Effects of orbital forcing, greenhouse gases and ice sheets on Saharan greening in past and future multi-millennia

Abstract. Climate archives reveal alternating arid and humid conditions in North Africa during the last several million years. Most likely the dry phases resembled current hyper-arid landscapes, whereas the wet phases known as African humid periods (AHPs) sustained much more surface water and greater vegetated areas that “greened” large parts of the Sahara region. Previous analyses of sediment cores from the Mediterranean Sea showed the last five AHPs differed in strength, duration and rate of change. To understand the causes of such differences we perform transient simulations of the past 190 000 years with the Earth system model of intermediate complexity CLIMBER-2. We analyse the amplitude and rate of change of the modelled AHP responses to changes in orbital parameters, greenhouse gases (GHGs) and ice sheets. In agreement with estimates from Mediterranean Sea sapropels, we find the model predicts a threshold in orbital forcing for Sahara greening and occurrence of AHPs. Maximum rates of change in simulated vegetation extent at AHP onset and termination correlate strongly with the rate of change of the orbital forcing. As suggested by available data for the Holocene AHP, the onset of modelled AHPs usually happens faster than termination. A factor separation analysis confirms the dominant role of the orbital forcing in driving the amplitude of precipitation and vegetation extent for past AHPs. Forcing due to changes in GHGs and ice sheets is only of secondary importance, with a small contribution from synergies with the orbital forcing. Via the factor separation we detect that the threshold in orbital forcing for AHP onset varies with GHG levels. To explore the implication of our finding from the palaeoclimate simulations for the AHPs that might occur in a greenhouse-gas-induced warmer climate, we extend the palaeoclimate simulations into the future. For the next 100 000 years the variations in orbital forcing will be smaller than during the last 100 millennia, and the insolation threshold for the onset of late Quaternary AHPs will not be crossed. However, with higher GHG concentrations the predicted threshold drops considerably. Thereby, the occurrence of AHPs in upcoming millennia appears to crucially depend on future concentrations of GHGs.

[1]  P. Valdes,et al.  Paleoclimate-conditioning reveals a North Africa land–atmosphere tipping point , 2021, Proceedings of the National Academy of Sciences.

[2]  L. François,et al.  Early Holocene greening of the Sahara requires Mediterranean winter rainfall , 2021, Proceedings of the National Academy of Sciences.

[3]  G. Schmiedl,et al.  Nature and dynamics of North African humid and dry periods during the last 200,000 years documented in the clay fraction of Eastern Mediterranean deep-sea sediments , 2021 .

[4]  A. Timmermann,et al.  Drivers of river reactivation in North Africa during the last glacial cycle , 2021, Nature Geoscience.

[5]  M. Claussen,et al.  Influence of the representation of convection on the mid-Holocene West African Monsoon , 2021, Climate of the Past.

[6]  W. Peltier,et al.  African Humid Period Precipitation Sustained by Robust Vegetation, Soil, and Lake Feedbacks , 2020, Geophysical Research Letters.

[7]  A. Berg,et al.  The Greening of the Sahara: Past Changes and Future Implications , 2020 .

[8]  J. Kutzbach,et al.  African climate response to orbital and glacial forcing in 140,000-y simulation with implications for early modern human environments , 2020, Proceedings of the National Academy of Sciences.

[9]  M. Claussen,et al.  The end of the African humid period as seen by a transient comprehensive Earth system model simulation of the last 8000 years , 2020 .

[10]  C. Skinner,et al.  Modulation of Mid‐Holocene African Rainfall by Dust Aerosol Direct and Indirect Effects , 2019, Geophysical Research Letters.

[11]  J. Jungclaus,et al.  Northern Hemisphere Monsoon Response to Mid‐Holocene Orbital Forcing and Greenhouse Gas‐Induced Global Warming , 2019, Geophysical Research Letters.

[12]  A. Bory,et al.  Monsoon-driven Saharan dust variability over the past 240,000 years , 2019, Science Advances.

