CL1.2.4

During the mid-Holocene (6,000 years ago) the Northern Hemisphere experienced a reinforcement of the monsoonal regime, which led to the so-called “African Humid Period” (AHP) and to the greening of the Sahara region. Paleoclimate archives also show a gradual cooling of north-eastern Atlantic and the warming of the western subtropical Atlantic, eastern Mediterranean and northern Red Sea during the Holocene. These changes were likely accompanied by a positive-to-negative transition of the AO/NAO phase from mid-late Holocene into the pre-industrial period, leading to climate impacts in North America, Europe, the Mediterranean and Siberia.

However, inconsistencies still exist between proxies and model simulations of the Holocene climate. To explain the limitations of climate models, several studies pointed out the role of the vegetation feedback at tropical and higher latitudes. The objective of this study is to investigate the impact of the Green Sahara on the Northern Hemispheric mid-latitude atmospheric circulation and associated climate variability during the African Humid Period. To this aim, vegetated Sahara with reduced dust emission is prescribed into a coupled ocean-atmosphere model (the Green Sahara experiment).

Model simulations show a sizable impact on the main circulation features in the Northern Hemisphere when the Green Sahara is prescribed, especially during boreal summer, when the African monsoon develops. This study provides a first constraint on the Green Sahara influence on northern mid-latitudes, indicating new opportunities for understanding mid-Holocene climate anomalies in North America and Eurasia. However, inconsistencies between proxies and model simulations still persist in the Green Sahara experiment, indicating that more accurate simulations of the MH climate modifications are needed (e.g. prescribing realistic vegetation at mid and high latitudes, considering seasonal cycle in vegetation cover).

How to cite: Gaetani, M., Messori, G., Alvarez Castro, M. C., Zhang, Q., and Pausata, F. S. R.: Mid-Holocene climate at mid-latitudes: modelling the impact of the Green Sahara, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2031, https://doi.org/10.5194/egusphere-egu22-2031, 2022.

The Green Sahara Period, spanning about 11,500 to 5,000 years ago, offers an opportunity to test the ability of climate models to simulate large-scale changes in northern African climate through the strengthening of the West African Monsoon. In this study, we evaluate the performance of four models in simulating the mid-Holocene (6,000 BP), namely – EC-Earth, iCESM, CCSM4-Toronto, and the GISS ModelE2.1-G. Two scenarios are considered for each model – a standard PMIP scenario simulated with the mid-Holocene orbital parameters and greenhouse gas concentrations with vegetation prescribed to pre-industrial conditions, as well as a Green-Sahara scenario which additionally considers factors such as enhanced vegetation, reduced dust, presence of lakes, and land and soil feedbacks. All mid-Holocene scenarios capture an increase in monsoonal precipitation in northern Africa. However, a comparison of the two mid-Holocene scenarios reveals significantly higher precipitation in northern Africa for all the Green-Sahara scenarios relative to the PMIP scenarios – an observation consistent across all models. Accompanied by a strengthened Saharan Heat Low, these changes in the West African Monsoon are also linked to polar amplification, a stronger Indian Summer Monsoon and alterations to the Walker circulation. Model results are in agreement with pollen-based SAT records, multi-proxy SST records and African lake level records. This comparison indicates that a realistic simulation of the mid-Holocene Green Sahara requires consideration of multiple factors in addition to orbital and greenhouse gas forcings.

How to cite: Tiwari, S., Ramos, R., Pausata, F. S. R., LeGrande, A. N., Griffiths, M. L., Beltrami, H., Chandan, D., de Vernal, A., Litchmore, D., Peltier, R., and Tabor, C. R.: Model performance in simulating the mid-Holocene Green Sahara, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3233, https://doi.org/10.5194/egusphere-egu22-3233, 2022.

