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Open Access 11–6 ka) in the Discussions Climate Climate ∼ of the Past of the 2,3 N (Berger, 1978). The early Holocene ◦ , and F. Muschitiello 1 1654 1653 ects that using a modern cloud data set has upon mid ff , M. Blaschek 1 over this period at 65 2 − C from 9 to 0 ka (Renssen et al., 2005) and has been attributed ◦ , H. Renssen 1 This discussion paper is/has beenPlease under refer review to for the the corresponding journal final Climate paper of in the CP Past (CP). if available. Department of Earth Sciences, Faculty of Earth and LifeBolin Sciences, Centre Vrije for Universiteit Climate Research,Department Stockholm University, of 106 Geological 91 Sciences, Stockholm, Stockholm Sweden University, 106 91 Stockholm, Sweden positive summer insolationin anomaly Northern also Africa had through a a strong strengthening impact of on the the summer vegetation monsoons, leading to The Holocene isNorthern characterized Hemisphere, bypersisted followed an up until the early by recent periodwas thermal most gradually of prevalent anthropogenically in maximum induced the declining warming. high This ( decreasing northern cooling latitudes global by with 3–4 July temperatures, temperatures northto of the 60 that orbitallydecreasing forced by reduction 42 Wm in summer insolation (Renssen et al., 2005, 2009), is sensitive to changes indebate Sahara of vegetation the type, response whichsurrounding of has future significance the precipitation in Sahara projections the for to future this climate region. change, considering the uncertainty 1 Introduction 0 ka was aa direct land–atmosphere result teleconnection, of increasinga surface the regional albedo increase desertification in in surface that theand pressure, a Sahara occurred the weakening leads in of polar to the thethe trade easterlies, winds, Sahara. atmosphere which the Through westerlies in toexperiments turn the we explored reduces Arctic. the a theand Additionally, early meridional through Holocene heat climate a simulations, transportedin and series our show by model that of our despite original targeted an conclusions apparent sensitivity are weakness robust. We conclude that interglacial climate Since the startThis of the has Holocene, beenreconstructed temperatures over accredited in the the to samehere Arctic period. the that have However, indicate we orbitally steadily present that declined. forced climate up modelling decrease to results in 42 % summer of the insolation cooling in the Arctic, over the period 9– 1 Amsterdam, 1081HV Amsterdam,2 the Netherlands 3 Received: 15 March 2014 – Accepted: 29Correspondence March to: 2014 F. J. – Davies Published: ([email protected]) 11 AprilPublished 2014 by Copernicus Publications on behalf of the European Geosciences Union. Abstract The impact of Sahara desertificationArctic on cooling during the Holocene F. J. Davies Clim. Past Discuss., 10, 1653–1673, 2014 www.clim-past-discuss.net/10/1653/2014/ doi:10.5194/cpd-10-1653-2014 © Author(s) 2014. CC Attribution 3.0 License. 5 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | latitude- ◦ 3 × ◦ ects on global climate. ff ect on the surface albedo, soil moisture ff N). ◦ 1656 1655 These simulations allowed us to simulate the natural vegetation evolution over the Previous studies have shown that drastic vegetation changes in the Amazon can relative vegetation fractions that were present in9, both 6, the and OG and 0 ka. OGGIS simulations Following at this, a series of equilibrium experiments were performed with were based on reconstructionsHowever, GIS by topographic Peltier changes (2004) wereonly not and minor accounted applied at for at because thethe the 50 spatial experimental changes year resolution are setup time of(2013). of steps. our In OGGIS model. both the For OGinteractively a and reader using OGGIS, more VECODE. is global detailed referred 9 description to to of 0 Blaschek ka vegetation andHolocene changes Renssen in were the calculated Sahara region. From these simulations we were able to evaluate the Schilt et al., 2010),conditions. In whilst addition, the another simulation solaras (OGGIS), and which OG, volcanic included plus the forcingsfluxes same additional were and forcings topography fixed Laurentide changes, at was (LIS)on performed. preindustrial and the The reconstructions LIS Greenland of meltwater Licciardi fluxes (GIS)Renssen et were (2013). Ice al. based The (1999) Sheet and associated those meltwater topographic for and the surface GIS albedo on Blaschek changes and of the LIS content, and net precipitation. 