[13]  M. Claussen,et al.  The end of the African humid period as seen by a transient comprehensive Earth system model simulation of the last 8000 years , 2019 .

[14]  Mark Maslin,et al.  Pastoralism may have delayed the end of the green Sahara , 2018, Nature Communications.

[15]  V. Brovkin,et al.  Simulation of climate, ice sheets and CO 2 evolution during the last four glacial cycles with an Earth system model of intermediate complexity , 2017 .

[16]  P. deMenocal,et al.  A climatic context for the out-of-Africa migration , 2017 .

[17]  D. Heslop,et al.  A 3 million year index for North African humidity/aridity and the implication of potential pan-African Humid periods , 2017 .

[18]  M. Claussen,et al.  Theory and Modeling of the African Humid Period and the Green Sahara , 2017 .

[19]  G. Schmiedl,et al.  Intensity of African Humid Periods Estimated from Saharan Dust Fluxes , 2017, PloS one.

[20]  P. deMenocal,et al.  Rainfall regimes of the Green Sahara , 2017, Science Advances.

[21]  A. Coe,et al.  Drivers of ecosystem and climate change in tropical West Africa over the past ∼540 000 years , 2016 .

[22]  U. Mikolajewicz,et al.  The timing of Mediterranean sapropel deposition relative to insolation, sea-level and African monsoon changes , 2016 .

[23]  Ricarda Winkelmann,et al.  Critical insolation-CO2 relation for diagnosing past and future glacial inception , 2016, Nature.

[24]  G. Messori,et al.  Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period , 2016 .

[25]  H. Schellnhuber,et al.  Critical insolation–CO2 relation for diagnosing past and future glacial inception , 2016, Nature.

[26]  M. Kageyama,et al.  Evaluation of CMIP5 palaeo-simulations to improve climate projections , 2015 .

[27]  P. Gibbard,et al.  An optimized scheme of lettered marine isotope substages for the last 1.0 million years, and the climatostratigraphic nature of isotope stages and substages , 2015 .

[28]  J. Overpeck,et al.  The time-transgressive termination of the African Humid Period , 2015 .

[29]  E. Bauer,et al.  Sensitivity simulations with direct shortwave radiative forcing by aeolian dust during glacial cycles , 2014 .

[30]  M. Kageyama,et al.  Relative impacts of insolation changes, meltwater fluxes and ice sheets on African and Asian monsoons during the Holocene , 2013, Climate Dynamics.

[31]  A. Roberts,et al.  Dynamics of Green Sahara Periods and Their Role in Hominin Evolution , 2013, PloS one.

[32]  Yan Zhao,et al.  Evaluation of climate models using palaeoclimatic data , 2012 .

[33]  P. Fearnhead,et al.  Optimal detection of changepoints with a linear computational cost , 2011, 1101.1438.

[34]  M. Claussen,et al.  Implications of climate variability for the detection of multiple equilibria and for rapid transitions in the atmosphere-vegetation system , 2012, Climate Dynamics.

[35]  A. Ganopolski,et al.  The role of orbital forcing, carbon dioxide and regolith in 100 kyr glacial cycles , 2011 .

[36]  G. Krinner,et al.  Sahara and Sahel vulnerability to climate changes, lessons from Holocene hydrological data , 2011 .

[37]  R. S. Thompson,et al.  Pollen-based continental climate reconstructions at 6 and 21 ka: a global synthesis , 2011 .

[38]  S. Weber,et al.  The impact of varying ice sheets and greenhouse gases on the intensity and timing of boreal summer monsoons , 2011 .

[39]  A. Ganopolski,et al.  Aeolian dust modeling over the past four glacial cycles with CLIMBER-2 , 2010 .

[40]  M. Ziegler,et al.  The precession phase of the boreal summer monsoon as viewed from the eastern Mediterranean (ODP Site 968) , 2010 .

[41]  M. Claussen,et al.  Simulation of the last glacial cycle with a coupled climate ice-sheet model of intermediate complexity , 2009 .

[42]  M. Claussen,et al.  Coherent high- and low-latitude control of the northwest African hydrological balance , 2008 .

