It is widely believed that the Sahara desert is no more than ∼2–3 million years (Myr) old1, with geological evidence showing a remarkable aridification of north Africa at the onset of the Quaternary ice ages2,3,4. Before that time, north African aridity was mainly controlled by the African summer monsoon (ASM)5,6,7,8, which oscillated with Earth’s orbital precession cycles. Afterwards, the Northern Hemisphere glaciation added an ice volume forcing on the ASM, which additionally oscillated with glacial–interglacial cycles2. These findings led to the idea that the Sahara desert came into existence when the Northern Hemisphere glaciated ∼2–3 Myr ago. The later discovery, however, of aeolian dune deposits ∼7 Myr old9 suggested a much older age, although this interpretation is hotly challenged1 and there is no clear mechanism for aridification around this time. Here we use climate model simulations to identify the Tortonian stage (∼7–11 Myr ago) of the Late Miocene epoch as the pivotal period for triggering north African aridity and creating the Sahara desert. Through a set of experiments with the Norwegian Earth System Model10 and the Community Atmosphere Model11, we demonstrate that the African summer monsoon was drastically weakened by the Tethys Sea shrinkage during the Tortonian, allowing arid, desert conditions to expand across north Africa. Not only did the Tethys shrinkage alter the mean climate of the region, it also enhanced the sensitivity of the African monsoon to orbital forcing, which subsequently became the major driver of Sahara extent fluctuations. These important climatic changes probably caused the shifts in Asian and African flora and fauna observed during the same period4,12,13,14, with possible links to the emergence of early hominins in north Africa15,16.
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We thank N. Caud for her help in preparing the paper, and F. Guy for discussions on hominin emergence and evolution. This study was jointly supported by the Strategic and Special Frontier Project of Science and Technology of the Chinese Academy of Sciences (grant no. XDA05080803); the National 973 Program of China (grant no. 2010CB950102); the Earth System Modelling (ESM) project financed by Statoil, Norway; the Dynamics of Past Warm Climates (DYNAWARM) project financed by the Centre for Climate Dynamics (SKD) at the Bjerknes Centre; and the Aurora mobility program France–Norway financed by the Research Council of Norway.
The authors declare no competing financial interests.
Extended data figures and tables
a–d, Topography and bathymetry (metres) used in the Late Oligocene (a), Early Miocene (b) and Late Miocene (c), and modern (d) NorESM-L coupled simulations. e–h, The land–sea distribution and annual sea surface temperature (SST, °C) used in the Control (e), WTopen (f), PTopen (g) and TSwarm (h) CAM4 atmosphere-only experiments.
The red shading shows the zonal-mean temperature simulated in the Late Oligocene experiments, with the upper line for experiment LO-700CO2 and the lower line for experiment LO-560CO2. The dark blue shading shows the zonal-mean temperature simulated in the Early Miocene experiments, with the upper line for experiment EM-560CO2 and the lower line for experiment EM-420CO2. The light blue shading shows the zonal temperature simulated in the Late Miocene experiments, with the upper line for experiment LM-560CO2 and the lower line for experiment LM-350CO2. The vertical lines show the Late Miocene temperature reconstructions compiled in refs 47 and 48.
a, LO-700CO2; b, EM-420CO2; c, LM-350CO2; d, MD-280CO2; e, LO-560CO2; f, EM-560CO2; g, LM-560CO2; h, MD-560CO2. The arid desert climate includes BWh and BWk. The semiarid steppe climate includes BSh and BSk. Detailed information about Köppen climate classification can be found in ref. 50 or at http://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification.
Changes in annual precipitation (mm, shading) and 850-hPa summer winds (m s−1, arrows) caused by the closing of the West Tethys (Control minus WTopen) (a), the closing of the Paratethys (WTopen minus PTopen) (b), the cooling of the Tethys (PTopen minus TSwarm) (c), and all three factors (Control minus TSwarm) (d). Only changes that are significant at the 95% confidence level (two-tailed unequal t-test) are shown.
a, b, The 850-hPa summer winds (m s−1) simulated in the TSwarm experiment (a) and the Control experiment (b). c, Changes in sea level pressure (Pa, shading) and 850-hPa summer winds (m s−1, arrows) between the Control and the TSwarm experiments. Only changes that are significant at the 95% confidence level (two-tailed unequal t-test) are shown.
a, b, Comparison of the simulated summer ITCZ in the Control (blue) and TSwarm (red) experiments (a) and in the Control (blue) and IPflat (green) experiments (b). The shaded areas show the range of maximum and minimum ITCZ position6 and the dots show the position of climatological mean ITCZ. c, d, Changes in sea level pressure (Pa, shading) and 850-hPa summer winds (m s−1, arrows) between the Control and TSwarm experiments (c) and between the Control and IPflat experiments (d). e, f, Changes in annual precipitation (mm, shading) and 850-hPa summer winds (m s−1, arrows) between the Control and TSwarm experiments (e) and between the Control and IPflat experiments (f). Only changes that are significant at the 95% confidence level (two-tailed unequal t-test) are shown.
Differences (W m−2) between the Control and TSwarm experiments (a, c, e) and the Control and IPflat experiments (b, d, f). a, b, Net energy balance of the atmospheric column. c, d, Net surface heat flux. e, f, Surface latent heat flux. Negative values indicate less heat into the atmosphere. Only changes that are significant at the 95% confidence level (two-tailed unequal t-test) are shown.
Extended Data Figure 8 Climate sensitivity to increased Northern Hemisphere summer insolation before and after Tethys shrinkage.
Response in sea level pressure (Pa, shading) and 850-hPa summer winds (m s−1, arrows) to a change in orbital parameters from today to 6 kyr ago (higher minus lower Northern Hemisphere summer insolation) for continental configuration with a large Tethys (a) and a modern land–sea distribution (b). Only changes that are significant at the 95% confidence level (two-tailed unequal t-test) are shown.
Extended Data Figure 9 Energy balance response to increased Northern Hemisphere summer insolation before and after Tethys shrinkage.
Differences (W m−2) caused by the change in orbital parameters from today to 6 kyr ago (higher minus lower Northern Hemisphere summer insolation) for continental configuration with a large Tethys (a, c, e) and modern land–sea distribution (b, d, f). a, b, Net energy balance of the atmospheric column. c, d, Net top-of-the-atmosphere heat flux. e, f, Net surface heat flux. Positive values indicate more heat into the atmosphere for a–d and more heat from the atmosphere into the surface for e and f. Only changes that are significant at the 95% confidence level (two-tailed unequal t-test) are shown. Although the change in orbital parameters leads to similar increases in the top-of-the-atmosphere heat flux under the two continental configuration conditions (c versus d), the large Tethys produces larger increases in atmosphere-to-surface heat fluxes (e versus f) and these surface fluxes dominate the net energy balance (a versus b).
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Zhang, Z., Ramstein, G., Schuster, M. et al. Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene. Nature 513, 401–404 (2014) doi:10.1038/nature13705
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