2.2 Experimental design In order to calculateduring the contribution the of Sahara Holocene,simulation desertification (OG) a from on 9 cooling to series 0 in kasettings the was of (Berger, performed, Arctic forced 1978) experiments with appropriate and orbital were parameter greenhouse designed. gas concentrations An (Loulergue initial et al., transient 2008; (Fichefet and Morales Maqueda, 1997,longitude 1999), and realistic and bathymetry. has The aform vegetation dynamic resolution component, global of VECODE, vegetation model, 3 is and aplant is reduced capable functional of types, simulating the trees dynamics andet of grasses, two al., 2002). as These well vegetation as types have desert an as e a dummy type (Brovkin 1995; Deleersnijder et al., 1997) coupled to a thermodynamic-dynamic sea-ice model computed as a functionequation, of this free-surface dataset. general The ocean oceanic component, circulation CLIO, model is a (Deleersnijder primitive- and Campin, We applied LOVECLIM, an2010), which earth is model comprisedmodel. of of intermediate a The complexity coupledatmosphere (Goosse atmospheric atmosphere, with et ocean, component, sea-ice al., aand and horizontal ECBilt, vegetation 200 hPa T21 ishydrological (Haarsma truncation cycle. et a and Clouds al.,1995) are three quasi-geostrophic 1996; prescribed from vertical dynamical based Rossow Opsteegh layers, on (1996), et 800, the with 500, al., ISCCP the D2 1998) total dataset and (1983– upward includes and a downward full radiative fluxes the Arctic (defined herein as north of 66.5 2 Model and experimental design 2.1 Model influence winter precipitationabout in due to the large-scale circulation North changesValdes, in 2000). the Atlantic Therefore, mid with and and the highit latitudes Europe, Sahara is (Gedney being and which entirely the are plausibleshift world’s in largest that brought vegetation non-polar changes during desert, the inAccordingly, Holocene, in its could this surface have study, profound albedo,how we e caused Sahara have by vegetation performed the changes climate large-scale during model experiments the to Holocene analyse have contributed to cooling in Street-Perrott, 1985). This(AHP), phase which is lasted often untiland referred the the mid-Holocene, subsequent to rate although aset at the al., the 2000; which exact Kröpelin African it timing etof occurred Humid of al., the is 2008). long-term its Period Following still decline demise the in a AHP, summer the contentious insolation, Sahara, issue evolved under into the (deMenocal a influence desert environment. enhanced precipitation and grassland vegetation in the Sahara region (Kutzbach and 5 5 25 15 20 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | http: E and ◦ C is due ◦ erence in ff C (17 %) is C occurring ◦ ◦ W–35 ◦ C). ◦ C between 9 and 6 ka and without this ◦ C (Figs. 1a and 2a), with 0.7 ◦ 1658 1657 ects of insolation changes. Therefore it can be ff ect of 0.2 ff C from 9 to 6 ka (Fig. 1b). However, the di ◦ C due to desertification in the Sahara and the remainder ◦ C between 6 and 0 ka. Of this total cooling, 2.1 ◦ C, with 0.4 ), resulting in a total of 18 equilibrium experiments. All equilibrium ◦ ect, the warming would have even been higher (1.3 ff ect of the LIS and GIS, leading to a delayed thermal maximum over most ff N. ◦ The second phase, 6 to 0 ka, of the OGGIS equilibrium experiment was identical The cooling in the Arctic is a consequence of Sahara desertification, which invokes In the OGGIS transient experiment, the 9 ka climate is relatively cold due to the From the OG and OGGIS equilibrium experiments we were able to deduce the meridional temperature gradient dueAs the a relatively strong result, cooling over the the Icelandic Sahara. Low stabilises, which in turns results in a weakening of heat loss increases, leading tosurface a pressure decrease in in surface thean temperatures and Sahara. increase an The increase in in decreasingAtlantic, the temperatures leading in surface to theAdditionally, an pressure we Sahara observe easterly (Fig. a cause zonal weakeningdecrease 3) of shift, in the with plus low the latitude an ana atmospheric trade extension weakening winds expansion, meridional and of of over heat a theatmospheric the net flux the mid-latitude overall heat Azores (Fig. westerlies. Tropical transport 4). high. This over In overall the weakening particular Atlantic of we Ocean the observe is winds consistent and with a decrease in desertification suppressed the warming bywas 15 responsible % for between 19 % 9 of to the 6 ka, observed cooling and in froma the 6 land–atmosphere Arctic. to teleconnection. 0 Due ka toin the our prescribed equilibrium desertification simulations, in over the the course Sahara of the Holocene net albedo and radiative Sahara vegetation had amoderating cooling e e to the OGtemperature of 2.1 equilibrium experiment,of the with cooling a duesaid to total that the for decrease localiseda the e direct in consequence OG of mean desertification experiment in from the annual Sahara. 