 The continental and marine pollen-based quantitative reconstructions and the West Saharan dust records of the last 150,000 years are excellent paleoclimate proxies to observe variations in humidity and aridity as well as to evaluate similarities and differences in the climate history of the West Mediterranean and West African regions. The quantitative mean annual precipitation reconstruction from the western Mediterranean (southern Iberia and Alboran Sea) shows that during the Last Interglacial and the Early Holocene, the mean annual precipitation in this region was around 600-650 mm/yr, 200 mm/yr higher than the recent values (ca. 450 mm/yr). With respect to the cold Heinrich Stadials (from HS1 to HS6), the West Mediterranean and West Sahara show similar climatic trends and are characterized by strong and synchronous droughts. In particular, during these periods the West Mediterranean stack suggests mean annual precipitation values of 200-350 mm/yr. Here, we aim to elucidate the climate mechanisms affecting both regions during the most arid events of the last glacial period (i.e., Heinrich Stadials) as well as the similarities/differences between the African Humid Periods and the West Mediterranean Humid Periods for the last interglacial-glacial cycle.

How to cite: Camuera Bidaurreta, J., Ramos-Román, M. J., Jiménez-Moreno, G., García-Alix, A., and Seppä, H.: Hydroclimatic variability in the West Sahara and West Mediterranean during the last 150,000 years, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3696, https://doi.org/10.5194/egusphere-egu22-3696, 2022.

During orbital precession minima, the Sahara was humid and more vegetated. Uncertainties remain over the climatic processes controlling the initiation, demise and amplitude of these African Humid Periods (AHPs). Here we study these processes using a series of transient simulations of the penultimate deglaciation and Last Interglacial period performed with an Earth system model of intermediate complexity (LOVECLIM). These results are compared to a transient simulation of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) at the end of deglacial millennial-scale events exerts a dominant control on the abrupt initiation of AHPs, as the AMOC modulates the position of the Intertropical Convergence Zone. In addition, residual Northern Hemispheric ice-sheets can delay the peak of the AHPs. Through its impact on Northern Hemispheric ice-sheets disintegration and thus AMOC, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt onset of the AHP during the Last Interglacial period. Finally, we show that the mean climate state modulates precipitation variability, with higher variability under wetter background conditions.

How to cite: Menviel, L., Govin, A., Avenas, A., Meissner, K., Grant, K., and Tzedakis, P.: Drivers of the evolution and amplitude of African Humid Periods, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6415, https://doi.org/10.5194/egusphere-egu22-6415, 2022.

The hyper-arid Saharan desert belt stretching across North Africa is an important part of the global climate system, with dust export shown to influence climate systems such as ENSO and distant monsoon systems. Understanding climate dynamics and potential future changes in this region is however difficult due to a paucity in both instrumental and high-resolution paleoclimate data. There is strong evidence for periods of increased rainfall across large parts of North Africa during the late Quaternary, termed ‘Green Sahara’ periods, which contribute to regional aquifer recharge and improved human population connectivity across the Sahara. There is, however, currently limited evidence regarding: i) precisely where and when rainfall occurred and; ii) the sources of moisture contributing to increased rainfall at the northern-most reaches of the Sahara.

In this study, we present new proxy reconstructions from the northern limits of the presently hyper-arid Sahara Desert, to identify moisture sources, timing and latitudinal extent of rainfall change during these so-called Green Sahara periods. We do this using several ancient fossil stalagmites collected from cave sites in the desert foothills of the central Saharan Atlas Mountains, Algeria. High-precision U-Th chronology and stable-isotope measurements on calcite samples from multiple cave sites contribute towards an east-west transect of records. Due to the locations of the caves, stalagmite growth periods and stable isotope records provide direct evidence of where and when there was significantly increased rainfall in this region, and help us to identify potential sources of moisture through time. We present these results, and their implications for a more detailed reconstruction of the occurrence of Green Sahara periods in northwest Africa.

How to cite: Couper, H., Day, C., Maouche, S., Deramchi, A., Carolin, S., Mason, A., Derder, M. E. M., Yelles Chaouche, A., and Henderson, G.: Northern Sahara speleothems record timing of rainfall and moisture source during Green Sahara periods, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8249, https://doi.org/10.5194/egusphere-egu22-8249, 2022.

