion of stratiform clouds is based upon the cloud water equation choice of how climate is used as input for climate change scenarios. following Sundqvist )1978) and Roeckner )1995). Sub-grid scale We deal with these two points in turn. condensation and cloud formation are taken into account by specifying appropriate thresholds of relative humidity. The soil 1. Henderson-Sellers )1993), in her pioneering work on biosphere- heat budget is solved by means of a ®ve-layer model. A re®ned atmosphere coupling, exchanged information between an bucket model )DuÈ menil and Todini 1992) is used for the compu- atmospheric model and a vegetation model at the end of each tation of soil moisture. Vegetation e€ects on land-surface ¯uxes year. Lofgren )1995b) did the same; however he used, after an are from Blondin )1989). To allow for the coupling with BIOME, initial phase without any coupling, a ®ve-year running mean of ECHAM was modi®ed in such a way that global data of surface the simulated climate. Pollard et al. )1998) took a similar ap- parameters )e.g. background albedo, roughness length, vegetation proach using a four-year running mean. Claussen )1994, 1997) fraction, LAI and forest ratio) are speci®ed for each biome type and Ciret and Henderson-Sellers )1995) after series of tests )Claussen 1994, 1997). Moreover, Claussen )1997) added the sur- concluded that the AGCM should be run for at least ®ve years face type `sand desert' to the list of biomes to take into account the to remove instability due to the simulated interannual vari- large di€erences between albedo values of various types of deserts. ability of the climate. Ciret and Henderson-Sellers )1995) added de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 647

that the appropriate frequency may depend on both the AGCM Harrison et al. )1998) ran a set of BIOME simulations, using and the vegetation model. For example, annual-mean based outputs from ECHAM to test whether the choice of procedure models are far less sensitive to interannual variability than more )anomaly or absolute) is likely to a€ect their estimates of the realistic biome models that respond to seasonal climate. For the magnitude of the simulated vegetation at 6 ka. They show that, experiments presented here the LMD model is run for 16 years, even though there are di€erences between the biome maps pro- and ECHAM for six years. In both cases the ®rst year of sim- duced by the various procedures, they do not signi®cantly a€ect ulation is considered as the time necessary for spin-up, mean estimates of the nature and magnitude of vegetation change as climates are then averaged over the last 15 years for LMD, and simulated with ECHAM-BIOME. We have conducted similar tests over the last 5 years for ECHAM. with LMD-BIOME and reached the same conclusion. We have 2. There are two ways to derive input climate ®elds for the vege- moreover performed an additional coupled simulation for the mid- tation model. In the ®rst )Prentice et al. 1994; Harrison et al. Holocene climate, using the absolute approach as in ECHAM- 1995; TEMPO 1996; de Noblet et al. 1996; Kutzbach et al. BIOME. First a control )CTRL) and a 6 ka )EXP1) experiment 1996; Texier et al. 1997) AGCM di€erences between the per- were carried out using, as input, the vegetation distribution re- turbed and the present climates are superimposed on a modern constructed from a simulation of present-day climate with land- climate data set )here: Leemans and Cramer 1991). This pro- surface conditions prescribed as modern. Mid-Holocene biomes cedure )herafter referred to as the ``anomaly'' approach) is were then reconstructed from EXP1 and prescribed in a second commonly used in climate impact studies. It was originally 6 ka experiment )EXP2). Biomes reconstructed from EXP2 were designed with the idea that the use of AGCM departures from then compared to the parallel biomes obtained using the anomaly present would reduce the e€ects of systematic model biases, and approach )HOL2). From the simulation of present-day climate we is employed here for the simulations conducted with the LMD can infer that this alternative version of the LMD-BIOME model is model. In the second procedure )hereafter referred to as the consistently )Fig. 2): ``absolute'' approach), used here for ECHAM, the vegetation model is run directly from the simulated climate )Henderson- A. Too warm in the high northern latitudes. The area of tundra is Sellers 1993; Claussen and Esch 1994; Claussen 1997, 1998; partially replaced by boreal forests; Claussen and Gayler 1997). B. Too dry in the tropics, with an unrealistic southward extension of the Sahara; ab present-day present-day + Green Sahara

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Fig. 4a±d Same as Fig. 3 but using the ECHAM AGCM 648 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara

C. Too dry in the northern mid-latitudes. The lack of available by Leemans and Cramer )1991), before being added to the present- water in the soil in this model prevents trees from growing and day climatology at this same ®ne resolution and used to force BI- allows grassland to be simulated in some regions that actually OME. The fractional area of each AGCM grid point, covered by support forest. each biome type, is then simply calculated )arithmetic weighted sum) and used in SECHIBA. The 6 ka climate simulated for the mid- and high northern latitudes is warmer throughout the growing season, and northern- summer monsoons were enhanced in Africa and Asia. The in- 2.5 Boundary conditions and experimental design creased temperatures through spring and summer led, in the anomaly approach, to the replacement of tundra by cold deciduous The boundary conditions )sea-surface temperatures, sea-ice dis- forests and taiga while, in the absolute approach, relatively warm- tribution, atmospheric CO2 concentration, orbital con®guration) adapted trees )cold and cool mixed forest) replaced relatively cold- were prescribed according to PMIP )Table 1), except for land- adapted trees )cool conifer forest and taiga). In Africa, the extent of surface conditions which were interactive. Two coupled simula- desert was reduced in both experiments by about the same amount. tions of the mid-Holocene climate, with di€erent initial condi- Comparing the biome distributions of HOL2 and EXP2 generally shows features that are similar to comparing the biome distribution of CTRL and present-day. We conclude that systematic biases Table 1 Boundary conditions used for the simulations of present present in the control simulation remain present when using the and 6 ka climates, as recommended within PMIP absolute approach, while they are removed )to ®rst order) when using the anomaly approach. But in any case, the direction and the Period Orbital CO2 SST and magnitude of the biome change does not depend on the choice of parameters sea ice coupling procedure. There is another minor di€erence in the way biomes are re- Present-day Eccentricity: 345 )ppm) Mean of 10 constructed, which will be evident to the reader in the following, 0.016724 AMIP years when comparing Figs. 3 and 4: the resolution at which biomes are Obliquity: 23.446 reconstructed )and plotted). For ECHAM, biomes are recon- Precession: 102.24 structed at the AGCM resolution since the land-surface model al- 6 ka Eccentricity: 280 )ppm) Same as for lows only one vegetation type per grid-point. SECHIBA on the 0.018682 present other hand allows the co-existence of all types. The climate Obliquity: 24.105 `anomalies' simulated by the LMD AGCM are therefore interpo- Precession: 0.87 lated to a much ®ner grid )0.5° * 0.5°) using the method developed

Fig. 5a±f Time evolution of monsoon rains in West Africa )20°W±30°E/Eq.±30°N) and over land only, a from Leemans and Cramer )1991), b as simu- lated by the ECHAM AGCM, c di€erences between the ECHAM model and the data, d as simulated by the LMD AGCM, e di€erences between the LMD model and the data, and f di€erences between the ECHAM and the LMD model de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 649

Fig. 6a±f Mean velocity potential at 850 hPa for June-July-August- simulated with the LMD model, e di€erences between the LMD September, a from ECMWF analyses, b assimulatedwiththe model and the data, and f di€erences between ECHAM and the LMD ECHAM model, c di€erences between ECHAM and the data, d as model tions, were performed with each model. In the ®rst simulation vegetation was initially set as for present-day )Figs. 3a and 4a). In 3 Simulation of present-day climate the second simulation the Sahara was arti®cially vegetated )her- after referred to as the green-Sahara experiment: Figs. 3b and 4b). 3.1 Precipitation In the ECHAM runs the green-Sahara initial condition consisted of homogeneous tropical rainforest replacing desert over the Sahara and the Arabian peninsula. )Another experiment with an The time evolution of monsoon rains in northern Africa initial cover of grassland instead of forest yielded the same results was compared to the land climatology of Leemans and and is not discussed further). In the LMD experiment the green- Cramer )1991; Fig. 5a). If we de®ne the intertropical Sahara initial condition consisted of xerophytic woods/scrub re- placing desert up to 23°N, and grassland from 23°Nto30°N. convergence zone )ITCZ) in Africa as the region where Equilibrium was reached after 3 to 7 iterations, depending on the rainfall rates exceed 4 mm/day, then its location and AGCM and the initial condition )Claussen and Gayler 1997; northward extent are correct in ECHAM )Fig. 5b), Texier et al. 1997). although the maximum values reached in August are 650 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara

Fig. 7a±c Mean sea-level pres- sure for June-July-August- September, a from ECMWF analyses, b as simulated with the ECHAM model, and c as simulated with the LMD mod- el. Wind ®eld at 850 hPa, represented using vectors,is superimposed on the pressure ®eld

slightly underestimated around 10°N )Fig. 5c). Never- already started its southward migration while in reality it theless, precipitation is greatly overestimated through- continues to progress northward. out the entire monsoon season )from April to October). During the early phase of the monsoon )April to Moreover the southward retreat of the monsoon, start- June/July), ECHAM shows lower precipitation rates ing in September, is slower than observed. than the LMD model. During the late phase )July to In the LMD model the northernmost latitude reached October), the picture is reversed )Fig. 5f). In summary, by the ITCZ is about 10°N )Fig. 5d) while in reality it north of 10°N, i.e. in the Sahel and the southern border goes beyond 12°N. The rates of rainfall are overesti- of the Sahara, the LMD model is too dry due to the mated during the ®rst half of the monsoon season )from unrealistically early retreat of the African summer May to August), and underestimated in the second half monsoon rains, while ECHAM is too wet in these same )Fig. 5e). This latter feature is due to the early retreat of regions due both to overestimated precipitation rates the monsoon in this model: in August the ITCZ has and a smoother retreat of the monsoon rains. de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 651

Fig. 8a±d Changes in annual rainfall )mid-Holocene minus present with a green-Sahara in the ECHAM model, c when starting with values) for the mid-Holocene equilibrium simulations, a when starting present-day vegetation in the LMD model, and d when starting with a with present-day vegetation in the ECHAM model, b when starting green-Sahara in the LMD model