9 In the Arctic to OGGIS experiment 0 Sahara ka, 17 % of the observed cooling was due to Sahara desertification alone (Figs. 1a and 2c). cooling e of the Arctic (Renssenwe find et a al., total 2009). warming Because of of 1.0 the cooler early Holocene climate, to direct orbital and greenhouse gas forcing (Figs. 1b and 2b), whilst 0.5 cover at 9, 6and and trace gas 0 ka levels taken for 9, from 6, both and OG 0 and ka, which OGGIS followed were the combined guidelines with of orbital PMIP3 ( fixed Saharan vegetation (Table 1). The relative percentages of Saharan vegetation is a total 9–0 kabetween cooling 9 in and the 6 ka Arctic and of 2.1 2.9 3 Results and discussion Our results for both the OGtwo and distinct OGGIS equilibrium phases, experiments 9–6 can ka be and separated into 6–0 ka (Fig. 1a and b). In the OG experiment there 9k9kEQ_OGGIS equilibrium experiments. the contribution of(Appendix A). Data Sahara presented are desertificationTo averages over investigate the an to last 500 “extreme”late years Arctic example of Holocene, of each we cooling simulation. also Sahara performeddesert during desertification 6 in between equilibrium the the simulations the that Saharaand mid Holocene had desert at and 100 are % 9, 0.2 grass 6,11–33 and and and 0.4 0 respectively. ka, The Sahara (Table 2). was defined In as LOVECLIM 15 the albedo of grass were kept constant, however LIS andwould GIS melt result fluxes in were the not included. constantprevent Including the addition oceans, them and in particular freshening the oftherefore, deep the oceans, rendering in ocean, reaching which those a quasi-equilibrium inthe particular state, turn melt would equilibrium fluxes experiments likely unrealistic. Neglecting resulted in a marginally warm early Holocene climate in our //pmip3.lsce.ipsl.fr/ Renssen et al., 2006). For the OGGIS equilibrium simulations LIS and GIS topography simulations were ran fromthe a control model, pre-industrial in simulation for particular 2500 the years, allowing deep oceans, to reach a quasi-equilibrium state (c.f. 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 2 − − 342]. · 0.4) − [(1 2 ects of insolation ff − (Kiehl and Trenberth, 1997). C (42 %) is attributable to the ◦ 2 − ects of the increase in albedo ff erence in solar radiation absorbed ff N, before gradually reducing to 0 PW ◦ N. ◦ 1660 1659 C (Fig. 2d), of which 1.7 ◦ 0.77)]. Hence, in theory the surface of a cloudy vegetated region and that · 0.23)]. Therefore, the solar radiation absorbed at the surface is 211 Wm (342 · − (1 · 0.2) − To explore this situation further we have performed a series of sensitivity experiments However, a particular weakness of LOVECLIM and our simulations that needs to Biome reconstructions of pollen and plant macrofossils show that during the mid- In the OGGIS simulation we see the same long range land–atmosphere–ocean over the Sahara. To achieve this we have taken the modern cloud cover that is In a cloud covered,23 % vegetated region, of with incomingthen surface solar albedo the radiation of downward is 0.2, solar[342 and reflected radiation assuming by reaching that clouds the[(1 surface (Kiehl of and the Trenberth,of Sahara 1997) a is cloudless 265 desert Wm solar environment radiation. absorb approximately similar amounts of incoming that included the addition of cloud cover that is representative of a vegetated region, in the two environments,at we can the easily two see surfaces0.4, the is di and quite we similar. Forradiation have instance, reaching a if the 100 % we surfaceThus, of assume cloudless, the the that desert total Sahara desert environment is solar albedo then 342 radiation is m the absorbed downward at solar the surface is 205 Wm a standard setup islower representative latent of heat cloud flux,thus cover higher reduced over sensible a cloud heatSahara desert flux cover. at and environment, Therefore, 6 low hence for and atmosphericin our 9 convection ka, too experiments and the high that prescribedreaching temperatures. contain the cloud With surface of a cover the the is Sahara vegetated changing inclusion at unrealistic albedo 6 of and and the 9 amount clouds, ka is of wouldIf the likely be solar reduced. we radiation to incoming However, due perform absorbed result to solar at a the the radiation simple surface calculation is likely of to the vary. total solar radiation absorbed at the surface temperatures decrease by 4.0 change in vegetation. be addressed isa the modern fact climatology. that Thus within cloud LOVECLIM cover clouds over the are Sahara prescribed in according all to our experiments with results show that from a 9 ka, 100 % “green” Sahara to a 0 ka, 100 % “desert” Sahara, impact of Sahara desertification has upon Arctic cooling over the Holocene. The Holocene the Sahara waset covered by al. grass (1999) and simulated shrubsfrom (Jolly a 90 et to decrease al., 10 in65 %. 