Social, biological and environmental dynamics have affected the way of humans out of Africa during the late Quaternary. Hence understanding the dynamics of these three factors and their interaction is a prerequisite for understanding human migration. Conceptual climate and environmental modeling is often based on individual paleo climate records which are sparsely distributed in space and time, and fewer and fewer records exist the further one looks into the past. Can climate system modeling provide a way forward? So far, different climate system models yield completely different patterns of past greening in the Sahara. None of the global models is able to generate local landscape changes like the emergence of gallery forests or wetlands that could provide green corridors or barriers for migration. Considering a spectrum of models of different complexity might be the way forward. Coarse-scale models can be used to explore multi-millennial-scale and continent-scale dynamics, thereby providing information on the large-scale effect of orbital forcing or the gross differences in the overall dynamics of the last African Humid Periods, for example. Global climate system models with a grid size of some 100 km, e.g. the CMIP models, yield interesting insight into large-scale atmospheric dynamics and regional heterogeneity, like differences between West and East Sahara weather and vegetation patterns. This type of models can also help reconciling seemingly divergent reconstructions, such as the discussion of abrupt vs gradual termination of Saharan greening some 5000 years ago. For a more detailed view on local landscape changes, regional climate models operating at km-scale are necessary to resolve the complex orography, mesoscale convection and related local climate changes. These models can currently be run over seasons only. However, development of a new generation of Earth system models bodes well for the potential use of global high-resolution simulations. In summary, we suppose that using the spectrum of climate system models will bring models and proxy data closer together and will advance our understanding of past climate change and human migration.

How to cite: Claussen, M., Dallmeyer, A., Duque Villegas, M., and Jungandreas, L.: Green Sahara spatial and temporal patterns, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9682, https://doi.org/10.5194/egusphere-egu22-9682, 2022.

Variations in the Earth's orbit are recognised as the main trigger for the hydrological changes that led to the periodic 'greening' of the Sahara region over the late Quaternary. However, the frequency and amplitude of the greening events as seen in the geological records cannot be predicted from orbital theory alone. To understand the changes in the proxy data it is also important to consider feedback mechanisms that arise from the complexity of the interactions between the vegetation, land, atmosphere and ocean components in the region. Yet discrepancies between state-of-the-art computer simulations of greening during African Humid Periods (AHPs) and proxy data still remain. We hypothesize that the effects of additional internal forcing from other climate drivers like atmospheric levels of greenhouse gases (GHGs) and extension of ice sheets may have had a greater impact than previously thought. Using two climate models of varying complexity and spatial resolution, CLIMBER-2 and MPI-ESM, we simulate several of the greening events in the Sahara within the last glacial cycle and study the effects of the orbital, GHGs and ice sheets forcings for every greening response. The results from CLIMBER-2 suggest that the critical insolation at the Tropics required for AHPs onset depends on atmospheric levels of GHGs, while the results from MPI-ESM show that the spatial pattern that develops during AHPs varies with all three forcing factors. These findings highlight the role that GHGs may play for the future of Saharan climate, when low--eccentricity orbits concur with high levels of atmospheric GHGs.

How to cite: Duque-Villegas, M., Claussen, M., Brovkin, V., and Kleinen, T.: Orbital and non-orbital drivers of late Quaternary African Humid Periods, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9906, https://doi.org/10.5194/egusphere-egu22-9906, 2022.

The last decades have seen rapid growth of renewable energy globally for accommodating the urgent need of mitigating climate change. Large-scale projects like solar farms are actively financed by transnational investors to get established in drylands like Sahara. The Earth-system model simulations on large-scale solar-farm scenarios show an increased regional rainfall and vegetation cover, analogue to a “green Sahara” that happened in the past. It will not only induce local climate and ecosystem changes but also prompt remote impacts globally through atmospheric teleconnections and ocean dynamics. This suggests that spatial tensions are inherent to climate change mitigation measures, where action in one place at a particular time impacts not only this place and the short time but place at distance and time in the future. Meanwhile, case studies in social sciences seem to suggest common unintended social consequences of the ongoing projects but no systematic assessment across these projects has been done. This study thus aims to pilot an interdisciplinary investigation of the multi-dimensional effects of large-scale renewable energy projects, mainly solar farms in drylands. Our literature review of the social effects across solar farms and other major types of renewable energy projects shows that, local host communities widely bear adverse social consequences from these projects despite there are benefits at regional, national, and transnational levels. Economic redistribution and social differentiation rapidly occur through land acquisition, livelihoods, compensation, and development programs, further dividing local communities and amplifying inequalities. These social effects could be further complicated by the likely local climate and ecosystem changes as shown by our Earth-system model simulations. Based on this combined analysis, we conclude that spatial tensions in the current climate change mitigation measures challenge the assumption of global common goods and the reach of global justices. We urge interdisciplinary research to combine their different expertise for developing integrated conceptual and methodological models, for better understanding the intersected effects of renewable energy projects on drylands, and for advising fair and just climate mitigation policy and measures.

