Description: The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 years ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in early to mid- Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. In this article, we present an 18,500 year-long paleoclimate record from Lake Tislit in Morocco (32°N) that provides the first quantitative reconstruction of rainfall seasonality in northern Africa. The Tislit record shows that increased humidity in the AHP extended up to the North Saharan and Mediterranean regions due to increased winter rainfall, rather than summer monsoon rainfall. Based on this observation of past climate, we propose that, as a response to the orbital forcing, the AHP included a strengthening and a southward shift of the Mediterranean winter rainfall system in addition to the intensified summer monsoon, with an overlap of these rainfall zones in the Sahara. Using a mechanistic vegetation model in early Holocene conditions, we show that this hypothetical seasonal distribution of rainfall yields a more realistic representation of the Green Sahara. This new conceptual framework should be taken into consideration in Earth System paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.

Description: This dataset of 10 files (D1 to D10) provides all data used to perform the model simulations, the reconstructed modern climate from pollen samples collected in Morocco and the related modern pollen samples. More specifically here follows the description of each file from D1 to D10: D1-prcMEAN1901_1930.dat is the modern monthly precipitation with additional 300mm all throughout Northern Africa, distributed over the summer months, used for the vegetation model simulation in figure 2a. D2-prcMEAN1901_1930.dat is the modern monthly precipitation with additional 300mm for the vegetation model simulation in figure 3a. This dataset includes additional 300mm precipitation below 18°N during the summer season, above 24°N during the winter season, and a progressive transition of the additional 300mm between 18°N and 24°N (see table S2). D3-biomeMEAN1901_1930.res corresponds to the simulated biomes values using HADCM3 climatology (figure 1b). D4-nppmthMEAN1901_1930.res corresponds to the simulated NPP values using HADCM3 climatology (figure 1c). D5-biomeMEAN1901_1930.res corresponds to the biomes values using precipitation scenario for the second CARAIB simulation (figure 2b). D6-nppmthMEAN1901_1930.res corresponds to the NPP values using precipitation scenario for the second CARAIB simulation (figure 2c). D7-biomeMEAN1901_1930.res corresponds to the biomes and NPP values using precipitation scenario for the third CARAIB simulation (figure 3b). D8-nppmthMEAN1901_1930.res corresponds to the NPP values using precipitation scenario for the third CARAIB simulation (figure 3c). D9-surf_data.csv is the modern pollen data set used for reconstructing the modern precipitation in Morocco (figure S5). D10-surf_prc.csv is the modern pollen-based reconstruction of precipitation (figure S5).

Description: Lack of constraint on spatial and long-term temporal variability of the El Niño southern Oscillation (ENSO) and its sensitivity to external forcing limit our ability to evaluate climate models and ENSO future projections. Current knowledge of Holocene ENSO variability derived from paleoclimate reconstructions does not separate the role of insolation forcing from internal climate variability. Using an updated synthesis of coral and bivalve monthly resolved records, we build composite records of seasonality and interannual variability in four regions of the tropical Pacific: Eastern Pacific (EP), Central Pacific (CP), Western Pacific (WP) and South West Pacific (SWP). An analysis of the uncertainties due to the sampling of chaotic multidecadal to centennial variability by short records allows for an objective comparison with transient simulations (mid-Holocene to present) performed using four different Earth System models. Sea surface temperature and pseudo-δ18O are used in model-data comparisons to assess the potential influence of hydroclimate change on records. We confirm the significance of the Holocene ENSO minimum (HEM) 3-6ka compared to low frequency unforced modulation of ENSO, with a reduction of ENSO variance of ∼50 % in EP and ∼80 % in CP. The approach suggests that the increasing trend of ENSO since 6ka can be attributed to insolation, while models underestimate ENSO sensitivity to orbital forcing by a factor of 4.7 compared to data, even when accounting for the large multidecadal variability. Precession-induced change in seasonal temperature range is positively linked to ENSO variance in EP and to a lesser extent in other regions, in both models and observations. Our regional approach yields insights into the past spatial expression of ENSO across the tropical Pacific. In the SWP, today under the influence of the South Pacific Convergence Zone (SPCZ), interannual variability was increased by ∼200 % during the HEM, indicating that SPCZ variability is independent from ENSO on millennial time scales.

Description: Marine gateways play a critical role in the exchange of water, heat, salt and nutrients between oceans and seas. As a result, changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate, as well as having a profound impact on local environmental conditions. Mediterranean-Atlantic marine corridors that pre-date the modern Gibraltar Strait, closed during the Late Miocene and are now exposed on land in northern Morocco and southern Spain. The restriction and closure of these Miocene connections resulted in extreme salinity fluctuations in the Mediterranean, leading to the precipitation of thick evaporites. This event is known as the Messinian Salinity Crisis (MSC). The evolution and closure of the Mediterranean-Atlantic gateways are a critical control on the MSC, but at present the location, geometry and age of these gateways are still highly controversial, as is the impact of changing Mediterranean outflow on Northern Hemisphere circulation. Here, we present a comprehensive overview of the evolution of the Late Miocene gateways and the nature of Mediterranean-Atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins. We also consider the possible impact of evolving exchange on both the Mediterranean and global climate and highlight the main enduring challenges for reconstructing past Mediterranean-Atlantic exchange.