1998). Sahara Claussen to However, in vegetation 20 % our fractionto over OG over capture the simulation, the the same vegetation Holocene general period. fractionthe pattern full Therefore, decreases of amplitude we from of vegetation state these changesvegetation changes. that in Therefore, on whilst this the suggests Arctic LOVECLIM Sahara that isfor cooling it the this able impact cannot could and of capture Sahara to be constrainHolocene greater environments. our These than results results we enable the us simulated to 17 extreme, place % early an estimated. (9 upper ka) limit To of and the account late potential (0 ka), changes, results in a warmingover over North the America. This North warmingpenetrating emanates Atlantic eastwards the and Arctic, accountingthe the OGGIS for experiment. rest the of observed northern warming between Europe 9 and and Eurasia, 6 ka as in well as transport from the equatoret to al., the 1997). Arctic In (Peixoto ourshown simulations, and to the Oort, weakening be 1992; of robust Zhangmeridional the (Fig. Northern and atmospheric Hemisphere Rossow 4), winds heat in are leading transport particular to widespread from the cooling Westerlies, the north which of low 60 results latitudes in toteleconnection a the reduced present. Arctic However,between (Fig. 9 the 4), and localised 6 ka, due e to the diminishing LIS and the localised e atmospheric heat transporttransport would (Bjerknes, be compensated 1964).increase by in This oceanic an heat is transport increase (0.058 indeedin PW in the at what Arctic) 7 oceanic is less we heat thanpoleward the observe heat atmospheric transport. decreases, (Fig. Mid-latitude resulting storms in 4), are an responsible overall for however decrease the of the majority of the heat the polar easterlies. The Bjerknes compensation theory suggests that a weakened 5 5 25 15 20 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ects ff 1 C. Importantly, ◦ GHG and Vegetation to the C respectively. This result ◦ + 0.8 ect is nearer the upper end of ff ± C due to ALL forcings C due to VEGETATION forcing C due to ORB & GHG forcings ◦ ◦ ◦ 11.1 = ∆ = ∆ = ∆ − and 1662 1661 ◦ erence between 6 and 0 ka with modern day cloud ff 0.8 ± C (Fig. 5b), hence with additional clouds in the Sahara ◦ 6k6kEQ_OGGIS 9k6kEQ_OGGIS 6k9kEQ_OGGIS C. With the addition of prescribed clouds at 6 ka (Fig. 5b) 11.0 ◦ − − − − ect that prescribed clouds have upon our initial simulations ff erence between 6 and 0 ka reduces slightly to 2–4 ff 9 ka to 6 ka 9k9kEQ_OGGIS 9k9kEQ_OGGIS 9k9kEQ_OGGIS – OGGIS: – ect of prescribed clouds during the early Holocene (9 ka). As can be seen To further test the e The first sensitivity experiment we performed allowed us to compare the temperature ff Equations to calculate the relative contributionscooling of in ORBG the Arctic, for OGGIS, OG and 100 % (extreme) runs. sensitive to Sahara desertification. Appendix A In conclusion we can say thatthe over Arctic the region course is of not theof only Holocene, desertification the driven in observed by the cooling localisedThis in Sahara insolation teleconnection initiate changes, accounts but a for that long betweenbetween the range 17 e land–atmosphere 9 and teleconnection. and 42 %this 0 of ka, range. the We with observed also Arctic itapparent cooling show weakness likely through of that a our seriesour the model, overall of with actual findings its sensitivity are e conclusive prescribed experimentswe and modern that robust, day performed. withstanding despite cloud the an Therefore, sensitivity data experiments set, it that can be stated that high-latitude interglacial climate is and interpretations. 4 Concluding remarks experiments we performed allow us to have greater confidence with our initial findings clouds results in a cooler Arctic. Therefore, we can say that the additional sensitivity we see aavailable slight for transportation cooling from in the low the to high Arctic, latitudes. whichwe is performed due toe an the additional reduction(Table of 1) sensitivity heat when that experiment. wesimulations is compare This 9k9kEQ_OG the temperature and experimentArctic changes 9k9kEQ_OG_clouds, tested temperatures in we the are the is Arctic see similar at that 9 to ka the the between mean 6 ka annual sensitivity experiments, where the simulation with prescribed the temperature di in both experiments thewarmer basic premise than that the theobserved late Sahara in during Holocene the the Arctic remains.a mid-Holocene occurs reduction In was in in temperature addition, the by Canadian 1 the Basin only and the temperature East change Siberian Sea, with reconstructions for thepossible Sahara to region verify duringproxy our based the temperature research early into results, this and but area. mid-Holocene, hopefully it theychange will is from encourage 6 not to further 0modern ka for dataset when and the the cloudbe other conditions seen when (Fig. at 5a) we 6 the ka artificially temperature aredataset di induced both ranges a prescribed from 6 on ka 3 the cloud to cover. 