[43]  David Archer,et al.  The millennial atmospheric lifetime of anthropogenic CO2 , 2008 .

[44]  V. Brovkin,et al.  Lowering of glacial atmospheric CO2 in response to changes in oceanic circulation and marine biogeochemistry , 2007 .

[45]  Yi Wang,et al.  On the cause of abrupt vegetation collapse in North Africa during the Holocene: Climate variability vs. vegetation feedback , 2006 .

[46]  A. Ganopolski,et al.  Simulation of climate phase lags in response to precession and obliquity forcing and the role of vegetation , 2005 .

[47]  Carlo Barbante,et al.  Eight glacial cycles from an Antarctic ice core , 2004, Nature.

[48]  S. Weber,et al.  The response of the African summer monsoon to remote and local forcing due to precession and obliquity , 2003 .

[49]  Victor Brovkin,et al.  Climate Change in Northern Africa: The Past is Not the Future , 2003 .

[50]  Victor Brovkin,et al.  Carbon cycle, vegetation, and climate dynamics in the Holocene: Experiments with the CLIMBER‐2 model , 2002 .

[51]  Frank Lunkeit,et al.  Earth system models of intermediate complexity: closing the gap in the spectrum of climate system models , 2002 .

[52]  H. Brumsack,et al.  Geological constraints on tidal dissipation and dynamical ellipticity of the Earth over the past three million years , 2001, Nature.

[53]  Stefan Rahmstorf,et al.  Rapid changes of glacial climate simulated in a coupled climate model , 2001, Nature.

[54]  C. Waelbroecka,et al.  Sea-level and deep water temperature changes derived from benthic foraminifera isotopic records , 2001 .

[55]  Victor Brovkin,et al.  CLIMBER-2: a climate system model of intermediate complexity. Part I: model description and performance for present climate , 2000 .

[56]  Joseph D. Ortiz,et al.  Abrupt onset and termination of the African Humid Period:rapid climate responses to gradual insolation forcing , 2000 .

[57]  Victor Brovkin,et al.  CLIMBER-2: a climate system model of intermediate complexity. Part II: model sensitivity , 2001 .

[58]  V. Brovkin,et al.  A new model for climate system analysis: Outline of the model and application to palaeoclimate simulations , 1999 .

[59]  Victor Brovkin,et al.  Simulation of an abrupt change in Saharan vegetation in the Mid‐Holocene , 1999 .

[60]  J. Jouzel,et al.  Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica , 1999, Nature.

[61]  V. Brovkin,et al.  On the stability of the atmosphere‐vegetation system in the Sahara/Sahel region , 1998 .

[62]  Stefan Rahmstorf,et al.  Simulation of modern and glacial climates with a coupled global model of intermediate complexity , 1998, Nature.

[63]  V. Brovkin,et al.  A continuous climate-vegetation classification for use in climate-biosphere studies , 1997 .

[64]  R. Greve.  A continuum–mechanical formulation for shallow polythermal ice sheets , 1997, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[65]  W. Peltier,et al.  Ice Age Paleotopography , 1994, Science.

[66]  Pinhas Alpert,et al.  Factor Separation in Numerical Simulations. , 1993 .

[67]  Ann Henderson-Sellers,et al.  Biosphere-atmosphere transfer scheme(BATS) version 1e as coupled to the NCAR community climate model , 1993 .

[68]  Thomas F. Stocker,et al.  A Zonally Averaged, Coupled Ocean-Atmosphere Model for Paleoclimate Studies , 1992 .

[69]  M. Rossignol-Strick.  African monsoons, an immediate climate response to orbital insolation , 1983, Nature.

[70]  J. Kutzbach,et al.  Monsoon Climate of the Early Holocene: Climate Experiment with the Earth's Orbital Parameters for 9000 Years Ago , 1981, Science.

[71]  André Berger,et al.  Long-term variations of daily insolation and Quaternary climatic changes , 1978 .

[72]  A. Vernekar.  Long-Period Global Variations of Incoming Solar Radiation , 1972 .