How to cite: Zhang, Q., Berntell, E., Lu, Z., and Zhang, Q.: Unpacking spatial tensions: An interdisciplinary analysis of large-scale solar farm effects in drylands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11740, https://doi.org/10.5194/egusphere-egu22-11740, 2022.

The Sahara is a vast, bare, intensely arid, dust exporting landscape today. Yet, in the early Holocene, the Sahara was green; a well-vegetated landscape crosscut by a network of rivers and lakes, populated by hippopotamuses, other megafauna and our early ancestors. Strong evidence also exists for multiple earlier Green Sahara Periods (GSPs), with their occurrence paced by variability in solar insolation. However, terrestrial climate archives used to provide direct evidence of past humid conditions are often plagued with intervals of erosion and/or non-deposition, while sapropels (organic-rich sediment layers in the Mediterranean Sea) only provide an indirect record of North African climate. Here, we explore how the expression of GSPs has changed across a range of global climate states, including warmer intervals than today, with new, detailed records of terrigenous inputs to North Atlantic deep-sea sediments situated underneath the Saharan dust plume. We document a long and sustained history of astronomically-paced oscillations between distinctly humid and arid conditions from at least 11 million years ago, with three distinct phases in the sensitivity of the relationship between astronomical forcing and African hydroclimate identified. Our data provide a new framework for assessing evolutionary outcomes on land, including implications for our hominid ancestors.

How to cite: Crocker, A., Naafs, B. D., Westerhold, T., James, R., Cooper, M., Röhl, U., Pancost, R., Xuan, C., Osborne, C., Beerling, D., and Wilson, P.: Green Sahara Periods in a warmer world: a proxy-based reconstruction of the last 11 Myr, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12442, https://doi.org/10.5194/egusphere-egu22-12442, 2022.

Proxy records have shown that the Mid-Holocene was a period of humid conditions across West Africa, with an enhanced West African Monsoon (WAM) reaching far into the Sahara region and with vegetation covering areas currently characterized by desert leading to conditions being referred to as the Green Sahara. However, General Circulation Models struggle with recreating this strengthened Mid-Holocene monsoon, and the results from the latest PMIP4 simulations showed a clear underestimation of the rainfall enhancement across the Sahel and Sahara region. Understanding what physical processes drive the variability of the WAM, and including those processes in our simulations, might aid in closing this gap. The vegetation-albedo feedback has long been viewed as an important process modulating the monsoon variability in West Africa, and simulations using prescribed vegetation to recreate a Green Sahara have exhibited a strengthening of the WAM and increased rainfall. However, these simulations represent an idealised vegetation cover based on proxy records found along the west coast of West Africa and do not take any environmental heterogeneity into account. Furthermore, this only represents a one-directional forcing by the vegetation on the climate, rather than the vegetation-albedo feedbacks. This might therefore over-/underestimate the changes of the WAM, as well as over-/understate the importance of the vegetation feedbacks. To address this, we have simulated the Mid-Holocene (~6 ka) climate using the high-resolution Earth System Model EC-Earth3-Veg. The results show that coupled dynamic vegetation reproduce a clear enhancement of the WAM when compared to simulations with a prescribed modern vegetation cover. This enhances warming of the Sahara region and deepens the Sahara Heat Low resulting in increased rainfall and strengthened monsoonal flow across West Africa. However, the enhancement is still below what can be viewed in proxy reconstructions which highlights the importance of investigating additional processes, such as including interactive aerosol-albedo feedbacks.

How to cite: Berntell, E. and Zhang, Q.: Vegetation feedbacks enhance the West African Monsoon during the Mid-Holocene, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12607, https://doi.org/10.5194/egusphere-egu22-12607, 2022.