5 As can Given that the Amazonthat region more consists than of likelythe trees, consisted mid as of and opposed grasses lateserves and to the Holocene, shrubs a purpose this of during vegetated assessing cloudsurface Sahara its the cover impact vegetated temperature. of is period Unfortunately, cloud probably cover in due over an a to vegetated over Sahara the estimation, on near but total still lack of proxy temperature prescribed in our model for the Amazon region and have applied it to the Sahara. 5 5 15 10 20 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 1 1 2 2 2 2 10.1029/2001GB001662 , 2013. C due to ALL forcings C due to VEGETATION forcing C due to ORB & GHG forcings C due to ALLC forcings due to VEGETATION forcing C due to ORB & GHG forcings ◦ ◦ ◦ ◦ ◦ ◦ C due to ALL forcings C due to VEGETATION forcing C due to ORB & GHG forcings ◦ ◦ ◦ = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ C due to ALL forcings C due to VEGETATION forcing C due to ORB & GHG forcings C due to ALL forcings C due to VEGETATION forcing C due to ORB & GHG forcings C due to ALL forcings C due to VEGETATION forcing C due to ORB & GHG forcings ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ 1664 1663 = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ = ∆ 10.5194/cp-9-1629-2013 0k6kEQ_OGGIS 0k0kEQ_OGGIS 9k0kEQ_OGGIS 0k9kEQ_OGGIS 0k0kEQ_OGGIS 6k0kEQ_OGGIS 6k100dEQ_OG 0k100gEQ_OG 0k100dEQ_OG − − − − − − − − − 0k0kEQ_OG 6k0kEQ_OG 0k6kEQ_OG 6k6kEQ_OG 9k6kEQ_OG 6k9kEQ_OG 0k0kEQ_OG 9k0kEQ_OG 0k9kEQ_OG GHG, ORB and prescribed Sahara vegetation. GHG, ORB, prescribed Sahara vegetation, LIS and GIS topography. − − − − − − − − − = = FJD, HR and MB are funded by the “European Communities 7th All forcings All forcings 6k100gEQ_OG 6k100gEQ_OG 2 6 ka to 0 ka 6k100gEQ_OG 6k6kEQ_OGGIS 9 ka to 0 ka 9k9kEQ_OGGIS 9k9kEQ_OGGIS 9k9kEQ_OGGIS 1 6 ka to 0 ka 6k6kEQ_OG 6k6kEQ_OG 6k6kEQ_OG 6 ka to 0 ka 6k6kEQ_OGGIS 6k6kEQ_OGGIS 9 ka to 6 ka 9k9kEQ_OG 9k9kEQ_OG 9k9kEQ_OG 9 ka to 0 ka 9k9kEQ_OG 9k9kEQ_OG 9k9kEQ_OG – – – – – – 100 %: OG: – – model, Clim. Past, 9, 1629–1643, doi: Abrupt onset and termination of theinsolation African forcing, Humid Quaternary Period: rapid Sci. climate Rev., responses 19, to 347–361, gradual 2000 of Greenland Ice Sheet melt and other forcings in a coupled atmosphere–sea-ice–ocean 2002 Simulation of an abruptLett., change 26, in 2037–2040, Saharan 1999. vegetation in the mid-Holocene, Geophys. Res. Atmos. 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D., D., Prentice, Lenderink, I.C, G., Bonnefille, and R., Liu, Ballouche, Q.: A., A Bengo, M., coupled Brenac, P., Buchet, G., Goosse, H., Brovkin, V., Fichefet, T., Haarsma, R., Huybrechts, P., Jongma, J., Mouchet, A., Fichefet, T. and Morales Maqueda, M. A.: Modelling the influence of snow accumulation and Fichefet, T. and Morales Maqueda, M. A.: Sensitivity of a global sea ice model to the treatment Deleersnijder, E., Beckers, J. M., Campin, J. M., El Mohajir, M., Fichefet, T., and Luyten, P.: Deleersnijder, E. and Campin, J.-M.: On the computation of the barotropic mode of a free- 5 5 30 25 20 15 10 30 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N) N) ◦ ◦ ) C) ± ◦ ) ) C) C) ± ± ◦ ◦ 12.111.3 0.8 10.9 0.8 12.7 0.8 12.5 0.8 14.5 0.9 0.9 10.5 0.8 − − − − − − − 13.212.2 1.0 11.8 0.8 11.5 0.8 13.5 0.8 13.0 0.9 12.8 0.9 0.9 13.112.2 1.0 11.8 0.8 11.5 0.8 11.6 0.8 11.2 0.8 11.0 0.8 11.1 0.8 0.8 13.913.5 0.9 0.9 13.913.5 1.0 1.0 − − − − − − − − − − − − − − − − − − − 1668 1667 OG Arctic temp. ( OGGIS Arctic temp. ( Simulation Arctic temp. ( GHG VEG TYPE NAME temp s.d. ( GHG VEG TYPE NAME temp s.d. ( + + GHG VEG TYPE NAME temp s.d. ( + ORB 0 ka6 ka6 ka 0 ka6 ka EQ9 ka9 ka 0 ka 0k0kEQ_OGGIS 9 ka 6 ka EQ 9 ka EQ 0 ka 6k0kEQ_OGGIS EQORB 6 ka 6k6kEQ_OGGIS EQ 9 ka0 6k9kEQ_OGGIS ka EQ 9k0kEQ_OGGIS EQ 9k6kEQ_OGGIS 6 9k9kEQ_OGGIS ka6 ka 0 ka6 ka EQ9 ka9 ka 0 ka 0k0kEQ_OG 9 ka 6 ka EQ9 ka 9 ka EQ 0 ka 6k0kEQ_OG EQ 6 ka 6k6kEQ_OG EQ 9 ka 6k9kEQ_OG EQ 9 ka 9k0kEQ_OG EQ 9k6kEQ_OG EQ 9k9kEQ_OG 9k9kEQ_OG_clouds 0 ka0 ka 6 ka 9 ka EQ EQ 0k6kEQ_OGGIS 0k9kEQ_OGGIS 0 ka0 ka 6 ka 9 ka EQ EQ 0k6kEQ_OG 0k9kEQ_OG Extreme experiments that were performed and the mean annual Arctic (66.5 Equilibrium experiments that were performed and the mean annual Arctic (66.5 6 ka6 ka6 ka0 ka0 ka 100 % Grass 100 % Grass EQ 100 % Desert EQ 100 % Grass EQ 6k100gEQ_clouds 100 % Desert 6k100gEQ EQ 6k100dEQ EQ 0k100gEQ 0k100dEQ ORB 9 ka 100 % Desert EQ 9k100dEQ 9 ka 100 % Grass EQ 9k100gEQ Table 2. temperature for each simulation. Table 1. temperature for each simulation. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