3.2 Velocity potential at 850 hPa In ECHAM the gradient essentially is north/south, while it has a strong east/west component both in LMD and Both models are compared to 1980±1987 ECMWF an- in the ECMWF data. Given that the large-scale ¯ow is alyses )Fig. 6a) in terms of velocity potential at 850 hPa. perpendicular to the isopotentials, the feature outlined They both overestimate the large-scale low-level con- here will induce a stronger meridional component of the vergence )upper-level divergence) in the East Asia/West ¯ow in ECHAM, while the zonal wind will be neces- Paci®c region, but underestimate the divergence )upper- sarily dominant in LMD allowing the advection of level convergence) in southern Africa and the Atlantic moisture further north into western Africa in ECHAM, Ocean )Fig. 6b±e). The peak of the convergent cell is as compared with LMD. smaller in ECHAM than in LMD )Fig. 6f). But west of these maximum values, over India, the Arabian Sea and the Horn of Africa, the low-level ¯ow in ECHAM 3.3 Sea-level pressure and associated surface winds is more convergent. Over the equatorial and North Atlantic, and over western North Africa, the low-level Sea-level pressure and surface winds )at 850 hPa), as ¯ow is less divergent in ECHAM, implying a weaker simulated by both models, are compared to ECMWF subsidence than in LMD, although both underestimate 10-year averaged data )Fig. 7). Both models )and espe- the intensity of the subsidence. cially LMD) overestimate the strength of the Asian low- Signi®cantly, these continental areas where the sub- pressure cell, extends this cell too far east and north and sidence is weaker in ECHAM are also those where do not capture the higher pressures over Tibet and the summer monsoon rainfall is greater. Masson and Jou- Himalayas. Over Africa, both models tend to simulate ssaume )1997), using two di€erent versions of the LMD too high pressure. Generally ECHAM simulates higher model, have also shown that the version simulating the pressure over Africa than in LMD, except in the western larger present-day monsoon rains in northern Africa was part of Northern Africa where ECHAM simulates more the one showing the weakest subsidence. precipitation than LMD. The main direction of the simulated gradient of ve- However, it is not the overall magnitude of the sea- locity potential is also quite di€erent in the two models. level pressure that drives moist surface ¯ow into 652 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara

Fig. 9a±d Zonal mean rainfall in northern Africa )20°W± 30°E), plotted from 5°Sto 30 °N, for the present-day simulations )LMD 0k and ECHAM 0k) and the mid-Holocene climates responding to orbital forcing alone when starting with )1) present-day vegetation )LMD 6k pd and ECHAM 6k pd), )2) a green-Sahara )LMD 6k gs and ECHAM 6k gs). Values are averaged for June, July and August for a the present and mid-Holocene climates, b the di€erences )mid-Holocene mi- nus present). Values are aver- aged for September, October and November for c the present and mid-Holocene climates, d the di€erences )mid-Holocene minus present)

Northern Africa and India, but rather the strength of the of the surface temperature maximum, which in turn meridional pressure gradient. The pattern of sea-level corresponds to the position of the continental thermal pressure in ECHAM is, although far from perfect, more low. The maximum warming simulated in ECHAM in realistic than that in LMD. The LMD model has an the western Sahara is located around 25°N, which is unrealistically strong meridional gradient, which forces about 5° further north than in LMD. This is why the the moist surface ¯ow to converge towards India and ITCZ extends further north in ECHAM )18°N) than in Indochina rather than Africa. The ECHAM model has a LMD )14°N). meridional gradient slightly stronger than data indicate In summary, for the simulation of present-day cli- over the Arabian Sea and the Indian subcontinent, but mate in West Africa )the region of main interest here), the gradient is weaker over north western Africa, where ECHAM simulates more summer rainfall than LMD the moist advected ¯ux converges. because the sea-level pressure pattern in ECHAM allows Braconnot et al. )in press) have shown, for many the penetration of water vapour into this region, while in models participating in PMIP and at di€erent time pe- LMD water vapour is forced to move further to the east. riods )present, 6 and 21 ka), that the northernmost ex- In addition the simulated subsidence is stronger in tension of the ITCZ is located immediately to the south LMD, inhibiting deep convection. Our purpose is not to de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 653

Fig. 10a±d Changes in velocity potential at 850 hPa )mid-Holocene ECHAM, b for LMD, c di€erences between ECHAM and the LMD minus present values) in June-July-August-September, in response to model, and d Di€erences of mid-Holocene velocity potential between orbital forcing alone when starting with present-day vegetation a for ECHAM and LMD ®nd out why both models are showing these di€erences Gayler )1997), and also applies here for the LMD in simulated present-day climates )there can be many model. Both models agree that there is only one reasons for this, including the prescribed land-surface stable state of the coupled atmosphere-vegetation characteristics which are quite di€erent), but rather how system at 6 ka. their di€erent behaviours for present-day climate impact on their response to climate change. 4.2 Changes in rainfall at 6 ka vegetation-climate equilibrium, and in response to orbital forcing alone 4 Simulation of the mid-Holocene climate The simulated enhancement of vegetation is associated 4.1 Biomes simulated at equilibrium in both models with a strong increase in annual rainfall )Fig. 8). Examination of the time evolution of precipi- Figures 3c±d and 4c±d show the equilibrium vegetation tation in northern Africa )not shown) shows that most distributions reached by the LMD and the ECHAM of the excess rain falls during the summer monsoon models in both experiments )starting with present-day season, that is from June to October. This feature vegetation and with a green-Sahara). Two important is in agreement with all published simulations of the features emerge: mid-Holocene climate. A. The ECHAM model grows vegetation in the west The response of both atmospheric models to orbital part of Africa, whatever the initial vegetation distri- forcing alone, without coupling, is displayed in Fig. 9. bution is. In LMD however, the vegetation migrates When vegetation is prescribed in the Sahara, in place zonally by only a few degrees of latitude. When of the present desert, the change in rainfall in northern compared to data )Jolly et al. 1998b), ECHAM is in Africa is considerably enhanced. Land-surface feed- closer agreement, but remains too dry in the east. backs are responsible for this ampli®cation )e.g. Street- B. In both models the equilibrium reached is insensitive Perrott et al. 1990; Kutzbach et al. 1996; Claussen 1997; to the initial vegetation prescribed in the Sahara. This Coe and Bonan 1997; Texier et al. 2000). The combi- particular feature has already been thoroughly dis- nation of lowered albedo, increased roughness length cussed for the ECHAM model, by Claussen and and increased leaf area index reinforces water vapour 654 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara

a ECHAM b LMD

50N

40N

30N

20N

10N

EQ

10S 20W 10W 0 10E 20E 30E 40E 50E 60E 70E 80E 90E 100E 20W 10W 0 10E 20E 30E 40E 50E 60E 70E 80E 90E

2

c ECHAM - LMD / Mean U d ECHAM - LMD / Mean

50N 0.5 -0.5 40N -0.5 0.5 -0.5 -0.5 0.5 0.5 0.5 0 0.5 0.5 0 -0. -1 30N 0 -0.5 -0.5 -1 -0.5 -1.5 0.5 -0.5 -1 -2 0.5 -1 -1 1 20N -2 -1.5 -1.5 -1.5 1.5 -2 0 0 10N 0.5 2 -1 0 1 1 - 1.5 -2 0 EQ -0.5 0.5 -0.5 0 -1 10S 20W 10W 0 10E 20E 30E 40E 50E 60E 70E 80E 90E 100E 20W 10W 0 10E 20E 30E 40E 50E 60E 70E 80E 90E

0

Fig. 11a±d Changes in 850 hPa wind )mid-Holocene minus present b LMD. Di€erences between both models are presented in c for the values) in June-July-August-September, in response to orbital forcing zonal component and in d for the meridional component alone when starting with present-day vegetation for a ECHAM, advection and convective activity above the regions Chad; this ®nding mirrors the previously noted tendency where the vegetation change is prescribed, as discussed of ECHAM to show more rainfall in West Africa than in Claussen and Gayler )1997) for the ECHAM model today. and in Texier et al. )2000) for the LMD model. In both models, even though rainfall is much larger in the green- Sahara experiments than in any others, it is not sucient 4.3 Changes in the tropical circulation, in response to maintain vegetation as initially prescribed. This con- to orbital forcing alone clusion was also noted by BrostroÈ m et al. )1998) using the NCAR CCM3 model. In the equilibrium simulation of ECHAM, tropical rainforest is replaced by grassland 4.3.1Velocity potential and associated surface winds and xerophytic woods/scrub in west Africa, and by desert in the east, southward to about 20/25°N. In the Mid-Holocene changes in 850 hPa velocity potential LMD model this feature is even more striking since most during the summer monsoon season are displayed in of the Sahara returns to desert with no east-west dif- Fig. 10a, b. They show, for both models, increased low- ferential. The southward retreat of vegetation in both level convergence )or decreased divergence) to the west cases is not sudden but gradual. of the present-day convergent cell, and decreased con- Comparison of the response of both models shows vergence to the east. East Africa is then experiencing that the overall intensity of mid-Holocene changes, in more convergent ¯ow, and West Africa less divergent response to orbital forcing alone, is larger in the LMD ¯ow. This can be interpreted as a change in the Walker model than in ECHAM. This suggests that the African circulation, as discussed by Masson and Joussaume monsoon in LMD is more sensitive to orbital forcing )1997). than it is in ECHAM. Only during the late phase of the Di€erences between the models )Fig. 10c) show that monsoon season )September till November; Fig. 9c±d), the intensity of these changes is overall larger in LMD and outside the ITCZ, to the north, does ECHAM than in ECHAM. But when considering di€erences be- produce a larger precipitation increase. The di€erence tween models during the mid-Holocene )Fig. 10d), the between the models is most substantial west of Lake picture is the same as for the present, i.e. a much weaker de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 655