S VEG (0– tion ons, EG ibed ibed phy, phy, melt melt ii aa rr aa VV (c) GHG (Orbital + as veget t Topogr m simulat melt, GIS melt, e gases), e gases), and presc h OGGIS equilibrium e u u s s B B LISTOPO (Orbital and (b) + HG, OR Greenhou region suc Iceshe ide tation, LIS S equilibri t S GHG (0–9 ka); t G G e e GHG + + OG and rbital and rbital ORB b) OGGI ahara veg he Sahara nd Lauren includes: t t O d O d (a) S S s a a erent forcings; ORB ff (b) B+GHG ( e Gases s outside rescribed rescribed or OG thi a) OG an r r s s r p r p R R F F . .

σ 1 σ 1 1670 1669 ± Greenhou rcings; O G, ORB, nd ALL ( Arctic fo nt ±1 other facto o o o a a e e e e H ALL (0–9 ka); H (a) nge in th rbital and rbital OTHER ( different f 9 to 6ka) ars repres cludes: G

a (Vegetation changes in the Sahara), OTHER (other factors a b b n n f O f O S erature ch e Sahara), GIS this i es). Error es). Error STOPO ( ibutions o ibutions the period the period pp II hh rr rr gg GG lated tem +GHG+L on; for O elevant fo elevant anges in t aphy chan lative cont lative u u i i r r r r B B h h Sim 100 % Desert Sahara – 100 % Green Sahara (0 ka) forcings upon global climate. tion c tion a (d) ing the re re 1. ges), OR LIS topog ch is only ara vegetat get w n u w n h h Simulated temperature change in the Arctic for gu Surface temperature anomaly plots from the OG equilibrium simulation from 9 to 0 ka