Fig. 12a±d The 700 hPa wind in June-July-August-September for the forcing alone when starting with present-day vegetation for c present-day climate simulated by a ECHAM, b LMD. Changes in the ECHAM, d LMD 700 hPa wind )mid-Holocene minus present), in response to orbital subsidence in ECHAM west of Lake Chad. The 6 ka African monsoons in today's climate )Fontaine et al. large-scale low-level convergent ¯ow therefore remains 1995). The simulated AEJ increase in ECHAM is at a more westerly position in ECHAM than in LMD. stronger than in LMD. The strongly northward surface ¯ow in ECHAM is re- The tropical easterly jet, around 200 hPa, is a large- inforced at 6 ka )Fig. 11) and contributes to the model's scale signature of the Asian monsoon. This is of the simulation of a greatly extended monsoon rainfall region same order of magnitude in both models for present-day in the west. Claussen et al. )1998) also discussed the fact climate )Fig. 13a, b). In ECHAM it is strongly increased that a relatively far eastward simulated position of the at 6 ka )Fig. 13c) while in LMD the increase is less, main low-level convergent cell in summer )as seen in and con®ned to southern Asia )Fig. 13d). Thus there LMD) might lock the simulated atmospheric circulation are suggestive parallels between the African and Asian in northern Africa into a ``dry'' mode. monsoon responses in the two models; however, the mechanistic relationships between these phenomena are still unclear, as shown in a recent analysis of AGCM 4.3.2 Easterly tropospheric jets simulations of easterly tropospheric jet by DuÈ menil and Bauer )1998). The African easterly jet )AEJ) around 700 hPa is a mid- tropospheric signature of the African monsoon. The simulated AEJ for today is stronger in ECHAM than in 5 Conclusion LMD )Fig. 12a, b), with a centre of action located fur- ther north. At 6 ka, the AEJ is increased in both models, Recent theoretical and modelling studies have under- north of its present-day maximum )Fig. 12c, d), and lined the importance of feedback between atmosphere is displaced to higher altitude: increases at 600 and and vegetation at regional and global scales. A consen- 500 hPa are larger than at 700 hPa )not shown). This sus has emerged that the climate of the mid-Holocene feature is in agreement with observations of strong cannot realistically be simulated if this feedback is 656 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara

Fig. 13a±d Same as Fig. 12 but at the 200 hPa level well as large-scale divergence and convergence, and these di€erences are manifested both in present and 6 ka ignored. All relevant model studies, sensitivity experi- simulations. Furthermore, while both models yield an ments and coupled model studies, performed so far apparently realistic representation of present-day vege- point to the conclusion that di€erences in the Earth's tation pattern in northern Africa, these underlying dif- orbit between today and the mid-Holocene caused ferences in simulated atmospheric dynamics consistently changes in climate and that these changes have been determine di€ering regional sensitivities to external ampli®ed by the vegetation-atmosphere feedback. forcing. These di€erences thus a€ect not only the However, the magnitude of this ampli®cation is dis- `baseline' )control) simulations, but also the behaviour puted. While one group )Claussen and Gayler 1997) ®nd of the models when subjected to external forcing, and a realistically strong greening of the western Sahara, the response of the models to the inclusion of interactive little ampli®cation is seen in Texier et al.'s )1997) model vegetation. The sensitivity of the coupled models to or- results. Therefore, we have compared both model ex- bital forcing then di€ers, in ways that can be explained periments to address the following questions: can the in terms of model biases. By the same token, using the di€erence be assigned to the procedure of coupling at- anomaly approach does not eliminate the consequences mosphere and vegetation models? If not, can we assess of biases in the control simulation. the performance of the atmospheric models with respect The second question is more dicult to tackle. Both to the realism of circulation features that determine the models are more or less realistic, although they have magnitude of the feedback? their own systematic errors. For example, while the The ®rst question has been answered by demon- LMD model yields a less realistic surface pressure pat- strating that the coupled model results are robust tern, the ECHAM model obtains too strong an African against details of the coupling procedure. Our analysis monsoon. As ECHAM yields too strong an African indicates unambiguously that it is the simulation of monsoon for today, we cannot exclude that ECHAM tropical circulation in the atmospheric models which might have produced the desired strong feedback for the governs the e€ectiveness of the vegetation feedback. We wrong reason, i.e. because of too strong a sensitivity in have shown pronounced di€erences between the two western Africa, although the mid-Holocene greening of models in terms of simulated surface pressure patterns as the Sahara has been captured more or less realistically de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 657 by this model. The sensitivity experiment of BrostroÈ m Ciret C, Henderson-Sellers A )1995) `Static' vegetation and dy- et al. )1998) also suggested, like the LMD-BIOME namic global climate: preliminary analysis of the issues time steps and time scales. J Biogeogr 22: 843±856 simulation, that vegetation feedback is important Claussen M )1994) On coupling global biome models with climate but insucient. Coupled atmosphere-ocean experiments models. Clim Res 4: 203±221 )Kutzbach and Liu 1997; Hewitt and Mitchell 1998; Claussen M )1997) Modeling biogeophysical feedback in the Braconnot et al. 2000) have demonstrated that addi- African and Indian Monsoon region. Clim Dyn 13: 247± tional possible feedbacks to North African climates at 257 Claussen M )1998) On multiple solutions of the atmosphere-vege- 6 ka may be due to changes in sea-surface temperatures. tation system in present-day climate. Global Change Biol 4: Recent attempts at coupled simulations of the atmo- 549±559 sphere-ocean-vegetation system )Ganopolski et al. 1998; Claussen M, Esch M )1994) Biomes computed from simulated Braconnot et al. 1999) also point to the existence climatologies. Clim Dyn 9: 235±243 Claussen M, Gayler V )1997) The greening of Sahara during the of synergism between ocean and biosphere feedbacks. mid-Holocene: results of an interactive atmosphere ± biome Thus, at this point we are not fully able to quantify model. Global Ecol Biogeogr Lett 6: 369±377 the contributions of di€erent feedbacks to the 6 ka Claussen M, Brovkin V, Ganopolski A, Kubatzki C, Petoukhov V climate, but in performing model experiment isolating )1998) Modelling global terrestrial vegetation-climate interac- tion. Philos Trans R Soc London B 353: 53±63 the atmosphere-biosphere coupling we have learned Coe MT, Bonan GB )1997) Feedbacks between climate and surface useful lessons, above all, regarding the need for further water in northern Africa during the middle Holocene. J Geo- improvement in the simulation of atmospheric dynamics phys Res 102)D10): 11 087±11 101 in order to obtain correct regional climate sensitivity; COHMAP Members )1988) Climatic changes of the last 18 000 the necessity of including vegetation as an interactive years: observations and model simulations. Science 241: 1043± 1052 component of the climate system; and the value of pa- Dong B, Valdes PJ, Hall NMJ )1996) The changes of monsoonal leoclimate studies which help improve our understand- climates due to Earth's orbital perturbations and ice age ing of the strengths and limitations of climate models. boundary conditions. Paleoclimates 1: 203±240 Ducoudre N, Laval K, Perrier A )1993) SECHIBA, a new set of parametrizations of the hydrologic exchanges at the land- Acknowledgements We warmly thank Sylvie Joussaume, Pascale atmosphere interface within the LMD atmospheric general Braconnot and Sandy Harrison for the constructive discussions circulation model. J Clim 6)2): 248±273 which helped improved the ®rst version of the manuscript. We are DuÈ menil L, Todini E )1992) A rainfall-runo€ scheme for use in the grateful to Claudia Kubatzki for much helping in processing data Hamburg GCM. In: O'Kane JP )ed) Advances in theoretical for the ECHAM model. The LMD simulations were performed hydrology, European Geophysical Society Series on Hydro- at the Laboratoire des Sciences du Climat et de l'Environnement logical Sciences 1, Elsevier, Amsterdam, pp. 129±157 and sponsored by EEC, under contract ENV4-CT95±0075. The DuÈ menil L, Bauer HS)1998) The tropical easterly jet in a hierarchy ECHAM simulations were performed at the Max-Planck-Institute of GCMs and in reanalyses. Max-Planck-Institute for Meteo- for Meteorology. rology, Rep ISSN 0937-1060: 45 pp Dupont LM )1993) Vegetation zones in NW Africa during the Brunhes chron reconstructed from marine palynological data. Quat Sci Rev 12: 189±202 References Foley J, Kutzbach JE, Coe MT, Levis S)1994) Feedbacks between climate and boreal forests during the Holocene epoch. Nature Anon )1994) IGBP global modelling and data activities 1994±1998. 371: 52±54 Global Change Report 30 Foley J, Levis S, Prentice IC, Pollard D, Thompson SL )1998) Blondin C )1989) Research on land surface parameterization Coupling dynamic models of climate and vegetation. Global schemes at ECMWF. Proc Workshop on Parametrization of Change Biol 4)5): 561±580 ¯uxes over land surfaces. ECMWF, Reading UK Fontaine B, Janicot S, Moron V )1995) Rainfall anomaly patterns Brinkop S, Roeckner E )1995) Sensitivity of a general circulation and wind ®eld signals over West Africa in August )1958±1989). model to parameterizations of cloud-turbulence interactions in J Clim 8)6): 1503±1510 the atmospheric boundary layer. Tellus 47A: 197±220 Fouquart Y, Bonnel B )1980) Computations of solar heating of the Braconnot P, Joussaume S, de Noblet N, Ramstein G )in press) Earth's atmosphere: a new parametrization. Beitr Phys Atmos Mid-Holocene and last glacial maximum African monsoon 53: 35±62 changes as simulated within the paleoclimate modelling inter- Ganopolski A, Kubatzki C, Claussen M, Brovkin V, Petoukhov V comparison project. Global and planetary change )1998) The in¯uence of vegetation-atmosphere-ocean interac- Braconnot P, Marti O, Joussaume S, Leclainche Y )2000) Ocean tion on climate during the mid-Holocene. Science 280: 1916± feedback in response to 6 kyr BP insolation. Journal of Climate 1919 )in press) Harrison SP, Kutzbach JE, Prentice IC, Behling P, Sykes MT Braconnot P, Joussaume S, Marti O, de Noblet N )1999) Syner- )1995) The response of northern hemisphere extratropical gistic feedbacks from ocean and vegetation on the African climate and vegetation to orbitally-induced changes in inso- monsoon response to mid-Holocene insolation lation during the last interglacial: results of atmospheric gen- Braconnot et al. )99) Geophy. Res. Letters, Vol. 26 no. 16, pages eral circulation model and biome simulations. Quat Res 43: 2481±2484 )August 15) 174±184 BrostroÈ m A, Coe M, Harrison SP, Gallimore R, Kutzbach JE, Harrison SP, Jolly D, Laarif F, Abe-Ouchi A, Dong B, Herterich Foley J, Prentice IC, Behling P )1998) Land-surface processes K, Hewitt C, Joussaume S, Kutzbach JE, Micthell J, de Noblet and paleomonsoons in northern Africa. Geo Res Lett 25)19): N, Valdes P )1998) Intercomparison of simulated global vege- 3615±3618 tation distributions in response to 6 kyr BP orbital forcing. Charney JG )1975) Dynamics of deserts and drought in the Sahel. J Clim 11: 2721±2741 Q J R Meteorol Soc 101: 193±202 Harzallah A, Sadourny R )1995) Internal versus SST forced Cheddadi R, Yu G, Guiot J, Harrison SP, Prentice IC )1997) The atmospheric variability as simulated by an atmospheric general climate of Europe 6000 years ago. Clim Dyn 13: 1±9 circulation model. J Clim 8: 474±495 658 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara

Hewitt CD, Mitchell JFB )1998) A fully coupled GCM simulation to monsoon areas: a study with two versions of the LMD of the mid Holocene. Geophys Res Lett 25: 361±364 AGCM. J Clim 10: 2888±2903 Hense A, Kerschgens M, Raschke E )1982) An economical method Mitchell JFB, Grahame NS, Needham KJ )1988): Climate simu- for computing radiative transfer in circulation models. Q J R lations for 9000 years Before Present: seasonal variations and Meteorol Soc 108: 231±252 e€ects of the Laurentide Ice Sheet. J Geophys Res 93)D7): Henderson-Sellers A )1993) Continental vegetation as a dynamic 8283±8303 component of global climate model: a preliminary assessment. Morcrette JJ )1991) Radiation and cloud radiative properties in the Clim Change 23: 337±378 ECMWF operational weather forecast model. J Geophys Res Hoelzmann P, Jolly D, Harrison HP, Laarif F, Bonne®lle R, Pa- 96: 9121±9132 chur HJ )1998) Mid-Holocene land-surface conditions in de Noblet N, Prentice IC, Jousaume S, Texier D, Botta A, northern Africa and the Arabian peninsula: a data set for the Haxeltine A )1996) Possible role of atmosphere-biosphere in- analysis of biogeophysical feedbacks in the climate system. teractions in triggering the last glaciation. GRL 23)22): 3191± Global Biogeogr Cycl 12)1): 35±51 3194 Jolly D, Harrison SP, Damnati B, Bonne®lle R )1998a) Simulated Petit-Maire N )1989) Interglacial environments in presently hy- climate and biomes of Africa during the Late Quaternary: perarid Sahara: Paleoclimatic implications. In: Leinen, Sarn- comparison with pollen, and lake status data. Quat Sci Rev 17: thein )eds) Paleoclimatology and paleometeorology: modern 629±658 and past patterns of global atmospheric transport, Kluwer Jolly D, Prentice IC, Bonne®lle R, Ballouche A, Bengo M, Brenac Academic, Norwell, Mass., pp 637±661 P, Buchet G, Burney D, Cazet JP, Cheddadi R, Edorh T, Phillips P, Held IM )1994) The response of orbital perturbations in Elenga H, Elmoutaki S, Guiot G, Laarif F, Lamb H, Le zine an atmospheric model coupled to a slab ocean. J Clim 7: 767± AM, Maley J, Mbenza M, Peyron O, Reille M, Reynaud-Fer- 782 rera I, Riollet J, Ritchie JC, Roche E, Scott L, Ssemmanda I, Pollard D, Bergengren JC, Stillwell-Soller LM, Felzer B, Thomp- Straka H, Umer M, van Campo E, Vilimumbalo S, Vincens A, son SL )1998) Climate simulations for 10 000 and 6000 years Waller M )1998b) Biome reconstruction from pollen and plant BP using the GENESIS global climate model. Paleoclimates macrofossil data for Africa and the Arabian peninsula at 0 and 2)2±3): 183±218 6 ka. J Biogeogr 25: 1007±1028 Prentice IC, Webb III T )1998) BIOME 6000: reconstructing global Joussaume S, Taylor K )1995) Status of the Paleoclimate Model- mid-Holocene vegetation patterns from paleoecological re- ling Intercomparison Project )PMIP). Proc 1st Int AMIP cords. J Biogeogr 25: 997±1005 Scienti®c Conference, WCRP Report 92: 425±430 Prentice IC, Cramer W, Harrison SP, Leemans R, Monserud RA, Joussaume S, Taylor KE, Braconnot P, Mitchell JFB, Kutzbach Solomon AM )1992) A global biome model based on plant JT, Harrison SP, Prentice IC, Broccoli AJ, Abe-Ouchi A, physiology and dominance, soil properties and climate. J Bio- Bartlein PJ, Bon®ls C, Dong B, Guiot J, Herterich K, Hewitt geogr 19: 117±134 CD, Jolly D, Kim JW, Kislov A, Kitoh A, Loutre MF, Masson Prentice IC, Sykes MT, Lautenschlager M, Harrison SP, Deni- V, McAvaney B, McFarlane N, de Noblet N, Peltier WR, ssenko O, Bartlein PJ )1994) Modelling the increase in terres- Peterschmitt JY, Pollard D, Rind D, Royer JF, Schlesinger trial carbon storage after the last glacial maximum. Global Ecol ME, Syktus J, Thompson S, Valdes P, Vettoretti G, Webb RS, Biogeogr Lett 3: 67±76 Wyputta U )1999) Monsoon changes/regional climates for 6000 Ramanathan V, Cess RD, Harrison EF, Minis P, Barkstrom BR, years ago: results of 18 simulations from the Palaeoclimate Ahmad E, Hartmann D )1989) Cloud-radiative forcing and Modelling Intercomparison Project )PMIP). Geo Res Lett 26: climate: results from the Earth Radiation Budget Experiment. 859±862 Science 243: 57±63 Kutzbach JE, Street-Perrott FA )1985) Milankovitch forcing Rockel B, Raschke E, Weyres B )1991) A parameterization of ¯uctuations in the level of tropical lakes from 18 to 0 kyr BP. broad band radiative transfer properties of water, ice and mixed Nature 317: 130±134 clouds. Beitr Phys Atmos 64: 1±12 Kutzbach JE, Guetter PJ )1986) The in¯uence of changing orbital Roeckner E )1995) Parametrization of cloud radiative properties in parameters and surface boundary conditions on climate simu- the ECHAM4 model. Proc WCRP workshop on cloud micro- lations for the past 18 000 years. J Atmos Sci 43: 1726±1759 physics parametrizations in global atmospheric circulation Kutzbach JE, Liu Z )1997) Response of the African monsoon to models, May 23±25 1995, Kananaskis, Alberta, Canada, orbital forcing and ocean feedbacks in the middle Holocene. WCRP-90: 105±116, WMO/TD-713 Science 278: 440±443 Roeckner E, Arpe K, Bengtsson L, Brinkop S, DuÈ menil L, Kirk E, Kutzbach JE, Bonan G, Foley J, Harrison SP )1996) Vegetation Lunkeit F, Esch M, Ponater M, Rockel B, Sausen R, Schlese U, and soil feedbacks on the response of the African monsoon to Schubert S, Windelband M )1992) Simulation of the present- orbital forcing in the early to middle Holocene. Nature 384: day climate with the ECHAM model: impact of model physics 623±626 and resolution, Report 93, Max-Planck-Institut fuÈ r Meteorol- Kutzbach JE, Gallimore R, Harrison SP, Behling P, Selin R, Laarif ogie, Hamburg, 171 p F )1998) Climate simulations for the past 21 000 years. Quat Sci Sadourny R, Laval K )1984) January and july performance of the Rev 17: 473±506 LMD general circulation model. In: Berger A, Nicolis C )eds) Leemans R, Cramer W )1991) The IIASA database for mean New perspectives in climate modelling, Elsevier, Amsterdam, monthly values of temperature, precipitation, and cloudiness on pp 173±198 a global terrestrial grid. IIASA Research Report RR-91-18, Sarnthein M )1978) Sand deserts during glacial maximum and Laxenburg, Austria climatic optimum. Nature 272: 43±46 Lofgren BM )1995a) Sensitivity of land-ocean circulations, pre- Street FA, Grove AT )1976) Environmental and climatic implica- cipitation, and soil moisture to perturbed land surface albedo. tions of late Quaternary lake-level ¯uctuations in Africa. Na- J Clim 8: 2521±2542 ture 261: 385±390 Lofgren BM )1995b) Surface albedo-climate feedback simulated Street-Perrott FA, Harrison SP )1985) Lake levels and climate using two-way coupling. J Clim 8: 2543±2562 reconstructions. In: Hecht AD )eds) Paleoclimate analysis Louis JF )1979) A parametric model of vertical eddy ¯uxes in the and modeling. John Wiley and Sons, New York, pp 291± atmosphere. Boundary-Layer Meteorol 17: 187±202 340 Manabe S, Strickler RF )1964) Thermal equilibrium of the at- Street-Perrott FA, Perrott RA )1993) Holocene vegetation, lake mosphere with a convective adjustment. J Atmos Sci 21: 361± levels and climate of Africa. In: Wright HE Jr, Kutzbach JE, 385 Webb T III, Ruddiman WF, Street-Perrott FA, Bartlein PJ Masson V, Joussaume S)1997) Energetics of Mid-Holocene at- )eds) Global climates since the Last Glacial Maximum. mospheric circulation change in boreal summer from large scale University of Minnesota Press, Minneapolis, pp 318±356 de Noblet-Ducoudre et al.: Mid-Holocene greening of the Sahara 659