and Fi sho (Ve cha whi Sa 9 ka); and Values are 500 year means. showing the relative contributions of Fig. 2. outside the Sahara region such as vegetation changes), ORB and Greenhouse gases), VEG Fig. 1. simulations, showing the relative contributions of di Greenhouse Gases and Laurentide9–6 Icesheet ka) Topography, which and is only ALLOGGIS relevant (For this for the OG includes: period GHG, thistopography ORB, changes). includes: Error prescribed bars GHG, Sahara represent vegetation, ORB LIS and melt, prescribed GIS Sahara melt and vegetation; LIS for Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) 2 −

ntic ntic eric hted aa hh gg ects of changing ff

n in oo al atmosp North Atl a is highli u u r g vegetati n n Mean ann ) over the f the Saha f the f 2 2 - of changi s s (PW), ii) (PW), al extent o al extent maly (ms y y n o n o the effect g g nitude an ht,allowin ux anomal he latitudi l l g g g g T T

3

E) (9k0kEQ_OG–9k9kEQ_OG). The latitudinal

4 ◦ ential Hei ential t 1672 1671 t nic heat f Q_OG). wind ma a a l l E E

W–15 d Pa Geopo e h h ◦ s s OG-9k9k ean annua annual oce _ _ M M _OG 800 be visuali QQ oo and iii) e i) Mean e i) Mean ) (9k0kEQ ressure, t hh EQ-9k9kE EE p p k k 9k0 re 3. aly (PW) aly (PW) Sahara, on u W to 15° W icts both t gu º º m m Fi the Dep re 4. eference. flux ano ined as 60 r r t u t f f gu

Fi hea (de for Depicts both the (i) mean annual oceanic heat flux anomaly (PW), (ii) mean annual 9k0kEQ–9k9kEQ_OG 800 hPa Geopotential Height, allowing the e over the North Atlantic (defined as 60 Fig. 4. atmospheric heat flux anomaly (PW), and (iii)extent mean of annual the Sahara wind is magnitude highlighted anomaly for reference. (ms Fig. 3. vegetation in the Sahara, on pressure, to be visualised. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

d e ds- bb uu

ds prescri 0gEQ_clo u u 0 0 hara (6k1 ern day clo e a a d with mod uds – 0k S a o o scribed cl – 0k Sahar e e

5 with 6k pr een Sahara r r 6k Green Sahara with 6k prescribed clouds–0k 1673 (b) 6k Green Sahara–0k Sahara with modern day clouds een Sahara en a) 6k G r r (a) d b) 6k G nge betwe aa nn erature ch 100dEQ) a p p k k

Tem re 5. 00gEQ-0 00dEQ) u 1 1 gu

Fi (6k 0k1 Temperature change between Sahara (6k100gEQ_clouds–0k100dEQ). Fig. 5. prescribed (6k100gEQ–0k100dEQ) and