Street-Perrott FA, Mitchell JFB, Marchand DS, Brunner JS )1990) with palaeodata for northern Eurasia and Africa. Clim Dyn 13: Milankovitch and albedo forcing of the tropical monsoon: a 865±882 comparison of geological evidence and numerical simulations Texier D, de Noblet N, Braconnot P )2000) Sensitivity of the for 9000 y BP. Trans R Soc Edinburgh )Earth Sci) 81: 407±427 African and Asian monsoons to mid-Holocene insolation and Sundqvist )1978) A parametrization scheme for non-convective data-inferred surface changes. J Clim vol 13. pages 164±181 condensation including prediction of cloud water content. Tiedtke M )1989) A comprehensive mass-¯ux scheme for cumulus Q J R Meteorol Soc 104: 677±690 parameterization in large-scale models. Mon Weather Rev 117: TEMPO Members )1996) Potential role of vegetation feedback in 1779±1800 the climate sensitivity of high-latitude regions: a case study at Yu G, Harrison S)1996) An evaluation of the simulated water 6000 years BP. Glob Biogeochem Cycl 10)4): 727±736 balance of Eurasia and northern Africa at 6000 yr BP using Texier D, de Noblet N, Harrison SP, Haxeltine A, Jolly D, Jou- lake status data. Clim Dyn 12: 723±735 ssaume S, Laarif F, Prentice IC, Tarasov P )1997) Quantifying Zheng X, Eltahir EAB )1998) The role of vegetation in the the role of biosphere-atmosphere feedbacks in climate change: dynamics of west African monsoons. J Clim 11)8): 2078± coupled model simulations for 6000 years BP and comparison 2096